Loops in Python

Loops in Python are a fundamental concept that allow you to execute a block of code multiple times. In program logic, there comes multiple scenarios where we need to iterate over all the elements of a collection datatype. Those iterations over a collection can be done using loops.

Loops in Python

Python supports two types of loops: for loop and while loop

for loop

for loop in Python is used to iterate over a elements of a collection sequence (list, tuple, string, set, dictionary or other iterable objects). for loop allows to execute a block of code repeatedly for each element in the sequence. We can also use conditional statements, operators within that block of code.

for variable in sequence:
    # Code block to execute

• variable – for loop variable takes the value of the current item in the sequence.
• sequence – This is the iterable sequence to iterate over. This sequence can be a collection datatype, string.

for loop Example

course_list = ['Python', 'AWS', 'Java', 'Azure', 'Data Science']
for course in course_list:
    print(course)

for loop with range() method and if statement

for number in range(10):
    if number % 2 == 0:
        print(number)

range() method generates number from 0 to the given number. Using for loop we are iterating over the range of numbers and execute the code block for each number. if condition checks for remainder when number divided by 2 and if remainder is 0 then print that number.

Program Output

0
2
4
6
8

for loop with string

for letter in "shbytes":
    print(letter)

s
h
b
y
t
e
s

for loop with dictionary

print("dictionary iteration")
courses = {"c1": "Python", "c2": "AWS", "c3": "Azure", "c4": "Java"}
for key in courses:
    print("key - ", key, "value - ", courses[key])

In this program, we have defined a dictionary courses. Dictionary elements are key-value pairs. Using for loop we are iterating over the dictionary elements. We will get the key of each dictionary element and courses[key] we get the value of that key.

Program Output

dictionary iteration
key -  c1 value -  Python
key -  c2 value -  AWS
key -  c3 value -  Azure
key -  c4 value -  Java

In the similar way, we can iterate over other collection datatypes like Tuple and Set.

for loop with else statement

An else block can be used with a for loop. The code in the else block runs after the loop completes its iteration, unless the loop is terminated with a break.

for variable in sequence:
    # Code block to execute
else:
    # Code block for else

• for loop will iterate over the sequence and will execute the code block for all elements.
• else code block will be executed after the for loop completes its iteration.
• else code block will not be executed, if for loop is exited using break command in for loop code.

Example for loop with else

course_tuple = ('Python', 'Java', 'Data Science')
for course in course_tuple:
    print(course)
else:
    print("for loop completed")

Program Output

Python
Java
Data Science
for loop completed

while Loop

while loop in Python works with a specified condition and repeatedly execute a block of code as long as the given condition is True. while loop is particularly useful when the number of iterations is not known beforehand and the loop continues to run till the particular condition is True.

# initialize variable
while condition:
    # Code to execute repeatedly
    # update variable

• variable – Define a variable on which while loop condition will be checked. Initialize variable with initial value.
• condition – This is an expression using variable, that evaluates to True or False. while loop continues running as long as this condition is True.
• Block of code – This is the code block that will be executed repeatedly as long as the while loop condition is True
• update variable – Update variable for next iteration.

Example – while loop

counter = 0
while counter < 6:
    print("Counter value is", counter)
    counter += 1

In this program, we have defined a variable counter. We are using while loop with condition counter < 6. If this condition is True, then while loop code block will be executed. As a code block, we will print the current counter value and increase the counter value by 1. Based on the update counter value, again while loop condition will be checked and will repeat the steps till condition is True. while loop condition will remain True, till counter value reached 6.

Program Output – while loop

Counter value is 0
Counter value is 1
Counter value is 2
Counter value is 3
Counter value is 4
Counter value is 5

Nested loop

Similar to nested for loop, nested while loop can be used. We can have any level of nested loops. At 1 level of nested hierarchy, we can create 4 combinations using for loop and while loop.

1. for loop inside for loop
2. while loop inside for loop
3. while loop inside while loop
4. for loop inside while loop

for loop inside for loop

We can have for loop inside another for loop. We can use any level of nested for loops.

for variable in sequence:
    # Code block 2 to execute outer for loop
     for variable_inner in sequence_inner:
         # Code block to execute inner for loop
    # Code block 2 to execute outer for loop

Example – for loop inside for loop

print("Nested for loop")
for i in range(3):
    for j in range(2):
        print(f"i = {i}, j = {j}")

In this program, we are using for loop with variable i and elements from range(3). Inside this for loop, there is another for loop with variable j and elements from range(2). For each value of i, complete iteration of inner for loop with j will be executed.

