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for loops — Python Foundations

Day 14 · ~12 min●

A list of three numbers [10, 20, 30]. You want to print each one doubled. Without a loop, what does that look like?

print(numbers[0] * 2), print(numbers[1] * 2), print(numbers[2] * 2). Three lines that are almost identical.

And if the list grows to a hundred items, you write a hundred lines? That's where loops come in. for says "do this once for each item in this list":

python

for n in numbers:
    print(n * 2)

Three lines for three items, three lines for a hundred items, three lines for a million. The loop body runs once per item; n takes the value of the current item each time.

The colon, the indent — same as if?

Identical syntax. for variable in iterable: then an indented block. Python's structure is the same wherever blocks appear.

The variable n — does it have to be that letter?

Anything legal. for item in numbers:, for x in numbers:, for value in numbers: — pick a name that makes the loop body readable. Single letters are fine for short bodies; descriptive names help when the body is longer.

Can I loop over a string? Or only lists?

Anything iterable. Strings yield characters: for c in "hello" runs five times, with c being 'h', 'e', 'l', 'l', 'o'. You'll meet more iterables (dicts, ranges, generators) as you go.

`for` loops — iterate over a sequence

The shape:

python

for item in iterable:
    # body — runs once per item

item is a name you choose; it takes the value of each element in turn
iterable is anything Python can step through one item at a time — list, string, dict, range, ...
the colon and indentation work the same way as in if

Iterating a list

python

names = ["Ada", "Bob", "Cleo"]
for name in names:
    print(f"hello {name}")
# hello Ada
# hello Bob
# hello Cleo

Iterating a string

python

for c in "hi":
    print(c)
# h
# i

Strings iterate character by character.

`range(n)` — N-many iterations

When you want a loop that runs N times without a list to iterate, range(n) produces 0, 1, ..., n-1:

python

for i in range(5):
    print(i)
# 0
# 1
# 2
# 3
# 4

range(start, stop) and range(start, stop, step) exist for finer control.

Combining with `if`

Loops and conditionals nest naturally:

python

for n in [1, 2, 3, 4, 5]:
    if n % 2 == 0:
        print(f"{n} is even")

Indentation makes the structure visible at a glance.

Common gotcha: modifying a list while looping

python

for item in items:
    items.append(...)    # don't do this — undefined behaviour

If you need to build a new list while iterating, use a fresh list:

python

result = []
for item in items:
    result.append(transform(item))

Day 14 · ~12 min●

A list of three numbers [10, 20, 30]. You want to print each one doubled. Without a loop, what does that look like?

print(numbers[0] * 2), print(numbers[1] * 2), print(numbers[2] * 2). Three lines that are almost identical.

And if the list grows to a hundred items, you write a hundred lines? That's where loops come in. for says "do this once for each item in this list":

python

for n in numbers:
    print(n * 2)

Three lines for three items, three lines for a hundred items, three lines for a million. The loop body runs once per item; n takes the value of the current item each time.

The colon, the indent — same as if?

Identical syntax. for variable in iterable: then an indented block. Python's structure is the same wherever blocks appear.

The variable n — does it have to be that letter?

Can I loop over a string? Or only lists?

Anything iterable. Strings yield characters: for c in "hello" runs five times, with c being 'h', 'e', 'l', 'l', 'o'. You'll meet more iterables (dicts, ranges, generators) as you go.

`for` loops — iterate over a sequence

The shape:

python

for item in iterable:
    # body — runs once per item

item is a name you choose; it takes the value of each element in turn
iterable is anything Python can step through one item at a time — list, string, dict, range, ...
the colon and indentation work the same way as in if

Iterating a list

python

names = ["Ada", "Bob", "Cleo"]
for name in names:
    print(f"hello {name}")
# hello Ada
# hello Bob
# hello Cleo

Iterating a string

python

for c in "hi":
    print(c)
# h
# i

Strings iterate character by character.

`range(n)` — N-many iterations

When you want a loop that runs N times without a list to iterate, range(n) produces 0, 1, ..., n-1:

python

for i in range(5):
    print(i)
# 0
# 1
# 2
# 3
# 4

range(start, stop) and range(start, stop, step) exist for finer control.

Combining with `if`

Loops and conditionals nest naturally:

python

for n in [1, 2, 3, 4, 5]:
    if n % 2 == 0:
        print(f"{n} is even")

Indentation makes the structure visible at a glance.

Common gotcha: modifying a list while looping

python

for item in items:
    items.append(...)    # don't do this — undefined behaviour

If you need to build a new list while iterating, use a fresh list:

python

result = []
for item in items:
    result.append(transform(item))

`for` loops — iterate over a sequence

Iterating a list

Iterating a string

`range(n)` — N-many iterations

Combining with `if`

Common gotcha: modifying a list while looping

`for` loops — iterate over a sequence

Iterating a list

Iterating a string

`range(n)` — N-many iterations

Combining with `if`

Common gotcha: modifying a list while looping

`for` loops — iterate over a sequence

Iterating a list

Iterating a string

`range(n)` — N-many iterations

Combining with `if`

Common gotcha: modifying a list while looping

Sign up to practice

`for` loops — iterate over a sequence

Iterating a list

Iterating a string

`range(n)` — N-many iterations

Combining with `if`

Common gotcha: modifying a list while looping

Sign up to practice

for loops — iterate over a sequence

Iterating a list

Iterating a string

range(n) — N-many iterations

Combining with if

Common gotcha: modifying a list while looping

for loops — iterate over a sequence

Iterating a list

Iterating a string

range(n) — N-many iterations

Combining with if

Common gotcha: modifying a list while looping

for loops — iterate over a sequence

Iterating a list

Iterating a string

range(n) — N-many iterations

Combining with if

Common gotcha: modifying a list while looping

Sign up to practice

for loops — iterate over a sequence

Iterating a list

Iterating a string

range(n) — N-many iterations

Combining with if

Common gotcha: modifying a list while looping

Sign up to practice

`for` loops — iterate over a sequence

`range(n)` — N-many iterations

Combining with `if`

`for` loops — iterate over a sequence

`range(n)` — N-many iterations

Combining with `if`

`for` loops — iterate over a sequence

`range(n)` — N-many iterations

Combining with `if`

`for` loops — iterate over a sequence

`range(n)` — N-many iterations

Combining with `if`