Map and Filter

Higher-Order Functions: Map and Filter

Higher-order functions take other functions as arguments. Two of the most important are map and filter.

map applies a function to every element of a list:

#eval [1, 2, 3, 4, 5].map (fun x => x * 2)
-- [2, 4, 6, 8, 10]

filter keeps only elements that satisfy a predicate:

#eval [1, 2, 3, 4, 5, 6].filter (fun x => x % 2 == 0)
-- [2, 4, 6]

You can chain them together:

#eval [1, 2, 3, 4, 5, 6].filter (fun x => x % 2 == 0)
       |>.map (fun x => x * x)
-- [4, 16, 36]

Your Turn

1. Use map to square every element of [1, 2, 3, 4, 5]
2. Use filter to keep only numbers greater than 3 from [1, 2, 3, 4, 5, 6]