1585412160
How to Use and Write Express Middleware
Middleware is an often misunderstood topic since it sounds and appears very complicated, but in reality middleware is actually really straightforward. The entire idea of middleware is to execute some code before the controller action that sends the response and after the server gets the request from the client. Essentially it is code that executes in the middle of your request, hence the name middleware. Before I get too in depth on the details of middleware, though, I want to setup a basic Express server with two routes.
Setting Up An Express Server
To get started working with a Node.js project you will need to run npm init -y. This will create a basic package.json file with all of the default values filled in for you. From there the next thing to do is install Express by running npm i express. Lastly, we need to create a server.js file with the following code.
const express = require('express')
const app = express()
app.get('/', (req, res) => {
res.send('Home Page')
})
app.get('/users', (req, res) => {
res.send('Users Page')
})
app.listen(3000, () => console.log('Server Started'))
This server.js file simply sets up a server on port 3000 that has two routes, a home page route and a users page route. The last thing to do is run node server.js to start up the application and if everything worked you should see a message in the console saying Server Started. You can then open up any browser to localhost:3000 and you should see the message Home Page. If you go to localhost:3000/users you should then see the message Users Page.
That is all the basic setup we will need for the rest of this article. As we make changes you will need to restart your server in the console to see the changes take effect.
What Is Middleware?
I talked briefly about middleware as functions that execute after the server receives the request and before the controller action sends the response, but there are a few more things that are specific to middleware. The biggest thing is that middleware functions have access to the response (res) and request (req) variables and can modify them or use them as needed. Middleware functions also have a third parameter which is a next function. This function is important since it must be called from a middleware for the next middleware to be executed. If this function is not called then none of the other middleware including the controller action will be called.
This is all a bit difficult to understand just from text so in the next section we are going to create a logging middleware that will log the url of the request a user makes.
How To Create Logging Middleware
As I mentioned in the previous section, middleware takes three parameters, req, res, and next, so in order to create middleware we need to create a function that has those three inputs.
const express = require('express')
const app = express()
app.get('/', (req, res) => {
res.send('Home Page')
})
app.get('/users', (req, res) => {
res.send('Users Page')
})
function loggingMiddleware(req, res, next) { console.log('Inside Middleware')}
app.listen(3000, () => console.log('Server Started'))
We now have the shell of a basic middleware function defined with some placeholder content, but the application is not using it. Express has a few different ways you can define middleware to be used, but for this example we will make this middleware execute before every single controller action by adding it to the application level. This can be done by using the use function on the app variable like this.
const express = require('express')
const app = express()
app.use(loggingMiddleware)
app.get('/', (req, res) => {
res.send('Home Page')
})
app.get('/users', (req, res) => {
res.send('Users Page')
})
function loggingMiddleware(req, res, next) {
console.log('Inside Middleware')
}
app.listen(3000, () => console.log('Server Started'))
The application is now using the middleware that we defined and if we restart our server and navigate to any of the pages in our app you will notice that in the console the message Inside Middleware appears. This is great, but there is a slight problem. The application now loads forever and never actually finishes the request. This is because in our middleware we are not calling the next function so the controller action never gets called. We can fix this by calling next after our logging.
const express = require('express')
const app = express()
app.use(loggingMiddleware)
app.get('/', (req, res) => {
res.send('Home Page')
})
app.get('/users', (req, res) => {
res.send('Users Page')
})
function loggingMiddleware(req, res, next) {
console.log('Inside Middleware')
next()}
app.listen(3000, () => console.log('Server Started'))
Now if you restart the server you will notice that everything is logging correctly, and the web page is properly loading. The next thing to do is to actually log out the URL that the user is accessing inside the middleware. This is where the req variable will come in handy.
const express = require('express')
const app = express()
app.use(loggingMiddleware)
app.get('/', (req, res) => {
res.send('Home Page')
})
app.get('/users', (req, res) => {
res.send('Users Page')
})
function loggingMiddleware(req, res, next) {
console.log(`${new Date().toISOString()}: ${req.originalUrl}`) next()
}
app.listen(3000, () => console.log('Server Started'))
The logging middleware is now working 100% correctly on all the routes in the application, but we have only scratched the surface on the usefulness of middleware. In the next example we are going to take a look at creating a simple authorization middleware for the users page.
