Loops in Python – For, While and Nested Loops

Python programming language provides two types of Python loopshecking time. In this article, we will look at Python loops and understand their working with the help of examp – For loop and While loop to handle looping requirements. Loops in Python provides three ways for executing the loops.

While all the ways provide similar basic functionality, they differ in their syntax and condition-checking time. In this article, we will look at Python loops and understand their working with the help of examples.

While Loop in Python

In Python, a while loop is used to execute a block of statements repeatedly until a given condition is satisfied. When the condition becomes false, the line immediately after the loop in the program is executed.

Python While Loop Syntax:

while expression:
    statement(s)

All the statements indented by the same number of character spaces after a programming construct are considered to be part of a single block of code. Python uses indentation as its method of grouping statements. 

Let’s learn how to use a while loop in Python with Examples:

Example of Python While Loop 

Let’s see a simple example of a while loop in Python. The given Python code uses a ‘while' loop to print “Hello Geek” three times by incrementing a variable called ‘count' from 1 to 3.

count = 0
while (count < 3):
    count = count + 1
    print("Hello Geek")

Hello Geek
Hello Geek
Hello Geek

Using else statement with While Loop in Python

The else clause is only executed when your while condition becomes false. If you break out of the loop, or if an exception is raised, it won’t be executed. 

Syntax of While Loop with else statement:

while condition:
     # execute these statements
else:
     # execute these statements

Here is an example of while loop with else statement in Python:

The code prints “Hello Geek” three times using a ‘while' loop and then, after the loop, it prints “In Else Block” because there is an “else” block associated with the ‘while' loop.

count = 0
while (count < 3):
    count = count + 1
    print("Hello Geek")
else:
    print("In Else Block")

Hello Geek
Hello Geek
Hello Geek
In Else Block

Infinite While Loop in Python

If we want a block of code to execute infinite number of time, we can use the while loop in Python to do so.

The code uses a ‘while' loop with the condition (count == 0). This loop will only run as long as count is equal to 0. Since count is initially set to 0, the loop will execute indefinitely because the condition is always true.

count = 0
while (count == 0):
    print("Hello Geek")

Note: It is suggested not to use this type of loop as it is a never-ending infinite loop where the condition is always true and you have to forcefully terminate the compiler.

For Loop in Python

For loops are used for sequential traversal. For example: traversing a list or string or array etc. In Python, there is “for in” loop which is similar to foreach loop in other languages. Let us learn how to use for loops in Python for sequential traversals with examples.

For Loop Syntax:

for iterator_var in sequence:
    statements(s)

It can be used to iterate over a range and iterators.

Example:

The code uses a Python for loop that iterates over the values from 0 to 3 (not including 4), as specified by the range(0, n) construct. It will print the values of ‘i' in each iteration of the loop.

n = 4
for i in range(0, n):
    print(i)

0
1
2
3

Example with List, Tuple, String, and Dictionary Iteration Using for Loops in Python

We can use for loop to iterate lists, tuples, strings and dictionaries in Python.

The code showcases different ways to iterate through various data structures in Python. It demonstrates iteration over lists, tuples, strings, dictionaries, and sets, printing their elements or key-value pairs.

The output displays the contents of each data structure as it is iterated.

print("List Iteration")
l = ["geeks", "for", "geeks"]
for i in l:
    print(i)
    
print("\nTuple Iteration")
t = ("geeks", "for", "geeks")
for i in t:
    print(i)
    
print("\nString Iteration")
s = "Geeks"
for i in s:
    print(i)
    
print("\nDictionary Iteration")
d = dict({'x':123, 'y':354})
for i in d:
    print("%s  %d" % (i, d[i]))
    
print("\nSet Iteration")
set1 = {1, 2, 3, 4, 5, 6}
for i in set1:
    print(i),

List Iteration
geeks
for
geeks

Tuple Iteration
geeks
for
geeks

String Iteration
G
e
e
k
s

Dictionary Iteration
xyz  123
abc  345

Set Iteration
1
2
3
4
5
6

Iterating by the Index of Sequences

We can also use the index of elements in the sequence to iterate. The key idea is to first calculate the length of the list and in iterate over the sequence within the range of this length. See the below

Example: This code uses a ‘for' loop to iterate over a list and print each element. It iterates through the list based on the index of each element, obtained using ‘range(len(list))'. The result is that it prints each item in the list on separate lines.

list = ["geeks", "for", "geeks"]
for index in range(len(list)):
    print(list[index])

geeks
for
geeks

Using else Statement with for Loop in Python

We can also combine else statement with for loop like in while loop. But as there is no condition in for loop based on which the execution will terminate so the else block will be executed immediately after for block finishes execution. 

In this code, the ‘for' loop iterates over a list and prints each element, just like in the previous example. However, after the loop is finished, the “else” block is executed. So, in this case, it will print “Inside Else Block” once the loop completes.

list = ["geeks", "for", "geeks"]
for index in range(len(list)):
    print(list[index])
else:
    print("Inside Else Block")

geeks
for
geeks
Inside Else Block

Nested Loops in Python

Python programming language allows to use one loop inside another loop which is called nested loop. Following section shows few examples to illustrate the concept. 

Nested Loops Syntax:

for iterator_var in sequence:
   for iterator_var in sequence:
       statements(s)
   statements(s)

The syntax for a nested while loop statement in the Python programming language is as follows: 

while expression:
   while expression: 
       statement(s)
   statement(s)

A final note on loop nesting is that we can put any type of loop inside of any other type of loops in Python. For example, a for loop can be inside a while loop or vice versa.