Program Output – for loop inside for loop

Nested for loop
i = 0, j = 0
i = 0, j = 1
i = 1, j = 0
i = 1, j = 1
i = 2, j = 0
i = 2, j = 1

while loop inside while loop

# initialize variable_1
while condition_1:
    # Code to execute repeatedly
    # initialize variable_2
    while condition_2:
         # inner loop - code to execute repeatedly
         # update variable_2
    # update variable_1

• For each iteration of outer while loop, complete iteration of inner while loop will be executed.

Example – while loop inside while loop

counter_1 = 0
while counter_1 < 3:
    counter_2 = 0
    while counter_2 < 2:
        print("counter_1 is", counter_1, "and counter_2 is", counter_2)
        counter_2 += 1
    counter_1 += 1

In this program, outer while loop condition is based on variable counter_1 and inner while loop condition is based on variable counter_2. For each iteration of outer while loop, complete inner while loop will be executed. We will print the value of variables counter_1 and counter_2.

Program Output – while loop inside while loop

counter_1 is 0 and counter_2 is 0
counter_1 is 0 and counter_2 is 1
counter_1 is 1 and counter_2 is 0
counter_1 is 1 and counter_2 is 1
counter_1 is 2 and counter_2 is 0
counter_1 is 2 and counter_2 is 1

Nested loops combinations

for loop and while loop can be used with each other to create a nested loops. There are 4 combinations (for 1 level of nested) that can be created as nested loops.

• for loop inside for loop
• while loop inside for loop
• while loop inside while loop
• for loop inside while loop

Infinite loops

Any loop can run indefinitely, if the given condition never becomes False. In case of while loop, if the given condition never becomes False then while loop can run indefinitely. This is called an infinite loop.

# initialize variable
while True:
    # Code to execute repeatedly
    # update variable

If we notice in this code, while loop condition is True always. Since, this while loop condition will never become False and it will run indefinitely.

Problems with Infinite loop

• If the loop will keep on running indefinitely, then program execution will not move forward and will stuck a particular code
• Indefinite loop execution will consume the high memory and CPU resources, which can cause program to fail.

Uses of Infinite loop

• Infinite loops are used in scenarios where we don’t know the condition or the condition are dynamic, that keeps on changing based on the code block execution.
• Infinite loops are also used in monitoring or health check scenarios, where program continuously checks for application health status.

To stop an infinite loop, we can use a break statement, or manually interrupt the program (e.g., with Ctrl + C in most environments).

loop control statements

Python provides three loop control statements – break, continue, and pass. These statements are used to control the flow of loops and other blocks of code. Each of these statement serves a specific purpose in managing how loops operate or how blocks of code are executed.

break Statement

• break statement in Python is used to exit a loop prematurely.
• Whenever a break statement is encountered inside a loop, the loop is immediately terminated, and the program continues with the next statement after the loop.
• break statement can be used with both for loop and while loop.

for num in range(8):
    if num == 5:
        break                # Exit the loop when num is 5
    print(num)

In this program, we are using for loop to iterate through the elements from range() method. if condition check for num == 5. if num value is 5, then break statement will be executed. break statement will stop the execution of the for loop and exit from for loop. Values will be printed from 0 to 4.

Program Output – break Statement

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4

continue Statement

• continue statement is used to skip the rest of the code inside the loop for the current iteration and move on to the next iteration.
• Unlike break, it doesn’t terminate the loop but rather skips only the current iteration and moves to the next iteration.

for num in range(6):
    if num == 2:
        continue                # Skip the iteration when num is 2
    print(num)

In this program, we are using for loop to iterate through the elements from range() method. if condition check for num == 2. if num value is 2, then continue statement will be executed. continue statement will skip the current iteration of for loop and will move on to next iteration. It means it will skip printing of value 2 and will print all other values from 0 to 5.

Program Output – continue Statement

0
1
3
4
5

pass Statement

• pass statement is a placeholder that does nothing when executed.
• It is used in situations where a statement is syntactically required, but you do not want to execute any code.
• pass statement does not stop or skip any iteration of the loop.
• It is often used in empty classes, functions, or conditional blocks as a temporary placeholder.

for num in range(4):
    if num == 2:
        pass         # Do nothing, just pass
    print(num)

Program Output – pass Statement

0
1
2
3

Summary

In this article, we learned about for loop while loop and explored different scenarios with loops. We also explored loop control statements break, continue and pass.