# Matrix algebra operations using recursion

This post is inspired by a couple of exercises from the classical book, SICP. I found them pretty interesting as they were just using recursion & some common list operations to multiply matrices !! I also wanted to try out the Jupyter notebook kernel for MIT Scheme.

#### Representation & Problem Statement

Suppose we represent vectors v = ( vi ) as sequences of numbers, and matrices m = ( mij ) as sequences of vectors (the rows of the matrix). For example, the matrix

 1 2 3 4 5 6 7 8 9 10 11 12

is represented as the sequence `((1 2 3 4) (5 6 7 8) (9 10 11 12))`. With this representation, we can use sequence operations to concisely express the basic matrix and vector operations.

We will look at the following 4 basic operations on matrices:

• `(dot-product v w)` returns the sum ∑i = vi wi
• `(matrix-*-vector m v)` returns the vector `t` , where ti = ∑j mij vj
• `(transpose m)` returns the matrix `n` , where nij = mji
• `(matrix-*-matrix m n)` returns the matrix `p` , where pij = ∑k mik nkj

Dot product of 2 vectors in this notation can be done by using 2 higher order functions, `map` and `fold`, both of which are implemented using recursion.

Implementation of `map`:

``````(define (map proc items)
(if (null? items)
nil
(cons (proc (car items))
(map proc (cdr items)))))
``````

Implementation of `fold-right`:

``````(define (fold-right op initial sequence)
(if (null? sequence)
initial
(op (car sequence)
(fold-right op initial (cdr sequence)))))
``````

We can use either `fold-left` or `fold-right` for `dot-product`.

``````;; Define dot product of 2 vectors of equal length
(define (dot-product v w)
(fold-right + 0 (map * v w))
)

;; testing our function
(define vec1 (list 1 2 3) )
(define vec2 (list 1 1 1) )

(dot-product vec1 vec2)
``````
``````6
``````

Calculating a dot product was really easy with a couple of higher order functions! Let’s work with matrices now. We will now right a function to multiply a matrix and a vector:

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

;; testing the function
(define mat1 (list (list 1 0 0) (list 0 1 0) (list 0 0 1)))
(matrix-*-vector mat1 vec1)
``````
``````(1 2 3)
``````

Let’s look at transpose now! For this, we will need to implement a helper function, `accumulate-n`, which is similar to `fold` except that it takes as its third argument a sequence of sequences, which are all assumed to have the same number of elements.

``````;; Defining helper functions for transpose to
;; apply the operation op to combine all the first elements of the sequences,
;; all the second elements of the sequences, and so on,
;; and returns a sequence of the results.
(define (accumulate-n op init seqs)
(if (null? (car seqs))
'()
(cons (fold-right op init (map car seqs))
(accumulate-n op init (map cdr seqs))
)
)
)

(define (transpose mat)
(accumulate-n cons '() mat)
)

;; testing transpose
(define mat2 (list (list 1 2 3) (list 4 5 6) (list 7 8 9)))
(transpose mat2)
``````
``````((1 4 7) (2 5 8) (3 6 9))
``````

Now, let’s use this transpose function to do matrix multiplication:

``````; Matrix multiplication
(define (matrix-*-matrix m n)
(let ((n-cols (transpose n)))
(map (lambda (m-row)(matrix-*-vector n-cols m-row))
m)
)
)

;; For testing
(matrix-*-matrix mat2 mat1)
``````
``````((1 2 3) (4 5 6) (7 8 9))
``````

We can even write recursive procedures to create some special kinds of matrices, for eg, an identity matrix. Here is a recursive procudure to create an identity matrix of length `n`:

``````;; Create an identity matrix of length n
; N -> [List-of [List-of N]]
(define (identityM n)
(letrec   ;; Documentation for letrec : https://groups.csail.mit.edu/mac/ftpdir/scheme-7.4/doc-html/scheme_3.html
(
;; N -> [List-of [List-of N]]
(generate-matrix (lambda (row)
(cond
((= row 0) '())
(else (cons (generate-row row n)
(generate-matrix (- row 1)))))))

;; N N -> [List-of N]
(generate-row (lambda (row col)
;; col goes from column n to 0
(cond
((= col 0) '())
(else (cons (if (= row col) 1 0)
(generate-row row (- col 1)))))))
)
(generate-matrix n)
)
)

(identityM 3)
``````
``````((1 0 0) (0 1 0) (0 0 1))
``````

If you found these functions interesting, I’de definitely encourage to go read SICP. I wrote about why I’m reading SICP here.

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