In computer programming, we use the if...else
statement to run one block of code under certain conditions and another block of code under different conditions.
For example, assigning grades (A, B, C) based on marks obtained by a student.
There are three forms of if...else
statements in C++.
if
statementif...else
statementif...else if...else
statementThe syntax of the if
statement is:
if (condition) {
// body of if statement
}
The if
statement evaluates the condition
inside the parentheses ( )
.
condition
evaluates to true
, the code inside the body of if
is executed.condition
evaluates to false
, the code inside the body of if
is skipped.Note: The code inside { }
is the body of the if
statement.
How if Statement Works
// Program to print positive number entered by the user
// If the user enters a negative number, it is skipped
#include <iostream>
using namespace std;
int main() {
int number;
cout << "Enter an integer: ";
cin >> number;
// checks if the number is positive
if (number > 0) {
cout << "You entered a positive integer: " << number << endl;
}
cout << "This statement is always executed.";
return 0;
}
Output 1
Enter an integer: 5
You entered a positive number: 5
This statement is always executed.
When the user enters 5
, the condition number > 0
is evaluated to true
and the statement inside the body of if
is executed.
Output 2
Enter a number: -5
This statement is always executed.
When the user enters -5
, the condition number > 0
is evaluated to false
and the statement inside the body of if
is not executed.
The if
statement can have an optional else
clause. Its syntax is:
if (condition) {
// block of code if condition is true
}
else {
// block of code if condition is false
}
The if..else
statement evaluates the condition
inside the parenthesis.
How if...else Statement Works
If the condition
evaluates true
,
if
is executedelse
is skipped from executionIf the condition
evaluates false
,
else
is executedif
is skipped from execution// Program to check whether an integer is positive or negative
// This program considers 0 as a positive number
#include <iostream>
using namespace std;
int main() {
int number;
cout << "Enter an integer: ";
cin >> number;
if (number >= 0) {
cout << "You entered a positive integer: " << number << endl;
}
else {
cout << "You entered a negative integer: " << number << endl;
}
cout << "This line is always printed.";
return 0;
}
Output 1
Enter an integer: 4
You entered a positive integer: 4.
This line is always printed.
In the above program, we have the condition number >= 0
. If we enter the number greater or equal to 0, then the condition evaluates true
.
Here, we enter 4. So, the condition is true
. Hence, the statement inside the body of if
is executed.
Output 2
Enter an integer: -4 You entered a negative integer: -4. This line is always printed.
Here, we enter -4. So, the condition is false
. Hence, the statement inside the body of else
is executed.
The if...else
statement is used to execute a block of code among two alternatives. However, if we need to make a choice between more than two alternatives, we use the if...else if...else
statement.
The syntax of the if...else if...else
statement is:
if (condition1) {
// code block 1
}
else if (condition2){
// code block 2
}
else {
// code block 3
}
Here,
condition1
evaluates to true
, the code block 1
is executed.condition1
evaluates to false
, then condition2
is evaluated.condition2
is true
, the code block 2
is executed.condition2
is false
, the code block 3
is executed.How if...else if...else Statement Works
Note: There can be more than one else if
statement but only one if
and else
statements.
// Program to check whether an integer is positive, negative or zero
#include <iostream>
using namespace std;
int main() {
int number;
cout << "Enter an integer: ";
cin >> number;
if (number > 0) {
cout << "You entered a positive integer: " << number << endl;
}
else if (number < 0) {
cout << "You entered a negative integer: " << number << endl;
}
else {
cout << "You entered 0." << endl;
}
cout << "This line is always printed.";
return 0;
}
Output 1
Enter an integer: 1
You entered a positive integer: 1.
This line is always printed.
Output 2
Enter an integer: -2
You entered a negative integer: -2.
This line is always printed.
Output 3
Enter an integer: 0
You entered 0.
This line is always printed.
In this program, we take a number from the user. We then use the if...else if...else
ladder to check whether the number is positive, negative, or zero.
If the number is greater than 0
, the code inside the if
block is executed. If the number is less than 0
, the code inside the else if
block is executed. Otherwise, the code inside the else
block is executed.
Sometimes, we need to use an if
statement inside another if
statement. This is known as nested if
statement.
Think of it as multiple layers of if
statements. There is a first, outer if
statement, and inside it is another, inner if
statement. Its syntax is:
// outer if statement
if (condition1) {
// statements
// inner if statement
if (condition2) {
// statements
}
}
Notes:
else
and else if
statements to the inner if
statement as required.if
statement can also be inserted inside the outer else
or else if
statements (if they exist).if
statements.// C++ program to find if an integer is positive, negative or zero
// using nested if statements
#include <iostream>
using namespace std;
int main() {
int num;
cout << "Enter an integer: ";
cin >> num;
// outer if condition
if (num != 0) {
// inner if condition
if (num > 0) {
cout << "The number is positive." << endl;
}
// inner else condition
else {
cout << "The number is negative." << endl;
}
}
// outer else condition
else {
cout << "The number is 0 and it is neither positive nor negative." << endl;
}
cout << "This line is always printed." << endl;
return 0;
}
Output 1
Enter an integer: 35
The number is positive.
This line is always printed.
Output 2
Enter an integer: -35
The number is negative.
This line is always printed.
Output 3
Enter an integer: 0
The number is 0 and it is neither positive nor negative.
This line is always printed.
In the above example,
if...else
statement to check whether num is not equal to 0
.true
, then the inner if...else
statement is executed.false
, the code inside the outer else
condition is executed, which prints "The number is 0 and it is neither positive nor negative."
if...else
statement checks whether the input number is positive i.e. if num is greater than 0.true
, then we print a statement saying that the number is positive.false
, we print that the number is negative.Note: As you can see, nested if...else
makes your logic complicated. If possible, you should always try to avoid nested if...else
.
If the body of if...else
has only one statement, you can omit { }
in the program. For example, you can replace
int number = 5;
if (number > 0) {
cout << "The number is positive." << endl;
}
else {
cout << "The number is negative." << endl;
}
with
int number = 5;
if (number > 0)
cout << "The number is positive." << endl;
else
cout << "The number is negative." << endl;
The output of both programs will be the same.
Note: Although it's not necessary to use { }
if the body of if...else
has only one statement, using { }
makes your code more readable.
In certain situations, a ternary operator can replace an if...else
statement.
If we need to make a choice between more than one alternatives based on a given test condition, the switch
statement can be used.
#cpluplus #c++