Unveiling the Power of for Loops in Python

In Python, the for loop is a versatile and fundamental construct that facilitates the iteration over a sequence of elements. Whether you’re working with lists, strings, or other iterable objects, the for loop simplifies the process of executing a set of statements for each item in the sequence. In this comprehensive guide, we’ll explore the syntax, functionality, and best practices associated with for loops in Python.

What Is a While Loop?

A while loop is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. The while loop can be thought of as a repeating if statement.

The while loop is used extensively in Python and alone with for and if-else loops, forms the basis of manipulating data in the language.

Syntax of while Loop in Python

The syntax of a while loop in Python programming language is −

while expression:
   statement(s)

while expression:
   statement(s)

Here, statement(s) may be a single statement or a block of statements. The condition may be any expression, and true is any non-zero value. The loop iterates while the condition is true.

When the condition becomes false, program control passes to the line immediately following the loop.

Diagram for while loop in Python:

  graph TD
    A((Start)) --> B(Initialize loop)
    B --> C{Condition}
    C -- True --> D(Process loop)
    D --> E{Condition}
    E -- True --> D
    E -- False --> F((End))
    C -- False --> F
    F --> G((End))

Example of while Loop in Python

Here is an example of while loop in Python:

# while loop
numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11]
sum = 0
i = 0
while i < len(numbers):
    sum = sum + numbers[i]
    i = i+1
print("The sum is", sum)

# while loop
numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11]
sum = 0
i = 0
while i < len(numbers):
    sum = sum + numbers[i]
    i = i+1
print("The sum is", sum)

Output:

C:\Users\Your Name> python while_loop.py
The sum is 48

C:\Users\Your Name> python while_loop.py
The sum is 48

In this program, we initialize the sumsum variable to zero and iterate over all the items of the numbersnumbers list using the while loop and add them to the sumsum variable. Finally, we print the sumsum variable which contains the sum of numbers in the given list.

While loop with else

Same as with for loops, while loops can also have an optional else block.

The else part is executed if the condition in the while loop evaluates to False. The while loop can be terminated with a break statement. In such cases, the else part is ignored. Hence, a while loop’s else part runs if no break occurs and the condition is false.

Here is an example to illustrate this.

# while loop with else
numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11]
sum = 0
i = 0
while i < len(numbers):
    sum = sum + numbers[i]
    i = i+1
else:
    print("No items left in the list.")
print("The sum is", sum)

# while loop with else
numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11]
sum = 0
i = 0
while i < len(numbers):
    sum = sum + numbers[i]
    i = i+1
else:
    print("No items left in the list.")
print("The sum is", sum)

Output:

C:\Users\Your Name> python while_loop_else.py
No items left in the list.
The sum is 48

C:\Users\Your Name> python while_loop_else.py
No items left in the list.
The sum is 48

In this program, we have used the else statement with the while loop. This is a unique feature of Python, not found in most other programming languages. When the condition in the while loop evaluates to False, the block of statements in the else clause is executed. This means, the else part will be executed only if the loop condition is False.

Single Statement While Block

Similar to the if statement syntax, if your while block consists of only a single statement, you can place it on the same line as the while header.

# single statement while block
flag = 1
while (flag): print('Given flag is really true!')
print("Good bye!")

# single statement while block
flag = 1
while (flag): print('Given flag is really true!')
print("Good bye!")

Output:

C:\Users\Your Name> python single_statement_while_block.py
Given flag is really true!
Given flag is really true!
Given flag is really true!
...

C:\Users\Your Name> python single_statement_while_block.py
Given flag is really true!
Given flag is really true!
Given flag is really true!
...

In this program, we have used the single statement while block. Here, the flagflag variable is initialized to 11 and as long as flagflag is true, the printprint statement will be executed. This results in printing the string Given flag is really true!Given flag is really true! infinitely.

Python Infinite While Loop

Infinite while loop is an indefinite iteration. It is called infinite because it does not have any end point which means it will be executed forever unless the program is terminated.

# infinite while loop
var = 1
while var == 1:
    num = int(input("Enter a number  :"))
    print("You entered: ", num)
print("Good bye!")

# infinite while loop
var = 1
while var == 1:
    num = int(input("Enter a number  :"))
    print("You entered: ", num)
print("Good bye!")

Output:

C:\Users\Your Name> python infinite_while_loop.py
Enter a number  :5
You entered:  5
Enter a number  :6
You entered:  6
Enter a number  :7
You entered:  7
...

C:\Users\Your Name> python infinite_while_loop.py
Enter a number  :5
You entered:  5
Enter a number  :6
You entered:  6
Enter a number  :7
You entered:  7
...

