Question

I am flying blind on scheme, and I feel like once I answer this question the rest of my homework should progress smoothly.

I’m defining a function that takes a list as its sole argument, then returns the same list with the first element added to the rest. For instance:

(addFirst ‘(4 3 2 1))   =>   (8 7 6 5)

I have a feeling that I should be using the map and car functions here… but I just can’t seem to get it exactly right. My current version of this code looks like:

(define (addlist x) ;adds the first element of a list to all other elements
  (define a (car x)) ;a is definitely the first part of the list
  (map (+ a) x)
)

How can I make the addition function work this way? Obviously I can’t provide a list as a parameter, but should I be using car again or recursion?

Okay, for posterity, here’s the completed, correct, formatted code:

(define (addlist x) ;adds the first element of a list to all other elements
  (define a (car x)) ;a is definitely the first part of the list
  (map (lambda (y) (+ a y)) x)
)

Answer 1

Map takes a function and n lists. So you need to transform (+ a) into a function that takes one argument (as you want to map over one list).

(map (lambda (item) ... do something to add a to item ...) x)