Map, Filter, and Reduce (Fold)

Map, filter, and reduce (called fold in Haskell) are the three most important higher-order functions in functional programming. They provide powerful, concise ways to transform and process lists without writing explicit loops.

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Map

Map applies a function to every element of a list, producing a new list of the same length with the transformed values.

Concept

map f [a, b, c, d] = [f(a), f(b), f(c), f(d)]

Python

# Double every number
numbers = [1, 2, 3, 4, 5]
doubled = list(map(lambda x: x * 2, numbers))
# doubled = [2, 4, 6, 8, 10]

# Convert temperatures from Celsius to Fahrenheit
celsius = [0, 20, 37, 100]
fahrenheit = list(map(lambda c: c * 9/5 + 32, celsius))
# fahrenheit = [32.0, 68.0, 98.6, 212.0]

# Get lengths of strings
words = ["hello", "world", "hi"]
lengths = list(map(len, words))
# lengths = [5, 5, 2]

Haskell

-- Double every number
doubled = map (*2) [1, 2, 3, 4, 5]
-- doubled = [2, 4, 6, 8, 10]

-- Convert to uppercase
import Data.Char (toUpper)
upper = map toUpper "hello"
-- upper = "HELLO"