Basic Syntax

The syntax for a for loop is:

1
for variable in sequence:
2
    # loop body here

The variable takes on each value from the sequence one at a time, and the loop body executes once for each value. Let’s see this in action with range():

1
for i in range(5):
2
    print(i)

This will output:

1
0
2
1
3
2
4
3
5
4

The variable i takes on each value from range(5) (which is 0, 1, 2, 3, 4) one at a time, and we print it. Notice how we don’t need to manually increment i or check a condition—the for loop handles all of that for us automatically.

Let’s compare this to the equivalent while loop:

1
i = 0
2
while i < 5:
3
    print(i)
4
    i += 1

Both produce the same output, but the for loop is more concise and less error-prone. We don’t need to worry about forgetting to increment i or accidentally creating an infinite loop.

Using range() with for loops

The range() function is commonly used with for loops to iterate over sequences of numbers. Let’s see various examples:

1
# 0 1 2 3 4
2
for i in range(5):
3
    print(i, end=" ")
4
print()
5

6
# 2 3 4 5 6 7
7
for i in range(2, 8):
8
    print(i, end=" ")
9
print()
10

11
# 1 3 5 7 9
12
for i in range(1, 10, 2):
13
    print(i, end=" ")
14
print()
15

16
# 20 18 16 14 12 10 8 6 4 2
17
for i in range(20, 0, -2):
18
    print(i, end=" ")

For loops vs While loops

When should a for loop be used instead of a while loop? Here are the key differences:

1
# Print "Hello" 5 times
2
for i in range(5):
3
    print("Hello")

1
# Print numbers from 1 to 10
2
for i in range(1, 11):
3
    print(i)

1
# Sum numbers from 1 to 100
2
total = 0
3
for i in range(1, 101):
4
    total += i
5
print(total)

1
# Keep asking for input until user enters "quit"
2
response = ""
3
while response != "quit":
4
    response = input("Enter a command (or 'quit' to exit): ")

1
# Keep reading numbers until sum exceeds 100
2
total = 0
3
while total <= 100:
4
    num = int(input("Enter a number: "))
5
    total += num

1
# Keep asking for password until correct
2
while True:
3
    password = input("Enter password: ")
4
    if password == "correct":
5
        break

Control flow with for loops

Just like while loops, for loops support continue, break, and else statements. The continue statement skips the remainder of the current iteration and moves to the next value in the sequence:

1
for i in range(10):
2
    if i % 2 != 0:  # Skip odd numbers
3
        continue
4
    print(i, end=" ")

This will output: 0 2 4 6 8

Note

Notice how we don’t need to manually increment anything—the for loop automatically moves to the next value. This is simpler than the equivalent while loop:

1
i = 0
2
while i < 10:
3
    if i % 2 != 0:
4
        i += 1
5
        continue
6
    print(i, end=" ")
7
    i += 1

The break statement exits the loop immediately, even if there are more items in the sequence:

1
for i in range(10):
2
    if i == 5:
3
        break
4
    print(i, end=" ")

This will output: 0 1 2 3 4

The else clause in a for loop executes only if the loop completes normally (without a break):

1
for i in range(5):
2
    print(i, end=" ")
3
else:
4
    print("\nLoop completed normally")

If we use break, the else clause doesn’t execute:

1
n = int(input("Enter a number: "))
2
for i in range(2, n):
3
    if n % i == 0:
4
        print(n, "is not prime")
5
        break
6
else:
7
    print(n, "is prime")

If we find a divisor, we break and print "not prime". If the loop completes without finding any divisor (no break occurred), the else clause executes and prints "prime".

Nested for loops

Just like while loops, for loops can be nested. This is very useful for working with patterns, grids, or any situation where you need to iterate over multiple dimensions.

1
for i in range(4):
2
    for j in range(4):
3
        print("*", end="")
4
    print()

This will output:

1
****
2
****
3
****
4
****

When using break or continue in nested loops, they only affect the innermost loop.

1
for i in range(3):
2
    for j in range(i, i + 3):
3
        if j % 3 == 0:
4
            break  # Only breaks out of inner loop
5
        print("i = ", i, "j = ", j)

This will output:

1
i =  1 j =  1
2
i =  1 j =  2
3
i =  2 j =  2

Exercises

Solve these problems using for loops.

1
for i in range(1, 51):
2
    print(i)

1
total = 0
2
for i in range(1, 101):
3
    total += i
4
print(total)

1
5 x 1 = 5
2
5 x 2 = 10
3
5 x 3 = 15
4
...
5
5 x 10 = 50

1
n = int(input())
2
for i in range(1, 11):
3
    print(n, 'x', i, '=', n * i)

1
*
2
**
3
***
4
****

1
n = int(input())
2
for i in range(1, n + 1):
3
    for j in range(i):
4
        print('*', end='')
5
    print()

1
*****
2
*****
3
*****

1
width = int(input())
2
height = int(input())
3
for i in range(height):
4
    for j in range(width):
5
        print('*', end='')
6
    print()

1
n = int(input())
2
factorial = 1
3
for i in range(1, n + 1):
4
    factorial *= i
5
print(factorial)

1
****
2
***
3
**
4
*

1
n = int(input())
2
for i in range(n, 0, -1):
3
    for j in range(i):
4
        print('*', end='')
5
    print()

1
1
2
12
3
123
4
1234

1
n = int(input())
2
for i in range(1, n + 1):
3
    for j in range(1, i + 1):
4
        print(j, end='')
5
    print()