Python · Beginner

Loops in Python: for and while

In short: A for loop runs its block once for each item in a sequence such as a list, a string or a range(); a while loop repeats its block for as long as a condition stays true. Inside either loop, break leaves immediately and continue skips to the next round.

Repeating work

Loops let one block of code run many times, either once per item in a collection or until some condition changes. Python has two loop statements and they answer different questions.

for item in sequence: is the loop for "do this with each thing". Python takes the items one at a time, binds the loop variable to each in turn and runs the block. The sequence can be a list, a string (one character per round), a tuple, a range() or anything else iterable. You never manage an index or a counter yourself, which removes the most common source of off-by-one errors. When you do need numbers, range(n) produces 0 up to but not including n, range(a, b) produces a up to b, and range(a, b, step) counts in steps; range(1, 6) is 1, 2, 3, 4, 5. When you need both the position and the item, enumerate(sequence) yields pairs, and enumerate(sequence, start=1) counts from one.

while condition: is the loop for "keep going until". Before each round Python evaluates the condition; if it is true the block runs, then the condition is checked again. The block must eventually make the condition false, usually by changing a variable it depends on, or the loop never ends. A while fits situations where the number of rounds is unknown in advance: reading input until a sentinel word, retrying until something succeeds, halving a number until it drops below a limit.

Two statements change the flow inside a loop. break ends the loop at once and execution resumes after it. continue abandons the current round and goes straight to the next test or the next item. A loop may also carry an else: block, which runs when the loop finishes without hitting break; it is uncommon but occasionally the clearest way to say "nothing was found".

Choose for when you have a collection or a known count, while when you have a stopping condition. A while True: with a break inside is the idiom for loops whose exit test sits in the middle of the body.

Syntax

for item in sequence:
    block

for i in range(start, stop, step):   # stop is excluded
    block

for index, item in enumerate(sequence, start=0):
    block

while condition:
    block                 # must eventually make condition false

break      # leave the loop now
continue   # skip to the next round

range() with one argument starts at 0 and steps by 1. The loop variable keeps its last value after a for loop ends.

for versus while

for over a list, with enumerate and range

Four loops over a bus route, a range and a string.

stops = ["Harbour", "Market", "Station"]
for stop in stops:
    print("Next stop:", stop)
for number, stop in enumerate(stops, start=1):
    print(number, stop)
total = 0
for value in range(1, 6):
    total += value
print("Sum of 1 to 5:", total)
for letter in "bus":
    print(letter.upper(), end=" ")
print()

Next stop: Harbour
Next stop: Market
Next stop: Station
1 Harbour
2 Market
3 Station
Sum of 1 to 5: 15
B U S

The first loop needs no index at all. enumerate supplies a running number alongside each item, unpacked into two loop variables. range(1, 6) stops before 6, so the sum is 1 + 2 + 3 + 4 + 5. A string is a sequence of characters, so the last loop visits each letter; end=" " makes print stay on the same line, and the bare print() finishes it.

while with a changing condition

A tank fills in rounds of 35 litres; then a number is halved until it reaches 1.

level = 0
rounds = 0
while level < 100:
    level += 35
    rounds += 1
    print("Round", rounds, "level", level)
print("Full after", rounds, "rounds")

n = 40
while n > 1:
    n //= 2
print("Ended with", n)

Round 1 level 35
Round 2 level 70
Round 3 level 105
Full after 3 rounds
Ended with 1

Each round adds to level, moving it toward the exit condition; after three rounds level < 100 is false and the loop stops without running the block a fourth time. The second loop halves n (40, 20, 10, 5, 2, 1) until it is no longer greater than 1: the number of rounds depends on the data, which is exactly when while is the right tool.

break and continue while reading input

Readings arrive one per line until the word stop. The input is 12, -3, 8, stop, 20.

total = 0
kept = 0
while True:
    line = input()
    if line == "stop":
        break
    value = int(line)
    if value < 0:
        continue
    total += value
    kept += 1
print("Kept", kept, "readings, total", total)

Kept 2 readings, total 20

The loop has no natural end condition, so it runs as while True and leaves through break when the sentinel arrives. Negative readings are skipped with continue, which jumps back to the top without touching total or kept. The 20 after stop is never read, because the loop had already ended.

Common mistakes

• A while loop whose condition never changes

  Why it goes wrong: while count < 5: with no line that changes count runs forever, and the program appears to hang.

  Fix: Make sure something in the block moves toward the exit, or use for with range() when the count is known.

   Python · fixCopy

  count = 0
  while count < 5:
      print(count)
      count += 1

• Off-by-one with range()

  Why it goes wrong: range(1, 10) stops at 9 because the end value is excluded. A loop that should include the last number silently misses it.

  Fix: Add one to the end: range(1, 11) for 1 through 10, or compute the bound from the data with len().

• Removing items from a list while looping over it

  Why it goes wrong: Deleting shifts the later items left, so the loop skips the item that followed each deletion.

  Fix: Build a new list of the items to keep, or loop over a copy.

   Python · fixCopy

  readings = [3, -1, 4, -2]
  kept = []
  for r in readings:
      if r >= 0:
          kept.append(r)
  print(kept)  # [3, 4]

• Looping over indexes when you only need the items

  Why it goes wrong: for i in range(len(items)): print(items[i]) works, but the index variable serves no purpose and invites mistakes.

  Fix: Loop over the items directly; use enumerate() when you need the position as well.

Where you use this

Almost every exercise on this site reads several lines of input, and a loop is how you read them. When the first line tells you how many follow, for _ in range(n) reads exactly that many. Once the values are in, another loop totals them, finds the largest, or prints a formatted row per item. The pattern of a variable updated inside a loop, an accumulator such as total or largest, is the shape behind averages, counts and sums in every language.

n = int(input())
largest = None
for _ in range(n):
    value = int(input())
    if largest is None or value > largest:
        largest = value
print(largest)

Key points

• for visits each item of a sequence; while repeats until a condition is false.
• range(a, b) counts from a up to b - 1; range(n) starts at 0.
• enumerate() gives index and item together; pass start=1 to count from one.
• break exits the loop; continue skips the rest of the current round.
• A while body must change something the condition depends on.
• Never modify a list you are iterating over; build a new one instead.

Try it yourself

Read a number and print its times table from 1 to 5, one line per product, in the form 7 x 1 = 7. Use a for loop with range().

n = int(input())
# print n x 1 = ..., up to n x 5

n = int(input())
for i in range(1, 6):
    print(n, "x", i, "=", n * i)

Practise this

• Longest reading streakMedium

  Find the longest run of reading days from a compact activity string using a Python loop.

  LoopsNot started

Open the Python playground

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