while Loop vs for Loop in Python: Which to Use?

for or while: Which Loop Should You Use?

Python has two loops, and many jobs can be done with either one. The rule of thumb: use for when you know what to go through, and while when you know when to stop.

Here is the same job, printing 1 to 3, both ways:

12for n in range(1, 4):
    print(n)

1234n = 1
while n <= 3:
    print(n)
    n += 1

The for loop does the start, the check and the change for you, in one line. It can't forget the change, so it can't run forever by mistake. For this job, for wins.

Think of a teacher handing back tests: going through a known pile of papers is a for job. Waiting at the door until the last student arrives is a while job.

Pick the loop

A quick test

Ask yourself: "Before the loop starts, do I know how many rounds it will run, or which items it goes through?"

• Yes → for.
• No, I only know when to stop → while.

Common beginner doubt: "Is while the worse loop, then?" Not at all. It's the only one of the two that can wait for the user, or keep going until a goal is reached. Each loop is best at its own job.

for: you know what to go through. while: you know when to stop.

break and continue in a while Loop

You met break and continue in the for loops chapter. They work in a while loop too: break leaves the loop at once, and continue skips the rest of this round and goes back to the check.

123456n = 0
while n < 6:
    n += 1
    if n % 2 == 0:
        continue
    print(n)

For even numbers, continue skips the print() and jumps back to while n < 6:. Odd numbers reach the print().

Think of reading a book: continue is skipping a boring page and going on with the next one. break is closing the book.

The continue trap

In a for loop, continue is safe: the loop moves to the next item by itself. In a while loop, you do the change. If continue jumps over it, the variable never changes again:

n = 0
while n < 6:
    if n % 2 == 0:
        continue      # jumps back with n still 0
    print(n)
    n += 1            # never reached: an infinite loop

n is 0, continue jumps back, n is still 0, and so on forever.

The fix: change first

Put the change at the top of the round, before any continue, as in the first example. Then every round moves the loop on, whatever happens below.

In a while loop, change the variable before any continue.

Using a while Loop with a List

A while loop can work through a list while it changes, for example taking items out until the list is empty. A list with items counts as true, and an empty list [] counts as false, just like an empty string in chapter 8.

12345tasks = ["wash", "cook", "shop"]
while tasks:
    task = tasks.pop(0)
    print("Doing:", task)
print("All done!")

while tasks: reads as "while there are tasks left". Each round, pop(0) (chapter 9) takes the first item out of the list. After three rounds the list is empty, and the loop stops.

Think of a to-do pile on your desk: you take the top sheet, do it, and stop when the pile is gone.

Front or back?

• tasks.pop(0) takes the first item: first in, first out, like a queue at a shop.
• tasks.pop() takes the last item: last in, first out, like a stack of plates.

Going through by position

You can also walk through a list by its index, with a counter. This is what a for loop does for you:

12345names = ["Sunny", "Rocky", "Lucky"]
i = 0
while i < len(names):
    print(i, names[i])
    i += 1

The check is i < len(names), not <=: the last index is one less than the length. Use <=, and the last round asks for names[3], which doesn't exist. That's an off-by-one error with a real error message:

12345names = ["Sunny", "Rocky", "Lucky"]
i = 0
while i <= len(names):
    print(names[i])
    i += 1

For plain "go through every item", for name in names: is shorter and safer. Use while when the list changes as you go, or when you stop early for a reason that depends on the items.

while my_list: keeps going until the list is empty.

A Menu Loop: Putting It All Together

Many real programs are one big menu loop: wait for a command, do it, and wait again, until the user says quit. It's while True, input(), an if/elif chain and a break, all in one.

1234567891011121314balance = 0
while True:
    command = input()
    if command == "quit":
        print("Bye!")
        break
    elif command == "add":
        amount = int(input())
        balance += amount
        print("Added", amount)
    elif command == "show":
        print("Balance:", balance)
    else:
        print("Unknown command:", command)

Read the Input box from top to bottom: add reads one more line for the amount, show prints the balance, fly isn't a command, and quit ends the loop.

Think of a coffee machine: it waits for a button, makes the drink, and waits again, until someone switches it off.

How it's built

• while True:: the program doesn't know how many commands are coming.
• One input() at the top of each round reads the next command.
• An if/elif chain (chapter 8) picks what to do. Only one branch runs per round.
• quit comes first, with its break: the way out is easy to find.
• A final else handles typos, so a wrong command doesn't break anything.
• balance starts before the loop, so it remembers its value from round to round.

Each command reads its own extra lines

add needs an amount, so it calls input() once more. The other commands don't. That's why the Input box above has 8 lines for 6 commands. When you test a menu loop, make sure the last line is quit, or you'll meet the EOFError from topic 3.

Common beginner doubt: "Why while True and not while command != "quit"?" Both can work. With while True, the command is read in one place, and the stop check sits right next to the other commands, so the whole menu reads like a list.

A menu loop: read a command, pick with if/elif, break on quit.

Practice: while or for

Solve these to lock in what you learned in this topic. Each one runs in your browser.

1. 98.Python Program to Count the Steps of the Collatz SequenceMediumSolve
2. 99.Python Program to Make a Number Guessing GameHardSolve