Advanced Middleware Example
To get started we need to create another function to use as middleware.
const express = require('express')
const app = express()
app.use(loggingMiddleware)
app.get('/', (req, res) => {
res.send('Home Page')
})
app.get('/users', (req, res) => {
res.send('Users Page')
})
function loggingMiddleware(req, res, next) {
console.log(`${new Date().toISOString()}: ${req.originalUrl}`)
next()
}
function authorizeUsersAccess(req, res, next) { console.log('authorizeUsersAccess Middleware') next()}
app.listen(3000, () => console.log('Server Started'))
This is just a shell of a function to be used as middleware, but we can add it to our users page route now in order to ensure that our middleware is only being executed on the users page route. This can be done by adding the function as a parameter to the app.get function for the users page.
const express = require('express')
const app = express()
app.use(loggingMiddleware)
app.get('/', (req, res) => {
res.send('Home Page')
})
app.get('/users', authorizeUsersAccess, (req, res) => { res.send('Users Page')
})
function loggingMiddleware(req, res, next) {
console.log(`${new Date().toISOString()}: ${req.originalUrl}`)
next()
}
function authorizeUsersAccess(req, res, next) {
console.log('authorizeUsersAccess Middleware')
next()
}
app.listen(3000, () => console.log('Server Started'))
Now if you restart the server and go to the users page you should see the message authorizeUsersAccess Middleware, but if you go to the home page this message will not show up. We now have middleware that only executes on a single route in the application. The next thing to do is fill in the logic of this function so that if the user does not have access to the page they will get an error message instead.
const express = require('express')
const app = express()
app.use(loggingMiddleware)
app.get('/', (req, res) => {
res.send('Home Page')
})
app.get('/users', authorizeUsersAccess, (req, res) => {
res.send('Users Page')
})
function loggingMiddleware(req, res, next) {
console.log(`${new Date().toISOString()}: ${req.originalUrl}`)
next()
}
function authorizeUsersAccess(req, res, next) {
if (req.query.admin === 'true') { next() } else { res.send('ERROR: You must be an admin') }}
app.listen(3000, () => console.log('Server Started'))
This middleware now checks to see if the query parameter admin=true is in the URL and if it is not an error message is shown to the user. You can test this by going to http://localhost:3000/users and you will see an error message explaining that you are not a admin. If you instead go to http://localhost:3000/users?admin=true you will be sent the normal users page since you set the query parameter of admin to true.
One other thing that is really useful with middleware is the ability to send data between middleware. There is no way to do this with the next function, but you can modify the req or res variables to set your own custom data. For example in the previous example if we wanted to set a variable to true if the user was a admin we could easily do that.
const express = require('express')
const app = express()
app.use(loggingMiddleware)
app.get('/', (req, res) => {
res.send('Home Page')
})
app.get('/users', authorizeUsersAccess, (req, res) => {
console.log(req.admin) res.send('Users Page')
})
function loggingMiddleware(req, res, next) {
console.log(`${new Date().toISOString()}: ${req.originalUrl}`)
next()
}
function authorizeUsersAccess(req, res, next) {
if (req.query.admin === 'true') {
req.admin = true next()
} else {
res.send('ERROR: You must be an admin')
}
}
app.listen(3000, () => console.log('Server Started'))
This code sets an admin variable on the req object which is then accessed in the controller action for the users page.
Middleware Additional Information
This is the majority of everything you need to know about middleware functions, but there a few extra things that are important to know.
1. Controller Actions Are Just Like Middleware
One thing you may have noticed is that controller actions which have a req, and res variable are very similar to middleware. That is because they are essentially middleware, but with no other middleware that comes after them. They are the end of the chain which is why there are never any next calls inside the controller action.
2. Calling next Is Not The Same As Calling return
By far the biggest mistake I see developers make when working with middleware is that they treat the next function as if it exited out of the middleware. Take for example this middleware.
function middleware(req, res, next) {
if (req.valid) {
next()
}
res.send('Invalid Request')
}
At face value this code looks correct. If the request is valid then the next function is called and if it isn’t valid then it is sending an error message. The problem is that the next function does not actually return from the middleware function. This means that when next is called the next middleware will execute and that will continue until no more middleware is left to execute. Then after all the middleware after this middleware is done executing the code will pick back up right after the next call in each of the middleware. That means that in this middleware the error message will always be sent to the user which is obviously not what you want. An easy way to prevent this is by simply returning when you call next
function middleware(req, res, next) {
if (req.valid) {
return next() }
res.send('Invalid Request')
}
Now the code will no longer execute after calling next since it will return out of the function. An easy way to see this issue in action is with the following code.
const express = require('express')
const app = express()
app.get('/', middleware, (req, res) => {
console.log('Inside Home Page')
res.send('Home Page')
})
function middleware(req, res, next) {
console.log('Before Next')
next()
console.log('After Next')
}
app.listen(3000, () => console.log('Server Started'))
When you run this code and go to the home page the console will print out the following messages in order.
Before Next
Inside Home Page
After Next
Essentially what is happening is the middleware is called and it logs out the before statement. Then next is called so the next set of middleware is called which is the controller action where the home page message is logged. Lastly the controller action finishes executing so the middleware then executes the code after next which logs out the after statement.
