CODEWITHSUNDEEP
matrixclojure
nenad
nenad

Reputation: 409

Clojure generating matrices

I am going throw the Clojure for Machine Learning book and I dont understand one part of one function.

 (defn square-mat
  "Creates a square matrix of size n x n whose
   elements are all e. Accepts an option argument
   for the matrix implementation"
   [n e & {:keys [implementation]
    :or {implementation :persistent-vector}}]
   (let [repeater #(repeat n %)]
   (matrix implementation (-> e repeater repeater))))

(defn id-mat
 "Creates an identity matrix of n x n size"
 [n]
 (let [init (square-mat :clatrix n 0)
  identity-f (fn [i j n]
               (if (= i j) 1 n))]
 (cl/map-indexed identity-f init)))

In this second function, I don't understand this part (if ( = i j) than 1 else n). Why else n? n is size

Upvotes: 4

Views: 396

Answers (1)

Aleph Aleph
Aleph Aleph

Reputation: 5395

Look at the definition of clatrix/map-indexed. It takes a function of three arguments (the row index, the column index, and the element in that position), and a matrix.

In the inner function (fn [i j n] (if (= i j) 1 n), n is bound not to the number of the rows in the matrix, but to the value in the position (i, j) - which happens to be always zero because the init matrix was initialized with zeros.

Looks like the book is using a very confusing notation (since n is bound to the number of rows in the matrix in the outer function, it would be better to name the third argument of the inner function x or somehow else), but the function should still work.

Upvotes: 2

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