Loops

Loops in python are similar to loop in C, except python does not have a do while loop, but we can do a makeshift do while loop in python too.

For Loop:

A loop which runs “FOR” a specific number of repetitions or iterations.

for (int i = 0; i < 10; i++) {
 printf("%d\n", i);
}

Or you can write it like this,

for i in range(3):
    print(i)

range() can take up to three values: range(start, stop, step). The stop value is never included, so range(3) gives 0, 1, 2. If you leave out start it begins at 0, and if you leave out step it goes up by 1.

for i in range(1, 6):        # 1 2 3 4 5
    print(i)

for i in range(0, 10, 2):    # 0 2 4 6 8
    print(i)

for i in range(5, 0, -1):    # 5 4 3 2 1 (counting down)
    print(i)

A big difference from C is that Python’s for loop can go through the items of a collection directly, like a list or a string, without needing an index.

fruits = ["apple", "banana", "mango"]
for fruit in fruits:
    print(fruit)

for letter in "Python":
    print(letter)

If you need both the position and the item, use enumerate():

for index, fruit in enumerate(fruits):
    print(index, fruit)

Nested Loops

A loop inside another loop. The inner loop runs fully for every single run of the outer loop. A classic example is a multiplication table:

for i in range(1, 4):
    for j in range(1, 6):
        print(i * j, end="\t")
    print()

Output:

1	2	3	4	5
2	4	6	8	10
3	6	9	12	15

While Loop:

A loop which runs “WHILE” a certain condition is true, the number of iterations/repetitions aren’t fixed, only the boundary condition.

i = 1
while i <= 3:
    print(i)
    i += 1

Now here we change the value of i same as in C.

Be careful to always change something inside the loop that will eventually make the condition false. If you forget the i += 1 above, the condition i <= 3 stays true forever and you get an infinite loop. If that happens, press Ctrl + C in the terminal to stop it.

While loops are best when you don’t know beforehand how many times something needs to repeat. For example, asking the user for a password until they get it right:

password = ""
while password != "python123":
    password = input("Enter the password: ")
print("Access granted")

Another example, a simple countdown:

count = 5
while count > 0:
    print(count)
    count -= 1
print("Liftoff!")

Do While Loop

A loop to “DO (something) WHILE” a certain condition is true, again the number of iterations/repetitions are not fixed. The difference is the fact that a ‘Do While’ loop runs the code first then checks the conditions and repeat. Which is the opposite of the ‘While’ loop. You can say it assumes that the first/base condition is true by default. Since there is no built in Do While loop in python, we make it using While itself.

while True:
    # code
    if condition:
        break

Here while True makes the loop run endlessly, so the code inside always runs at least once, and the break at the end is what stops it when the condition is met. For comparison, this is how it looks in C:

do {
    // code
} while (condition);

A practical example is a menu that must be shown at least once, and keeps coming back until the user chooses to exit:

while True:
    print("1. Say Hello")
    print("2. Say Bye")
    print("3. Exit")
    choice = int(input("Enter your choice: "))

    if choice == 1:
        print("Hello!")
    elif choice == 2:
        print("Bye!")
    elif choice == 3:
        print("Exiting...")
        break

Loop Control: break, continue and pass

These three keywords change how a loop behaves.

• break stops the loop completely and moves on to the code after it.
• continue skips the rest of the current iteration and jumps to the next one.
• pass does nothing, it is just a placeholder where Python expects some code.

for i in range(1, 8):
    if i == 3:
        continue     # skip 3
    if i == 6:
        break        # stop at 6
    print(i)         # prints 1 2 4 5

The else Block on Loops

Python has a unique feature that C doesn’t: a loop can have an else block, which runs only if the loop finished normally, meaning it was not stopped by a break.

for n in range(2, 10):
    if n % 5 == 0:
        print("Found a multiple of 5:", n)
        break
else:
    print("No multiple of 5 found")

Quick Summary

Loop	Use it when	Python keyword
For	You know how many times, or are going through a collection	for
While	You only know the condition to keep going	while
Do While	The code must run at least once first	while True + break