3. Middleware Will Execute In Order
This may seem self-explanatory but when you define middleware it will execute in the order it is used. Take for example the following code.
const express = require('express')
const app = express()
app.use(middlewareThree)
app.use(middlewareOne)
app.get('/', middlewareTwo, middlewareFour, (req, res) => {
console.log('Inside Home Page')
res.send('Home Page')
})
function middlewareOne(req, res, next) {
console.log('Middleware One')
next()
}
function middlewareTwo(req, res, next) {
console.log('Middleware Two')
next()
}
function middlewareThree(req, res, next) {
console.log('Middleware Three')
next()
}
function middlewareFour(req, res, next) {
console.log('Middleware Four')
next()
}
app.listen(3000, () => console.log('Server Started'))
Since the app.use statements come first the middleware in those statements will be executed first in the order they were added. Next the app.get middleware is defined and again they will be executed in the order they are in the app.get function. This will lead to the following console output if ran.
Middleware Three
Middleware One
Middleware Two
Middleware Four
Conclusion
That is all there is to know about middleware. Middleware is incredibly powerful for cleaning up code and making things like user authorization and authentication much easier, but it can be used for so much more than just that because of the incredibly flexibility of middleware.
#node-js #javascript #web-development
What is GEEK
Buddha Community
1667425440
Pdf2gerb: Perl Script Converts PDF Files to Gerber format
pdf2gerb
Perl script converts PDF files to Gerber format
Pdf2Gerb generates Gerber 274X photoplotting and Excellon drill files from PDFs of a PCB. Up to three PDFs are used: the top copper layer, the bottom copper layer (for 2-sided PCBs), and an optional silk screen layer. The PDFs can be created directly from any PDF drawing software, or a PDF print driver can be used to capture the Print output if the drawing software does not directly support output to PDF.
The general workflow is as follows:
- Design the PCB using your favorite CAD or drawing software.
- Print the top and bottom copper and top silk screen layers to a PDF file.
- Run Pdf2Gerb on the PDFs to create Gerber and Excellon files.
- Use a Gerber viewer to double-check the output against the original PCB design.
- Make adjustments as needed.
- Submit the files to a PCB manufacturer.
Please note that Pdf2Gerb does NOT perform DRC (Design Rule Checks), as these will vary according to individual PCB manufacturer conventions and capabilities. Also note that Pdf2Gerb is not perfect, so the output files must always be checked before submitting them. As of version 1.6, Pdf2Gerb supports most PCB elements, such as round and square pads, round holes, traces, SMD pads, ground planes, no-fill areas, and panelization. However, because it interprets the graphical output of a Print function, there are limitations in what it can recognize (or there may be bugs).
See docs/Pdf2Gerb.pdf for install/setup, config, usage, and other info.
pdf2gerb_cfg.pm
#Pdf2Gerb config settings:
#Put this file in same folder/directory as pdf2gerb.pl itself (global settings),
#or copy to another folder/directory with PDFs if you want PCB-specific settings.
#There is only one user of this file, so we don't need a custom package or namespace.
#NOTE: all constants defined in here will be added to main namespace.
#package pdf2gerb_cfg;
use strict; #trap undef vars (easier debug)
use warnings; #other useful info (easier debug)
##############################################################################################
#configurable settings:
#change values here instead of in main pfg2gerb.pl file
use constant WANT_COLORS => ($^O !~ m/Win/); #ANSI colors no worky on Windows? this must be set < first DebugPrint() call
#just a little warning; set realistic expectations:
#DebugPrint("${\(CYAN)}Pdf2Gerb.pl ${\(VERSION)}, $^O O/S\n${\(YELLOW)}${\(BOLD)}${\(ITALIC)}This is EXPERIMENTAL software. \nGerber files MAY CONTAIN ERRORS. Please CHECK them before fabrication!${\(RESET)}", 0); #if WANT_DEBUG
use constant METRIC => FALSE; #set to TRUE for metric units (only affect final numbers in output files, not internal arithmetic)
use constant APERTURE_LIMIT => 0; #34; #max #apertures to use; generate warnings if too many apertures are used (0 to not check)
use constant DRILL_FMT => '2.4'; #'2.3'; #'2.4' is the default for PCB fab; change to '2.3' for CNC
use constant WANT_DEBUG => 0; #10; #level of debug wanted; higher == more, lower == less, 0 == none
use constant GERBER_DEBUG => 0; #level of debug to include in Gerber file; DON'T USE FOR FABRICATION
use constant WANT_STREAMS => FALSE; #TRUE; #save decompressed streams to files (for debug)
use constant WANT_ALLINPUT => FALSE; #TRUE; #save entire input stream (for debug ONLY)
#DebugPrint(sprintf("${\(CYAN)}DEBUG: stdout %d, gerber %d, want streams? %d, all input? %d, O/S: $^O, Perl: $]${\(RESET)}\n", WANT_DEBUG, GERBER_DEBUG, WANT_STREAMS, WANT_ALLINPUT), 1);
#DebugPrint(sprintf("max int = %d, min int = %d\n", MAXINT, MININT), 1);
#define standard trace and pad sizes to reduce scaling or PDF rendering errors:
#This avoids weird aperture settings and replaces them with more standardized values.
