Introduction Loop in Python with Examples

A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).

This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.

With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.

Python for Loop

The for loop in Python is used to iterate over a sequence (list, tuple, string) or other iterable objects. Iterating over a sequence is called traversal.

Syntax

for val in sequence:
	Body of for

Here val is a variable that is used for iterating over a . On every iteration it takes the next value from sequenceuntil the end of sequence is reached.

Flowchart of for Loop

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Python – For loop example

The following example shows the use of for loop to iterate over a list of numbers. In the body of for loop we are calculating the square of each number present in list and displaying the same.

	# Program to print squares of all numbers present in a list

# List of integer numbers
numbers = [1, 2, 4, 6, 11, 20]

# variable to store the square of each num temporary
sq = 0

# iterating over the given list
for val in numbers:
    # calculating square of each number
    sq = val * val
    # displaying the squares
    print(sq)

Output:

The range() function

In the above example, we have iterated over a list using for loop. However we can also use a range() function in for loop to iterate over numbers defined by range().

range(n): generates a set of whole numbers starting from 0 to (n-1).
For example:
range(8) is equivalent to [0, 1, 2, 3, 4, 5, 6, 7]

range(start, stop): generates a set of whole numbers starting from start to stop-1.
For example:
range(5, 9) is equivalent to [5, 6, 7, 8]

range(start, stop, step_size): The default step_size is 1 which is why when we didn’t specify the step_size, the numbers generated are having difference of 1. However by specifying step_size we can generate numbers having the difference of step_size.
For example:
range(1, 10, 2) is equivalent to [1, 3, 5, 7, 9]

Lets use the range() function in for loop:

Python for loop example using range() function

Here we are using range() function to calculate and display the sum of first 5 natural numbers.

# Program to print the sum of first 5 natural numbers

# variable to store the sum
sum = 0

# iterating over natural numbers using range()
for val in range(1, 6):
    # calculating sum
    sum = sum + val

# displaying sum of first 5 natural numbers
print(sum)

Output:

For loop with else block

A for loop can have an optional else block as well. The else part is executed if the items in the sequence used in for loop exhausts.

break statement can be used to stop a for loop. In such case, the else part is ignored.

Hence, a for loop’s else part runs if no break occurs.

Here is an example to illustrate this.

digits = [0, 1, 5]

for i in digits:
    print(i)
else:
    print("No items left.")

Output:

	
0
1
5
No items left.

Here, the for loop prints items of the list until the loop exhausts. When the for loop exhausts, it executes the block of code in the else and prints

Note: The else block only executes when the loop is finished.

Nested For loop in Python

When a for loop is present inside another for loop then it is called a nested for loop. Lets take an example of nested for loop.

	for num1 in range(3):
	for num2 in range(10, 14):
		print(num1, ",", num2)

Output:

Thanks for reading .

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