s c h e m a t i c s : c o o k b o o k

This Web

Other Webs

Schematics

RSS

Scheme Links

Filtering Lists

Problem

Solution

> (require (lib "list.ss"))
> (filter odd? '(1 2 3 4 5 6 7 8 9 0))
(1 3 5 7 9)
> (filter string? '(14 "abc" cons "xyz" 23))
("abc" "xyz")

You can supply your own predicate, as well. Here we filter a list of numbers that are greater than some limit we've defined:

> (let ((limit 12))
    (filter (lambda (x) (> x limit)) (map (lambda (n) (* n n)) '(1 2 3 4 5 6 7 8))))
(16 25 36 49 64)

Discussion

There are a few twists on this problem. SRFI-1's partition works like filter, except that it returns 2 lists, one with items that pass the filter, and the other with the rest of the items in the original list, e.g.

> (require (lib "1.ss" "srfi"))
> (filter string? '(14 "abc" cons "xyz" 23))
("abc" "xyz")
> (partition string? '(14 "abc" cons "xyz" 23))
("abc" "xyz")
(14 cons 23)

Also in SRFI-1, remove is the reverse of filter, returning a new list without the items satisfying the predicate supplied. In addition, there are ! versions of all these functions to modify a list in-place.

Also, SRFI-1's filter-map is another twist on filter, combining it with map. It's like a map that returns only the true values. Here we return the products of two lists, but only if both members of the lists are numbers:

> (filter-map (lambda (x y) (and (number? x) (number? y) (* x y))) '(a 1 b 3 c 7) '(cons 14 "abc"  "xyz" 23 9))
(14 63)

Comments about this recipe

Contributors

-- NoelWelsh - 02 Jun 2004

-- GordonWeakliem - 06 Aug 2004