In this program, we have used an infinite while loop. The condition of the while loop is always true, so the loop will run indefinitely. We have used the input()input() function inside the loop body to read the user input. The input()input() function reads a line entered on a console by an input device such as a keyboard and convert it into a string and returns it.

While Loop with String or List

In Python, while loops are used to iterate over a block of code as long as the test expression (condition) is true.

We generally use this loop when we don’t know beforehand, the number of times to iterate.

# while loop with string or list
name = "pynative"
while name != "":
    print("Current Letter :", name[0])
    name = name[1:]

# while loop with string or list
name = "pynative"
while name != "":
    print("Current Letter :", name[0])
    name = name[1:]

Output:

C:\Users\Your Name> python while_loop_with_string_or_list.py
Current Letter : p
Current Letter : y
Current Letter : n
Current Letter : a
Current Letter : t
Current Letter : i
Current Letter : v
Current Letter : e

C:\Users\Your Name> python while_loop_with_string_or_list.py
Current Letter : p
Current Letter : y
Current Letter : n
Current Letter : a
Current Letter : t
Current Letter : i
Current Letter : v
Current Letter : e

In this program, we have used the while loop to iterate over the characters in the string pynativepynative and print each character. We have used the namename variable to hold the string pynativepynative. The whilewhile loop iterates over the string as long as the string is not empty. The namename variable is updated in each iteration by removing the first character from it using the slice operation.

Nested while Loop

A nested while loop is a while loop inside another while loop.

# nested while loop
i = 2
while(i < 100):
    j = 2
    while(j <= (i/j)):
        if not(i % j): break
        j = j + 1
    if (j > i/j) : print(i, " is prime")
    i = i + 1
print("Good bye!")

# nested while loop
i = 2
while(i < 100):
    j = 2
    while(j <= (i/j)):
        if not(i % j): break
        j = j + 1
    if (j > i/j) : print(i, " is prime")
    i = i + 1
print("Good bye!")

Output:

C:\Users\Your Name> python nested_while_loop.py
2 is prime
3 is prime
5 is prime
7 is prime
11 is prime
13 is prime
...

C:\Users\Your Name> python nested_while_loop.py
2 is prime
3 is prime
5 is prime
7 is prime
11 is prime
13 is prime
...

In this program, we have used a nested while loop to find the prime numbers between 2 and 100. The outer while loop iterates from 2 to 100 and the inner while loop iterates from 2 to i/ji/j. If the remainder of i/ji/j is 0, the control breaks out of the inner while loop and ii is incremented by 1. If the remainder of i/ji/j is not 0, the control breaks out of the inner while loop and prints the value of ii which is a prime number. The value of ii is incremented by 1 in each iteration of the outer while loop.

Reverse while Loop

In Python, we can use reverse while loop to print numbers from n to 1.

# reverse while loop
n = 10
while n > 0:
    print(n)
    n = n-1
print("Loop ended.")

# reverse while loop
n = 10
while n > 0:
    print(n)
    n = n-1
print("Loop ended.")

Output:

C:\Users\Your Name> python reverse_while_loop.py
10
9
8
7
6
5
4
3
2
1
Loop ended.

C:\Users\Your Name> python reverse_while_loop.py
10
9
8
7
6
5
4
3
2
1
Loop ended.

In this program, we have used a reverse while loop to print numbers from 10 to 1. The value of nn is initialized to 10 and the while loop iterates until nn becomes 0. The value of nn is decremented by 1 in each iteration of the while loop.

While Loop with multiple conditions

In Python, we can use multiple conditions in a while loop.

# while loop with multiple conditions
i = 1
while i <= 5 and i >= 0:
    print(i)
    i = i + 1
print("Loop ended.")

# while loop with multiple conditions
i = 1
while i <= 5 and i >= 0:
    print(i)
    i = i + 1
print("Loop ended.")

Output:

C:\Users\Your Name> python while_loop_with_multiple_conditions.py
1
2
3
4
5
Loop ended.

C:\Users\Your Name> python while_loop_with_multiple_conditions.py
1
2
3
4
5
Loop ended.

In this program, we have used multiple conditions in a while loop. The while loop iterates until ii becomes 6. The value of ii is incremented by 1 in each iteration of the while loop.

Conclusion

The while loop is a powerful tool in Python, offering a flexible way to repeat a block of code based on a specified condition. It is particularly useful when the number of iterations is unknown beforehand or when executing code until a certain condition is met. By understanding the syntax, potential pitfalls, and best practices associated with while loops, you can leverage their capabilities effectively in your Python programs.

As you continue your Python programming journey, practice using while loops in various scenarios, and explore how they can enhance the responsiveness and efficiency of your code. For more insights and practical examples, check out our tutorials on Python Central Hub!