Example: This Python code uses nested ‘for' loops to create a triangular pattern of numbers. It iterates from 1 to 4 and, in each iteration, prints the current number multiple times based on the iteration number. The result is a pyramid-like pattern of numbers.

from __future__ import print_function
for i in range(1, 5):
    for j in range(i):
        print(i, end=' ')
    print()

1 
2 2 
3 3 3 
4 4 4 4 

Loop Control Statements

Loop control statements change execution from their normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. Python supports the following control statements. 

Continue Statement

The continue statement in Python returns the control to the beginning of the loop.

Example: This Python code iterates through the characters of the string ‘geeksforgeeks’. When it encounters the characters ‘e’ or ‘s’, it uses the continue statement to skip the current iteration and continue with the next character. For all other characters, it prints “Current Letter :” followed by the character. So, the output will display all characters except ‘e’ and ‘s’, each on a separate line.

for letter in 'geeksforgeeks':
    if letter == 'e' or letter == 's':
        continue
    print('Current Letter :', letter)

Current Letter : g
Current Letter : k
Current Letter : f
Current Letter : o
Current Letter : r
Current Letter : g
Current Letter : k

Break Statement

The break statement in Python brings control out of the loop.

Example: In this Python code, it iterates through the characters of the string ‘geeksforgeeks’. When it encounters the characters ‘e’ or ‘s’, it uses the break statement to exit the loop. After the loop is terminated, it prints “Current Letter :” followed by the last character encountered in the loop (either ‘e’ or ‘s’). So, the output will display “Current Letter :” followed by the first occurrence of ‘e’ or ‘s’ in the string.

for letter in 'geeksforgeeks':
    if letter == 'e' or letter == 's':
        break

print('Current Letter :', letter)

Current Letter : e

Pass Statement

We use pass statement in Python to write empty loops. Pass is also used for empty control statements, functions and classes.

Example: This Python code iterates through the characters of the string ‘geeksforgeeks’ using a ‘for' loop. However, it doesn’t perform any specific action within the loop, and the ‘pass' statement is used. After the loop, it prints “Last Letter :” followed by the last character in the string, which is ‘s’.

for letter in 'geeksforgeeks':
    pass
print('Last Letter :', letter)

Last Letter : s

How for loop works internally in Python?

Before proceeding to this section, you should have a prior understanding of Python Iterators.

Firstly, lets see how a simple for loops in Python looks like.

Example: This Python code iterates through a list called fruits, containing “apple”, “orange” and “kiwi.” It prints each fruit name on a separate line, displaying them in the order they appear in the list.

fruits = ["apple", "orange", "kiwi"]

for fruit in fruits:

    print(fruit)

Here we can see the for loops that iterates over iterable object fruit which is a list. Lists, sets, dictionaries are few iterable objects while an integer object is not an iterable object. For loops can iterate over any of these iterable objects.

This Python code manually iterates through a list of fruits using an iterator. It prints each fruit’s name one by one and stops when there are no more items in the list.

fruits = ["apple", "orange", "kiwi"]
iter_obj = iter(fruits)
while True:
    try:
        fruit = next(iter_obj)
        print(fruit)
    except StopIteration:
        break

apple
orange
kiwi

We can see that under the hood we are calling iter() and next() method. 

We have covered Python Loops in this article. We also saw how to use for loop, while loop and nested loop in Python. This article provides different use-case scenarios and examples to demonstrate working of loops and give clear understanding.

Learn More on Loops:

• Difference between for loop and while loop in Python
• Looping Techniques in Python
• Python loop-programs Archives

Python Loops – FAQs

Which loop is faster in Python?

In general, for loops tend to be faster than while loops in Python due to the way they are implemented. However, the difference in performance between for and while loops is often negligible for most practical purposes. The choice between them should be based on clarity and suitability for the specific task rather than performance concerns.

Why is Python slow in loops?

Python can be perceived as slow in loops when compared to lower-level languages like C or C++. This is primarily because Python is an interpreted language, meaning that each line of code is executed one by one at runtime. Additionally, Python’s dynamic typing and memory management can introduce overhead compared to statically-typed compiled languages.

How many loops are used in Python?

Python supports several types of loops:

• for loop: Iterates over a sequence (e.g., list, tuple, string, dictionary).
• while loop: Executes a block of code as long as a specified condition is true.
• Nested loops: One loop inside another loop.
• Comprehensions (e.g., list comprehensions, dictionary comprehensions): Concise ways to create lists, dictionaries, etc., using loops.

How to improve Python loops?

To optimize loops in Python, consider the following tips:

• Use list comprehensions: They are often faster than traditional for loops for creating lists.
• Minimize function calls: Move function calls outside loops if possible.
• Use built-in functions: Python’s built-in functions (e.g., map(), filter(), sum()) are optimized and can be faster than manually written loops.
• Profile your code: Use tools like cProfile to identify bottlenecks and optimize accordingly.
• Consider using libraries: Libraries like NumPy and pandas offer optimized functions for common tasks involving loops over arrays or data frames.

Is for loop bad in Python?

No, for loops are not inherently bad in Python. They are a fundamental construct for iteration and are widely used in Python programming. However, for certain tasks involving large datasets or performance-critical code, optimizing loop operations using techniques like list comprehensions, vectorized operations (in libraries like NumPy), or reducing function calls can improve efficiency.

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