adding together two 2d arrays in javascript

Question

I am having trouble this problem regarding 2d arrays. We must create a javascript function addArrays(al, a2) which takes in two 2d arrays of numbers, a1 and a2 and returns their "sum" in the sense of the matrix sum. If If a is array with the sum, then, for all rows i and all columns j, a[i][j] = a1[i][j] + a2[i][j].

I am completely stumped I am pretty sure my logic to find the sum is right but I don't know if anything is actually stored in the sum array or what this error means. Any help would be greatly appreciated.

This is the code I have so far:

var sum = new Array(a1.length);
for (var k = 0; k<sum.length; k++) {
    sum[k] = new Array(sum.length);
}

for (var l = 0; l<sum.length; l++) {
    for (var m = 0; m<sum.length[l]; m++) {
        sum[l][m].push(a1[l][m] + a2[l][m]);
    }
}

return sum;

We are given a test file:

 function start() {  
  var ar1 = [[1,2], [3,4]],
  ar11 = [[1,2], [3,4], [5,6]],
  ar12 = [[1,2,3], [3,4]],

  ar2 = [[6,7], [8,9]],
  ar21 = [[6,7], [8,9], [19,11]],
  ar22 = [[6,7], [8,9,10]],

  ar;

  try {
     alert( addArrays(ar1, ar2).toSource() );
  }
  catch (e) {
      alert( e );
  }}

When I run the program I keep getting the error: TypeError: addArrays(...).toSource is not a function

toSource is non-standard and doesn't work in your browser. Try console.log(addArrays(ar1, ar2)). — georg
– georg, Commented Jun 3, 2016 at 14:55
@georg it needs to be within alert. How do I do this with console.log? — Krae
– Krae, Commented Jun 3, 2016 at 15:02
As @georg points out, toSource() is a non-standard function that is not equally supported by browsers. MDN reference. According to the spec - Firefox appears to be the only browser with confirmed support. — trenthaynes
– trenthaynes, Commented Jun 3, 2016 at 15:18
I would also ask why your instructor is using non-standard JavaScript. — trenthaynes
– trenthaynes, Commented Jun 3, 2016 at 15:20

user2033671 · Accepted Answer · 2016-06-03 15:20:04Z

2

error: TypeError: addArrays(...).toSource is not a function

Means it cannot find the function toSource on what is beign returned by addArrays. Object.toSource is not part of any standard and should not be used.

From MDN

Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

Looking at the output of toSource it looks similar to JSON. You can change the test to utilize JSON.stringify to achieve a similar output.

 function start() {  
  var ar1 = [[1,2], [3,4]],
  ar11 = [[1,2], [3,4], [5,6]],
  ar12 = [[1,2,3], [3,4]],

  ar2 = [[6,7], [8,9]],
  ar21 = [[6,7], [8,9], [19,11]],
  ar22 = [[6,7], [8,9,10]],

  ar;

  try {
     alert( JSON.stringify(addArrays(ar1, ar2)) );
  }
  catch (e) {
      alert( e );
  }}

answered Jun 3, 2016 at 15:20

user2033671

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4 Comments

user2033671 Over a year ago

@user2415266 I typed it before I saw the comments, great minds think alike!

Danmoreng Over a year ago

wasn't really meant as critique, you put quite some effort into the answer

user2033671 Over a year ago

@user2415266 I wasn't taking it as such :)

Krae Over a year ago

@Austin Thank you! this worked, but now when I run it the text in the alert box says, "[[null,null],[null,null]]". Is my method calculating and creating the sum matrix correct?

Redu · Accepted Answer · 2016-06-03 15:36:10Z

2

For same size arrays you can invent some Array method like Array.prototype.matriceSum() as follows;

Array.prototype.matriceSum = function(a) {
  return this.reduce((p,c,i) => (p[i] = c.reduce((f,s,j) => (f[j]+= s,f),p[i]),p),a.slice());
};
var a1 = [[1,2,3],[2,2,2],[4,5,6]],
    a2 = [[6,6,6],[9,8,7],[6,5,4]];
    a3 = a1.matriceSum(a2);
console.log(JSON.stringify(a3));

This won't mutate the array it's called upon and return a new 2D array with the matrix addition of the two 2D arrays.

answered Jun 3, 2016 at 15:36

Redu

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