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

Operating on a Series of Integers

Problem

You want to perform an operation on all integers in a certain range.

Use the map, foldl, or for-each functions if you have a known list of (possibly non-contiguous) integers. Or, you may be interested in SRFI 42 (Eager List Comprehensions). Otherwise, try a do loop.

For example, to double all entries in a list:

> (define a (list 1 2 5 7 9 10 108))
> (map (lambda (x) (* 2 x)) a)
(2 4 10 14 18 20 216)
> (for-each (lambda (x) (printf "~a " (* 2 x))) a)
2 4 10 14 18 20 216

Or, to print all even integers less than 71:

> (require (lib "42.ss" "srfi"))
> (list-ec (: i 72) (if (even? i)) i)
(0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38
 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70)

Or use a boring do loop. For example, to iterate across all integers from 0 to 4:

> (do ((i 0 (+ i 1)))
    ((= i 5) i)
    (printf "~a " i))
0 1 2 3 4
5

Or, to iterate across all integers from 0 to 71, step 7:

> (do ((i 0 (+ i 7)))
    ((>= i 71) (printf "Done! ~a" i))
    (printf "~a " i))
0 7 14 21 28 35 42 49 56 63 70 Done! 77

Discussion

The first two methods use a more functional approach to loops.

The following code shows each technique. Here we only print the numbers we generate:

(require (lib "42.ss" "srfi"))

(printf "Infancy is: ")
(foldl
 (lambda (x y) (printf "~a " x))
 0
 (list 0 1 2))
(newline)

(printf "Toddling is: ")
(for-each
 (lambda (x)
   (printf "~a " x))
 (list 3 4))
(newline)

(printf "Childhold is: ")
(do-ec (:range i 5 12 1) (printf "~a " i))
(newline)

The program output looks like:

Infancy is: 0 1 2 
Toddling is: 3 4 
Childhold is: 5 6 7 8 9 10 11

PLT Scheme	SISC
Scheme48	SCM
MIT Scheme	scsh
JScheme	Kawa
Chicken	Guile
Bigloo	Tiny
Gambit	LispMe
Gauche	Chez

CookbookForm
TopicType:	Recipe
ParentTopic:	NumberRecipes
TopicOrder:	050

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

Operating on a Series of Integers

Problem

Solution

Discussion

See Also