#(I'm not sure how photoplotters handle strange sizes).
#Fewer choices here gives more accurate mapping in the final Gerber files.
#units are in inches
use constant TOOL_SIZES => #add more as desired
(
#round or square pads (> 0) and drills (< 0):
.010, -.001, #tiny pads for SMD; dummy drill size (too small for practical use, but needed so StandardTool will use this entry)
.031, -.014, #used for vias
.041, -.020, #smallest non-filled plated hole
.051, -.025,
.056, -.029, #useful for IC pins
.070, -.033,
.075, -.040, #heavier leads
# .090, -.043, #NOTE: 600 dpi is not high enough resolution to reliably distinguish between .043" and .046", so choose 1 of the 2 here
.100, -.046,
.115, -.052,
.130, -.061,
.140, -.067,
.150, -.079,
.175, -.088,
.190, -.093,
.200, -.100,
.220, -.110,
.160, -.125, #useful for mounting holes
#some additional pad sizes without holes (repeat a previous hole size if you just want the pad size):
.090, -.040, #want a .090 pad option, but use dummy hole size
.065, -.040, #.065 x .065 rect pad
.035, -.040, #.035 x .065 rect pad
#traces:
.001, #too thin for real traces; use only for board outlines
.006, #minimum real trace width; mainly used for text
.008, #mainly used for mid-sized text, not traces
.010, #minimum recommended trace width for low-current signals
.012,
.015, #moderate low-voltage current
.020, #heavier trace for power, ground (even if a lighter one is adequate)
.025,
.030, #heavy-current traces; be careful with these ones!
.040,
.050,
.060,
.080,
.100,
.120,
);
#Areas larger than the values below will be filled with parallel lines:
#This cuts down on the number of aperture sizes used.
#Set to 0 to always use an aperture or drill, regardless of size.
use constant { MAX_APERTURE => max((TOOL_SIZES)) + .004, MAX_DRILL => -min((TOOL_SIZES)) + .004 }; #max aperture and drill sizes (plus a little tolerance)
#DebugPrint(sprintf("using %d standard tool sizes: %s, max aper %.3f, max drill %.3f\n", scalar((TOOL_SIZES)), join(", ", (TOOL_SIZES)), MAX_APERTURE, MAX_DRILL), 1);
#NOTE: Compare the PDF to the original CAD file to check the accuracy of the PDF rendering and parsing!
#for example, the CAD software I used generated the following circles for holes:
#CAD hole size: parsed PDF diameter: error:
# .014 .016 +.002
# .020 .02267 +.00267
# .025 .026 +.001
# .029 .03167 +.00267
# .033 .036 +.003
# .040 .04267 +.00267
#This was usually ~ .002" - .003" too big compared to the hole as displayed in the CAD software.
#To compensate for PDF rendering errors (either during CAD Print function or PDF parsing logic), adjust the values below as needed.
#units are pixels; for example, a value of 2.4 at 600 dpi = .0004 inch, 2 at 600 dpi = .0033"
use constant
{
HOLE_ADJUST => -0.004 * 600, #-2.6, #holes seemed to be slightly oversized (by .002" - .004"), so shrink them a little
RNDPAD_ADJUST => -0.003 * 600, #-2, #-2.4, #round pads seemed to be slightly oversized, so shrink them a little
SQRPAD_ADJUST => +0.001 * 600, #+.5, #square pads are sometimes too small by .00067, so bump them up a little
RECTPAD_ADJUST => 0, #(pixels) rectangular pads seem to be okay? (not tested much)
TRACE_ADJUST => 0, #(pixels) traces seemed to be okay?
REDUCE_TOLERANCE => .001, #(inches) allow this much variation when reducing circles and rects
};
#Also, my CAD's Print function or the PDF print driver I used was a little off for circles, so define some additional adjustment values here:
#Values are added to X/Y coordinates; units are pixels; for example, a value of 1 at 600 dpi would be ~= .002 inch
use constant
{
CIRCLE_ADJUST_MINX => 0,
CIRCLE_ADJUST_MINY => -0.001 * 600, #-1, #circles were a little too high, so nudge them a little lower
CIRCLE_ADJUST_MAXX => +0.001 * 600, #+1, #circles were a little too far to the left, so nudge them a little to the right
CIRCLE_ADJUST_MAXY => 0,
SUBST_CIRCLE_CLIPRECT => FALSE, #generate circle and substitute for clip rects (to compensate for the way some CAD software draws circles)
WANT_CLIPRECT => TRUE, #FALSE, #AI doesn't need clip rect at all? should be on normally?
