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I have a list of lists(matrix) and i would like to access using (map or apply) the main diagonal.

The problem is larger, to make a matrix of a list of images removing the duplicates but i did that part.

(make-list (length (filter (lambda (x) (<= (image-height  x) max-height)) L)) 
           (filter (lambda (x) (<= (image-height  x) max-height)) L)) )

I don't have a clue how to access using map or apply the elements of the main diagonal..

Eggcellentos
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Nita Liviu
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1 Answers1

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Remember that map requires a procedure that takes as many arguments as you have lists to map. In this case we want to access the ith element of a list, this is the built-in procedure list-ref which takes a list L and an index i and returns the ith element of L. So then we just need the indexes of the diagonal elements. This is (list 0 1 2 ...) which we can generate using build-list.

(define (diagonal M)
  (define indexes (build-list (length M) values))
  (map list-ref M indexes))

You don't have to build the second list if you use recursion. In this case you build the diagonal in a recursive function where we take the leading element of the first row, then recur on the rest of the elements in the rest of the rows:

(define (diagonal M)
  (if (null? M)
      '()
      (cons (caar M)
            (diagonal (map cdr (cdr M))))))

It's going to take basically the same amount of operations in terms of building the diagonal, but the second one doesn't require a new list be generated up front and doesn't require determining the size of the matrix, so it's a little better.

law-of-fives
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