Python Loops

Loops repeat a block of code, so you do not have to write the same lines again and again.

The for Loop

A for loop goes through each item of a sequence (a list, a string, a range and so on):

foods = ["nihari", "haleem", "karahi"]
for food in foods:
    print("I like", food)

I like nihari
I like haleem
I like karahi

It also works on a string, one character at a time:

for letter in "Ali":
    print(letter)

A
l
i

The range() Function

range() gives a sequence of numbers. It is perfect when you want to repeat something a set number of times.

for i in range(1, 6):
    print(f"5 x {i} = {5 * i}")

5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25

The while Loop

A while loop runs as long as its condition is True:

savings = 0
month = 0
while savings < 10000:
    savings += 3000
    month += 1
print(f"Goal reached in {month} months with Rs {savings}")

Goal reached in 4 months with Rs 12000

Make sure something inside the loop changes, so the condition becomes False one day. Otherwise the loop runs forever.

break and continue

• break stops the loop completely
• continue skips the rest of this round and goes to the next one

for n in range(1, 10):
    if n == 6:
        break
    if n % 2 == 0:
        continue
    print(n)

1
3
5

Even numbers are skipped by continue, and the loop ends at 6 because of break.

Looping With an Index

enumerate() gives you both the position and the item:

toppers = ["Ayesha", "Bilal", "Sana"]
for position, name in enumerate(toppers, start=1):
    print(position, name)

1 Ayesha
2 Bilal
3 Sana

else With a Loop

A loop can have an else block. It runs when the loop finishes normally, without break:

for i in range(3):
    print(i)
else:
    print("Done")

0
1
2
Done

Nested Loops

A loop inside another loop runs fully for each round of the outer loop:

for row in range(1, 4):
    print("*" * row)

*
**
***

for size in ["S", "M"]:
    for color in ["red", "blue"]:
        print(size, color)

S red
S blue
M red
M blue

Exercise