For Loop in Python

For loop in Python is used to execute a block of code repeatedly. In this tutorial, we will learn how to use range(), break, continue, else and nested loops.

Watch this beginner-friendly Python for loop tutorial to understand how range(), break, continue, else with loops and nested for loops work with simple examples.

For loop with range

for x in range(5):
    print(x)

For loop in python to execute code block repeatedly using continue break and else with nested loops

Note that the 2nd line print(x) is placed after an indente of one space. Without that we will get an error asking expectd an indented block . The code to be executed within the block is to be kept with indenting as there is no end of for loop like other languages.
The output of above code is here

Here is a statement which is outside the loop because there is no indenting at the beginning of the line

for x in range(5):
    print(x)
print("I am outside the loop")

Output is here .

0
1
2
3
4
I am outside the loop

In the above code the range is from 0 to 5 ( excluding 5 ) , so we get output from 0 to 4.

Loop with range

Now we will define a range for the loop

for x in range(5, 10):
    print(x)

Output is here

We can add a increment value

for x in range(5, 25, 5):
    print(x)

Output is here

Break and continue in a for loop

When break statement is encountered the execution comes out of the loop. In case of continue the execution returns to the starting of the loop skipping the rest of the statements ( after continue ) and continues again.

In the codes give below , as soon as break is encountered the execution comes out of the loop, so 15 and 20 are not printed. But when continue is used in place of break the printing of 15 is skipped but the loop continues again so 20 is printed.
Break and continue in for loop

Using continue

for x in range(5, 25, 5):
    if (x == 15):
        continue
    print(x)

5
10
20

Using break

for x in range(5, 25, 5):
    if (x == 15):
        break
    print(x)

5
10

Using else with for loop

Here else part of the code is executed once the for loop execution is over. However if loop is terminated by using break then else part of the code is skipped.

Using continue

for x in range(5, 25, 5):
    if (x == 15):
        continue
    print(x)
else:
    print("Out of loop but with else")
print("Out of loop")

Output is here

5
10
20
Out of loop but with else
Out of loop

Code block inside else will be executed in case of continue but not executed in case of break.

using break

for x in range(5, 25, 5):
    if (x == 15):
        break
    print(x)
else:
    print("Out of loop but with else")
print("Out of loop")

Output is here ( else part is skipped )

5
10
Out of loop

Check how else in for loop is used to display all prime numbers upto 100.

num = 100 # change this value to get all prime numbers 
for i in range(2, num - 1):
    for j in range(2, i - 1):
        if (i % j == 0):
            break
    else:
        print(i, " is a prime number")

Output is

2  is a prime number
3  is a prime number
5  is a prime number
7  is a prime number
-----
-----
83  is a prime number
89  is a prime number
97  is a prime number

Using list

More on List.
For simple cases where a loop is used to create a new list, see Python list comprehension.

names = ["King", "Queen", "Jack & others"]
for str1 in names:
    print(str1)

Output is here

King
Queen
Jack & others

Iterating with Index Numbers (enumerate)

When you need both the index position and the corresponding value during iteration, combine the for...in construct with Python's built-in enumerate() function.

# Program to loop through a list while tracking the index
names = ["Alice", "Bob", "Charlie"]

for index, name in enumerate(names):
    print(f"Index {index}: {name}")

Dictionary Iteration (Key-Value Pairs)

To unpack and iterate through dictionary key-value pairs seamlessly, use the .items() method inside the for...in structure.

# Program to iterate over keys and values in a dictionary
student = {"name": "John", "age": 20, "grade": "A"}

for key, value in student.items():
    print(f"{key}: {value}")

Nested for loop

We can keep one for loop inside another

for i in range(5):
    for j in range(5):
        print('*', end='')
    print("")

Output is here

*****
*****
*****
*****
*****

More on Patterns using nested for loops Multiplication table using nested for loops
Using list

names=["King","Queen","Jack & others"]
ages=["child","Young","old"]
for str1 in names:
    for str2 in ages:
     print(str1,str2)

Output is here

King child
King Young
King old
Queen child
Queen Young
Queen old
Jack & others child
Jack & others Young
Jack & others old

Key Differences between for and while loops

for loop (Definite Iteration): Used when you know in advance how many times the code block needs to run, or when iterating through a sequence or collection (such as a list, tuple, string, dictionary, or range()).
while loop (Indefinite Iteration): Used when execution needs to continue repeatedly as long as a specified boolean condition remains True. The exact number of iterations is often unknown beforehand.

Comparison Summary

Feature	`for` Loop	`while` Loop
Control Mechanism	Iterates over items in an iterable or `range()`	Evaluates a boolean condition (`True` / `False`)
Iteration Count	Predetermined by the sequence length	Variable / dependent on condition evaluation
Counter Variable	Handled and updated automatically by Python	Must be initialized and modified manually
Primary Use Cases	Traversal of lists, ranges, strings, and datasets	Handling user input, game loops, dynamic conditions
Infinite Loop Risk	Very low	High (if condition fails to evaluate to `False`)

Download the above full source code from Github or run the code in your Google colab platform.

Questions only
https://github.com/plus2net/Python-basics/blob/main/online_class_for_loop_v1.ipynb

Python Basics - for loop #forloop #python
20-Feb-2024

View and Download for_loop ipynb file ( .html format )

While Loop if else : condition checking Recursion Python- Tutorials

Subhendu Mohapatra

Author & Instructor at plus2net

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