How to subtract these two arrays

Question

I am trying to subtract two matrices, I have a function that takes in those matrices and returns a new matrix that has been subtracted. I get this error in node.js: TypeError: (intermediate value).map is not a function

subtract(a, b) {

  return new Matrix(a.rows, a.cols).map((_, i, j) => a.data[i][j] - b.data[i][j]);
}

This is the function I use from my main file (note: I already have an instance of the class).

let m = new Matrix(2, 2);
m.data[0] = [10, 11];
m.data[1] = [12, 13];

let n = new Matrix(2, 2);
n.data[0] = [1, 2];
n.data[1] = [3, 4];

mat.subtract(m, n);

This is the class that I have created:

class Matrix {
    constructor(rows, cols) {
      this.rows = rows;
      this.cols = cols;
      this.index = 0;
      this.rowCount = 0;
  
      //this.matrixData = Array(this.rows).fill().map(() => Array(this.cols).fill(0));
      this.data = Array(this.rows).fill().map(() => Array(this.cols).fill(0));
    }
}

Answer 1

I'm going to recommend a complete code rewrite that focuses on a plain functions instead of classes and methods. We begin writing our matrix module below and will add an OOP-style interface in the second section of this post. -

// matrix.js

const matrix = rows =>
  ({ matrix, rows })

const empty = _ =>
  matrix([])

const subtract = (t1, t2) =>
  matrix(t1.rows.map((_, i) => subtractRow(t1.rows[i], t2.rows[i])))

const subtractRow = (r1, r2) =>
  r1.map((v, i) => v - r2[i])

function toString (t)
{ const w =
    Math.max(...t.rows.flat().map(_ => String(_).length))
  const s = 
    t.rows.map(r => r.map(_ => String(_).padStart(w, " ")).join(" "))
  return `[ ${s.join("\n  ")} ]`
}

export { empty, matrix, subtract, toString } 

Next we write our main module that uses the matrix module -

// main.js

import { matrix, subtract, toString } from "./matrix.js"

const m1 = matrix([[10, 11], [12, 13]])
const m2 = matrix([[1,2], [3,4]])

console.log(toString(m1))
console.log(toString(m2))
console.log(toString(subtract(m1, m2)))

[ 10 11
  12 13 ]

[ 1 2
  3 4 ]

[ 9 9
  9 9 ]

If you are more comfortable with an OOP-style interface, you we can add that to our matrix module easily. Notice how our Matrix class it is a simple wrapper around our existing plain functions -

// matrix.js (continued)

class Matrix
{ constructor(t = empty())
  { this.t = t }

  subtract(other)
  { return new Matrix(subtract(this.t, other.t)) }

  toString()
  { return toString(this.t) }

  static of(rows)
  { return new Matrix(matrix(rows)) }
}

export default Matrix

And here's our main module using our new Matrix interface -

// main.js

import Matrix from "./matrix.js"

const m1 = Matrix.of([[10, 11], [12, 13]])
const m2 = Matrix.of([[1,2], [3,4]])

console.log(m1.toString())
console.log(m2.toString())
console.log(m1.subtract(m2).toString())

[ 10 11
  12 13 ]

[ 1 2
  3 4 ]

[ 9 9
  9 9 ]

Chain for days, if you wish -

console.log(m1.subtract(m2).subtract(m2).toString())
console.log(m2.subtract(m1).subtract(m1).subtract(m1).toString())

[ 8 7
  6 5 ]

[ -29 -31
  -33 -35 ]

As you can see we only write our matrix module once and it is flexible enough to use in functional-style and OOP-style. To see this module technique used in another setting, see this answer where we build a linked list.

---

Wondering why we went through so much trouble in matrix.toString? It's so that the matrix can be nicely formatted even when element sizes differ -

import { matrix, toString } from "./matrix.js"

const m3 = matrix([
  [1,2,3],
  [11,22,33]
  [111,222,333]
])

console.log(toString(m3))

[   1   2   3
   11  22  33
  111 222 333 ]