RECT_COMPLETION => FALSE, #TRUE, #fill in 4th side of rect when 3 sides found
};
#allow .012 clearance around pads for solder mask:
#This value effectively adjusts pad sizes in the TOOL_SIZES list above (only for solder mask layers).
use constant SOLDER_MARGIN => +.012; #units are inches
#line join/cap styles:
use constant
{
CAP_NONE => 0, #butt (none); line is exact length
CAP_ROUND => 1, #round cap/join; line overhangs by a semi-circle at either end
CAP_SQUARE => 2, #square cap/join; line overhangs by a half square on either end
CAP_OVERRIDE => FALSE, #cap style overrides drawing logic
};
#number of elements in each shape type:
use constant
{
RECT_SHAPELEN => 6, #x0, y0, x1, y1, count, "rect" (start, end corners)
LINE_SHAPELEN => 6, #x0, y0, x1, y1, count, "line" (line seg)
CURVE_SHAPELEN => 10, #xstart, ystart, x0, y0, x1, y1, xend, yend, count, "curve" (bezier 2 points)
CIRCLE_SHAPELEN => 5, #x, y, 5, count, "circle" (center + radius)
};
#const my %SHAPELEN =
#Readonly my %SHAPELEN =>
our %SHAPELEN =
(
rect => RECT_SHAPELEN,
line => LINE_SHAPELEN,
curve => CURVE_SHAPELEN,
circle => CIRCLE_SHAPELEN,
);
#panelization:
#This will repeat the entire body the number of times indicated along the X or Y axes (files grow accordingly).
#Display elements that overhang PCB boundary can be squashed or left as-is (typically text or other silk screen markings).
#Set "overhangs" TRUE to allow overhangs, FALSE to truncate them.
#xpad and ypad allow margins to be added around outer edge of panelized PCB.
use constant PANELIZE => {'x' => 1, 'y' => 1, 'xpad' => 0, 'ypad' => 0, 'overhangs' => TRUE}; #number of times to repeat in X and Y directions
# Set this to 1 if you need TurboCAD support.
#$turboCAD = FALSE; #is this still needed as an option?
#CIRCAD pad generation uses an appropriate aperture, then moves it (stroke) "a little" - we use this to find pads and distinguish them from PCB holes.
use constant PAD_STROKE => 0.3; #0.0005 * 600; #units are pixels
#convert very short traces to pads or holes:
use constant TRACE_MINLEN => .001; #units are inches
#use constant ALWAYS_XY => TRUE; #FALSE; #force XY even if X or Y doesn't change; NOTE: needs to be TRUE for all pads to show in FlatCAM and ViewPlot
use constant REMOVE_POLARITY => FALSE; #TRUE; #set to remove subtractive (negative) polarity; NOTE: must be FALSE for ground planes
#PDF uses "points", each point = 1/72 inch
#combined with a PDF scale factor of .12, this gives 600 dpi resolution (1/72 * .12 = 600 dpi)
use constant INCHES_PER_POINT => 1/72; #0.0138888889; #multiply point-size by this to get inches
# The precision used when computing a bezier curve. Higher numbers are more precise but slower (and generate larger files).
#$bezierPrecision = 100;
use constant BEZIER_PRECISION => 36; #100; #use const; reduced for faster rendering (mainly used for silk screen and thermal pads)
# Ground planes and silk screen or larger copper rectangles or circles are filled line-by-line using this resolution.
use constant FILL_WIDTH => .01; #fill at most 0.01 inch at a time
# The max number of characters to read into memory
use constant MAX_BYTES => 10 * M; #bumped up to 10 MB, use const
use constant DUP_DRILL1 => TRUE; #FALSE; #kludge: ViewPlot doesn't load drill files that are too small so duplicate first tool
my $runtime = time(); #Time::HiRes::gettimeofday(); #measure my execution time
print STDERR "Loaded config settings from '${\(__FILE__)}'.\n";
1; #last value must be truthful to indicate successful load
#############################################################################################
#junk/experiment:
#use Package::Constants;
#use Exporter qw(import); #https://perldoc.perl.org/Exporter.html
#my $caller = "pdf2gerb::";
#sub cfg
#{
# my $proto = shift;
# my $class = ref($proto) || $proto;
# my $settings =
# {
# $WANT_DEBUG => 990, #10; #level of debug wanted; higher == more, lower == less, 0 == none
# };
# bless($settings, $class);
# return $settings;
#}
#use constant HELLO => "hi there2"; #"main::HELLO" => "hi there";
#use constant GOODBYE => 14; #"main::GOODBYE" => 12;
#print STDERR "read cfg file\n";
#our @EXPORT_OK = Package::Constants->list(__PACKAGE__); #https://www.perlmonks.org/?node_id=1072691; NOTE: "_OK" skips short/common names
#print STDERR scalar(@EXPORT_OK) . " consts exported:\n";
#foreach(@EXPORT_OK) { print STDERR "$_\n"; }
#my $val = main::thing("xyz");
#print STDERR "caller gave me $val\n";
#foreach my $arg (@ARGV) { print STDERR "arg $arg\n"; }Download Details:
Author: swannman
Source Code: https://github.com/swannman/pdf2gerb
License: GPL-3.0 license
1684376340
How to Use Of Not Equal To The Operator in Bash
Many types of operators exist in Bash to check the equality or inequality of two strings or numbers. The “-ne” and “!=” operators are used to check the inequality of two values in Bash. The single third brackets ([ ]) are used in the “if” condition when the “!=” operator is used to check the inequality. The double third brackets ([[ ]]) are used in the “if” condition when the “-ne” operator is used to check the inequality. The methods of comparing the string and numeric values using these operators are shown in this tutorial.
