3. Control flow and functions

Making decisions

fn sign(n):
    if n > 0:
        return "positive"
    elif n < 0:
        return "negative"
    else:
        return "zero"

print(sign(3))
print(sign(-8))
print(sign(0))
# => positive
# => negative
# => zero

The body of an if is not a separate scope — a variable you assign there is still around afterwards:

if true:
    message = "set inside the if"
print(message)
# => set inside the if

Repeating work

while repeats while a condition holds; for walks a list, string, map, or range:

n = 5
factorial = 1
while n > 1:
    factorial = factorial * n
    n = n - 1
print(factorial)
# => 120

for word in ["one", "two", "three"]:
    print(word)
# => one
# => two
# => three

range(n) gives [0, 1, ..., n-1]; range(a, b) and range(a, b, step) give the rest of what you would expect:

total = 0
for i in range(1, 101):
    total = total + i
print(total)
# => 5050

break leaves a loop; continue skips to its next turn.

Functions

fn defines one. Parameters are positional, and a call must pass exactly the right number:

fn greet(greeting, who):
    return greeting + ", " + who + "!"

print(greet("Hello", "world"))
# => Hello, world!

A function with no return (or a bare return) gives back nil.

Functions are values

You can pass a function to another function, return one, or store one in a list:

fn twice(f, x):
    return f(f(x))

fn inc(n):
    return n + 1

print(twice(inc, 10))
# => 12

Closures

A function defined inside another one remembers the variables around it — and can change them:

fn make_counter():
    count = 0
    fn next():
        count = count + 1
        return count
    return next

tick = make_counter()
print(tick())
print(tick())
print(tick())
# => 1
# => 2
# => 3