How does `map` behave that matrix multiplication fails in this example?

Question

I am trying to make a function that does matrix-vector multiplication in scheme. I am confused by the results. Here is my code that defines a function that computes the dot products of two vectors:

(define (accumulate op initial sequence)
  (if (null? sequence)
      initial
      (op (car sequence)
      (accumulate op initial (cdr sequence)))))

(define (dot-product v w)
  (accumulate + 0 (map * v w)))

This returns the expected dot product of vectors, like (list 2 3 4) and (list 1 1 1). But when I try to compute the product of a matrix and vector with

(define (matrix-*-vector m v)
  (map (lambda (x) dot-product v x) m))

this function will simply return the original matrix m, rather than the expected result. I'm extra confused because I can write this in Wolfram Language just fine and it behaves as expected:

myMatrixMult[m_,v_]:=Map[Function[x,Dot[v, x]], m]

(I don't think you need to be expert in either language to see the similarities).

What am I misapprehending about the scheme implementation? What needs to be done in order to define matrix-vector multiplication using map and dot-product?

Answer 1

You are missing parens in your lambda:

(lambda (x) dot-product v x)

never calls dot-product, it just returns x.

You need

(lambda (x) (dot-product v x))