Using the “!=” Operator
The “!=” operator can be used to check the inequality between two numeric values or two string values. Two uses of this operator are shown in the following examples.
Example 1: Checking the Inequality Between Numbers
Create a Bash file with the following script that takes a number input and check whether the input value is equal to 10 or not using the “!=” operator. The single third brackets ([ ]) are used in the “if” condition here.
#!/bin/bash
#Take a number
echo -n "Enter a number:"
read number
#Use '!=' operator to check the number value
if [ $number != 10 ]; then
echo "The number is not equal to 10."
else
echo "The number is equal to 10."
fiThe script is executed twice in the following output. Twelve (12) is taken as input in the first execution and “The number is not equal to 10” is printed. Ten (10) is taken as input in the second execution and “The number is equal to 10” is printed:
Example 2:
Create a Bash file with the following script that takes two string values and check whether the input values are equal or not using the “!=” operator. The single third brackets ([ ]) are used in the “if” condition here.
#!/bin/bash
#Take a number
echo -n "Enter the first string value: "
read str1
echo -n "Enter the second string value: "
read str2
#Use '!=' operator to check the string values
if [ "$str1" != "$str2" ]; then
echo "The strings are not equal."
else
echo "The strings are equal."
fiThe script is executed twice in the following output. The “Hello” and “hello” string values are taken as inputs in the first execution and these values are not equal because the string values are compared case-sensitively. In the next execution, the “hello” and “hello” string values are taken as equal inputs:
Using the “-ne” Operator
The “-ne” operator can be used to check the inequality between two numeric values but not can be used to compare the string values. Two uses of this operator to compare the numeric and string values are shown in the following examples.
Example 1:
Create a Bash file with the following script that takes the username as input. Next, the length of the input value is counted after removing the newline(\n) character. Whether the length of the username is equal to 8 or not is checked using the “-ne” operator. The double third brackets ([[ ]]) are used in the “if” condition here.
#!/bin/bash
#Take the username
echo -n "Enter username: "
read username
#Remove newline from the input value
username=`echo $username | tr -d '\n'`
#Count the total character
len=${#username}
#Use the '-ne' operator to check the number value
if [[ $len -ne 8 ]]; then
echo "Username must be 8 characters long."
else
echo "Username: $username"
fiThe script is executed twice in the following output. The “admin” is taken as input in the first execution and the “Username must be 8 characters long” is printed. The “durjoy23” is taken as input in the second execution and the “Username: durjoy23” is printed:
Example 2:
Create a Bash file with the following script that takes the username as input. Next, whether the input value is equal to “admin” or not is checked using the “-ne” operator. The double third brackets ([[ ]]) are used in the “if” condition here. The “-ne” operator does not work to compare two string values.
#!/bin/bash
#Take the username and password
echo -n "Enter username: "
read username
#Remove newline from the input value
username=`echo $username | tr -d '\n'`
#Use '-ne' operator to check the string values
if [[ "$username" -ne "admin" ]]; then
echo "Invalid user."
else
echo "Valid user."
fiThe script is executed twice in the following output. The “if” condition is returned true in both executions for the valid and invalid outputs which is a “wrong” output:
Conclusion
The method of comparing two values using the “!=” and “-ne” operators are shown in this tutorial using multiple examples to know the uses of these operators properly.
Original article source at: https://linuxhint.com/
1620729846
Why Use WordPress? What Can You Do With WordPress?
Can you use WordPress for anything other than blogging? To your surprise, yes. WordPress is more than just a blogging tool, and it has helped thousands of websites and web applications to thrive. The use of WordPress powers around 40% of online projects, and today in our blog, we would visit some amazing uses of WordPress other than blogging.
What Is The Use Of WordPress?
WordPress is the most popular website platform in the world. It is the first choice of businesses that want to set a feature-rich and dynamic Content Management System. So, if you ask what WordPress is used for, the answer is – everything. It is a super-flexible, feature-rich and secure platform that offers everything to build unique websites and applications. Let’s start knowing them:
1. Multiple Websites Under A Single Installation
WordPress Multisite allows you to develop multiple sites from a single WordPress installation. You can download WordPress and start building websites you want to launch under a single server. Literally speaking, you can handle hundreds of sites from one single dashboard, which now needs applause.
It is a highly efficient platform that allows you to easily run several websites under the same login credentials. One of the best things about WordPress is the themes it has to offer. You can simply download them and plugin for various sites and save space on sites without losing their speed.
