Control flow · Lesson 5 of 18 · 18 min

Loops and iteration

for, while, range, zip, and the iterator protocol.

Iterate collections without index arithmetic
Use enumerate, zip, and range fluently
Know how for really works

for is not a C for

`for item in collection:` asks the collection for an iterator (`iter(collection)`), then repeatedly calls `next` until `StopIteration`. Anything that implements that protocol is iterable: lists, strings, files, dicts, generators, your own classes.

You almost never need `for i in range(len(xs))`. Use `for x in xs` to see values, `for i, x in enumerate(xs)` when you also need the index, and `for a, b in zip(xs, ys)` to walk two sequences together.

The loop tools that replace indexes

letters = ["a", "b", "c"]
for i, ch in enumerate(letters, start=1):
    print(i, ch)
print(list(zip("abc", [10, 20, 30])))
print(list(range(3, 10, 2)))
for line in "alpha\nbeta\n".splitlines():
    print("line:", line)

while, else, and early exits

`while` is for loops where the end condition is not 'the collection is exhausted' — waiting, retrying, reading until a sentinel.

`break` leaves the loop. `continue` skips to the next iteration. Python has `for/else` and `while/else`: the `else` runs if the loop did not `break`. It is the right shape for 'search, else not found'.

for/else as search

def first_even(nums):
    for n in nums:
        if n % 2 == 0:
            return n
    return None

def first_even_else(nums):
    for n in nums:
        if n % 2 == 0:
            break
    else:
        return None
    return n

print(first_even([1, 3, 4, 6]), first_even_else([1, 3, 5]))

Check yourself

Quiz

1.What does `list(zip('ab', [1, 2, 3]))` produce?
2.The else clause on a for-loop runs when:
3.`for x in xs` is equivalent to iterating:

Practice

Exercise

Write `index_of(items, target)` that returns the first index of target, or -1 if it is missing. Use enumerate. Do not use list.index.

Tutor

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