2. WordPress Social Network
WordPress can be used for high-end projects such as Social Media Network. If you don’t have the money and patience to hire a coder and invest months in building a feature-rich social media site, go for WordPress. It is one of the most amazing uses of WordPress. Its stunning CMS is unbeatable. And you can build sites as good as Facebook or Reddit etc. It can just make the process a lot easier.
To set up a social media network, you would have to download a WordPress Plugin called BuddyPress. It would allow you to connect a community page with ease and would provide all the necessary features of a community or social media. It has direct messaging, activity stream, user groups, extended profiles, and so much more. You just have to download and configure it.
If BuddyPress doesn’t meet all your needs, don’t give up on your dreams. You can try out WP Symposium or PeepSo. There are also several themes you can use to build a social network.
3. Create A Forum For Your Brand’s Community
Communities are very important for your business. They help you stay in constant connection with your users and consumers. And allow you to turn them into a loyal customer base. Meanwhile, there are many good technologies that can be used for building a community page – the good old WordPress is still the best.
It is the best community development technology. If you want to build your online community, you need to consider all the amazing features you get with WordPress. Plugins such as BB Press is an open-source, template-driven PHP/ MySQL forum software. It is very simple and doesn’t hamper the experience of the website.
Other tools such as wpFoRo and Asgaros Forum are equally good for creating a community blog. They are lightweight tools that are easy to manage and integrate with your WordPress site easily. However, there is only one tiny problem; you need to have some technical knowledge to build a WordPress Community blog page.
4. Shortcodes
Since we gave you a problem in the previous section, we would also give you a perfect solution for it. You might not know to code, but you have shortcodes. Shortcodes help you execute functions without having to code. It is an easy way to build an amazing website, add new features, customize plugins easily. They are short lines of code, and rather than memorizing multiple lines; you can have zero technical knowledge and start building a feature-rich website or application.
There are also plugins like Shortcoder, Shortcodes Ultimate, and the Basics available on WordPress that can be used, and you would not even have to remember the shortcodes.
5. Build Online Stores
If you still think about why to use WordPress, use it to build an online store. You can start selling your goods online and start selling. It is an affordable technology that helps you build a feature-rich eCommerce store with WordPress.
WooCommerce is an extension of WordPress and is one of the most used eCommerce solutions. WooCommerce holds a 28% share of the global market and is one of the best ways to set up an online store. It allows you to build user-friendly and professional online stores and has thousands of free and paid extensions. Moreover as an open-source platform, and you don’t have to pay for the license.
Apart from WooCommerce, there are Easy Digital Downloads, iThemes Exchange, Shopify eCommerce plugin, and so much more available.
6. Security Features
WordPress takes security very seriously. It offers tons of external solutions that help you in safeguarding your WordPress site. While there is no way to ensure 100% security, it provides regular updates with security patches and provides several plugins to help with backups, two-factor authorization, and more.
By choosing hosting providers like WP Engine, you can improve the security of the website. It helps in threat detection, manage patching and updates, and internal security audits for the customers, and so much more.
#use of wordpress #use wordpress for business website #use wordpress for website #what is use of wordpress #why use wordpress #why use wordpress to build a website
1679971140
Using Singular Value Separation in Python and Numpy (linalg.svd)
In this pythonn - Numpy tutorial we will learn about Numpy linalg.svd: Singular Value Decomposition in Python. In mathematics, a singular value decomposition (SVD) of a matrix refers to the factorization of a matrix into three separate matrices. It is a more generalized version of an eigenvalue decomposition of matrices. It is further related to the polar decompositions.
In Python, it is easy to calculate the singular decomposition of a complex or a real matrix using the numerical python or the numpy library. The numpy library consists of various linear algebraic functions including one for calculating the singular value decomposition of a matrix.
In machine learning models, singular value decomposition is widely used to train models and in neural networks. It helps in improving accuracy and in reducing the noise in data. Singular value decomposition transforms one vector into another without them necessarily having the same dimension. Hence, it makes matrix manipulation in vector spaces easier and efficient. It is also used in regression analysis.
Syntax of Numpy linalg.svd() function
The function that calculates the singular value decomposition of a matrix in python belongs to the numpy module, named linalg.svd() .
The syntax of the numpy linalg.svd () is as follows:
numpy.linalg.svd(A, full_matrices=True, compute_uv=True, hermitian=False)
You can customize the true and false boolean values based on your requirements.
The parameters of the function are given below:
- A->array_like: This is the required matrix whose singular value decomposition is being calculated. It can be real or complex as required. It’s dimension should be >= 2.
- full_matrices->boolean value(optional): If set to true, then the Hermitian transpose of the given matrix is a square, if it’s false then it isn’t.
- compute_uv->boolen value(optional): It determines whether the Hermitian transpose is to be calculated or not in addition to the singular value decomposition.
- hermitian->boolean value(optional): The given matrix is considered hermitian(that is symmetric, with real values) which might provide a more efficient method for computation.
The function returns three types of matrices based on the parameters mentioned above:
- S->array_like: The vector containing the singular values in the descending order with dimensions same as the original matrix.
- u->array_like: This is an optional solution that is returned when compute_uv is set to True. It is a set of vectors with singular values.
- v-> array_like: Set of unitary arrays only returned when compute_uv is set to True.
It raises a LinALgError when the singular values diverse.
Prerequisites for setup
Before we dive into the examples, make sure you have the numpy module installed in your local system. This is required for using linear algebraic functions like the one discussed in this article. Run the following command in your terminal.
pip install numpyThat’s all you need right now, let’s look at how we will implement the code in the next section.
To calculate Singular Value Decomposition (SVD) in Python, use the NumPy library’s linalg.svd() function. Its syntax is numpy.linalg.svd(A, full_matrices=True, compute_uv=True, hermitian=False), where A is the matrix for which SVD is being calculated. It returns three matrices: S, U, and V.
Example 1: Calculating the Singular Value Decomposition of a 3×3 Matrix
In this first example we will take a 3X3 matrix and compute its singular value decomposition in the following way:
#importing the numpy module
import numpy as np
#using the numpy.array() function to create an array
A=np.array([[2,4,6],
[8,10,12],
[14,16,18]])
#calculatin all three matrices for the output
#using the numpy linalg.svd function
u,s,v=np.linalg.svd(A, compute_uv=True)
#displaying the result
print("the output is=")
print('s(the singular value) = ',s)
print('u = ',u)
print('v = ',v)The output will be:
the output is=
s(the singular value) = [3.36962067e+01 2.13673903e+00 8.83684950e-16]
u = [[-0.21483724 0.88723069 0.40824829]
[-0.52058739 0.24964395 -0.81649658]
[-0.82633754 -0.38794278 0.40824829]]
v = [[-0.47967118 -0.57236779 -0.66506441]
[-0.77669099 -0.07568647 0.62531805]
[-0.40824829 0.81649658 -0.40824829]]
Example 1
Example 2: Calculating the Singular Value Decomposition of a Random Matrix
In this example, we will be using the numpy.random.randint() function to create a random matrix. Let’s get into it!
#importing the numpy module
import numpy as np
#using the numpy.array() function to craete an array
A=np.random.randint(5, 200, size=(3,3))
#display the created matrix
print("The input matrix is=",A)
#calculatin all three matrices for the output
#using the numpy linalg.svd function
u,s,v=np.linalg.svd(A, compute_uv=True)
#displaying the result
print("the output is=")
print('s(the singular value) = ',s)
print('u = ',u)
print('v = ',v)The output will be as follows:
The input matrix is= [[ 36 74 101]
[104 129 185]
[139 121 112]]
the output is=
s(the singular value) = [348.32979681 61.03199722 10.12165841]
u = [[-0.3635535 -0.48363012 -0.79619769]
[-0.70916514 -0.41054007 0.57318554]
[-0.60408084 0.77301925 -0.19372034]]
v = [[-0.49036384 -0.54970618 -0.67628871]
[ 0.77570499 0.0784348 -0.62620264]
[ 0.39727203 -0.83166766 0.38794824]]
Example 2
Suggested: Numpy linalg.eigvalsh: A Guide to Eigenvalue Computation.
Wrapping Up
In this article, we explored the concept of singular value decomposition in mathematics and how to calculate it using Python’s numpy module. We used the linalg.svd() function to compute the singular value decomposition of both given and random matrices. Numpy provides an efficient and easy-to-use method for performing linear algebra operations, making it highly valuable in machine learning, neural networks, and regression analysis. Keep exploring other linear algebraic functions in numpy to enhance your mathematical toolset in Python.
Article source at: https://www.askpython.com
1623050167
Mad Libs: Using regular expressions
From Tiny Python Projects by Ken Youens-Clark
Everyone loves Mad Libs! And everyone loves Python. This article shows you how to have fun with both and learn some programming skills along the way.
Take 40% off Tiny Python Projects by entering fccclark into the discount code box at checkout at manning.com.
When I was a wee lad, we used to play at Mad Libs for hours and hours. This was before computers, mind you, before televisions or radio or even paper! No, scratch that, we had paper. Anyway, the point is we only had Mad Libs to play, and we loved it! And now you must play!
We’ll write a program called mad.py which reads a file given as a positional argument and finds all the placeholders noted in angle brackets like <verb> or <adjective> . For each placeholder, we’ll prompt the user for the part of speech being requested like “Give me a verb” and “Give me an adjective.” (Notice that you’ll need to use the correct article.) Each value from the user replaces the placeholder in the text, and if the user says “drive” for “verb,” then <verb> in the text replaces with drive . When all the placeholders have been replaced with inputs from the user, print out the new text.
#python #regular-expressions #python-programming #python3 #mad libs: using regular expressions #using regular expressions