Javascript 2D array function

Question

I have a 2D array, which contains either true or false. I made a function that returns the number of neighbors (in all 8 directions) in that array, which are true. But for some unknown reason, it does not work corectly (returns wrong number of neighbors). (Yes, I'm making Conway's Game of Life.)

function neighbors(seq, x, y) {
   var cnt = 0;
   try {
    if (seq[y-1][x]){
        cnt++;
    }
   }
   catch(err) {

   }
   try {
    if (seq[y][x-1]){
        cnt++;
    }
   }
   catch(err) {

   }
   try {
    if (seq[y][x+1]){
        cnt++;
    }
   }
   catch(err) {

   }
   try {
    if (seq[y+1][x]){
        cnt++;
    }
   }
   catch(err) {

   }
   try {
    if (seq[y-1][x+1]){
        cnt++;
    }
   }
   catch(err) {

   }
   try {
    if (seq[y-1][x-1]){
        cnt++;
    }
   }
   catch(err) {

   }
   try {
    if (seq[y+1][x-1]){
        cnt++;
    }
   }
   catch(err) {

   }
   try {
    if (seq[y+1][x+1]){
        cnt++;
    }
   }
   catch(err) {

   }        
   return cnt;
}

This code was basically translated from my Python code, that works.

Replace try...catch with normal execution flow seq[y-1] && seq[y-1][x+1]. Js doesn't throw when array index is out of bound. — Yury Tarabanko
– Yury Tarabanko, Commented Jun 4, 2018 at 11:03

Nina Scholz · Accepted Answer · 2018-06-04 11:09:54Z

4

You could take an object with all needed directions for counting.

Then check if the index is a key of the array and check the nested level. Add all and return that value.

function neighbors(seq, i, j) {
    var directions = [{ x: 0, y: -1 }, { x: -1, y: -1 }, { x: -1, y: 0 }, { x: -1, y: 1 }, { x: 0, y: 1 }, { x: 1, y: 1 }, { x: 1, y: 0 }, { x: 1, y: -1 }];

    return directions.reduce((r, { x, y }) => {
            x += i;
            y += j;
            return r + (x in seq && seq[x][y] || 0);
        }, 0);
}

answered Jun 4, 2018 at 11:09

Nina Scholz

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Comments

ymutlu · Accepted Answer · 2018-06-04 11:23:18Z

You can simplify function like below.

var seq = [[false,false,false,false],
           [false,true,false,false],
           [false,true,false,false],
           [false,true,false,false]]
function neighbors(seq, x, y) {
   var cnt = 0;
   var controls = [{x:0,y:1},{x:0,y:-1},
                    {x:1,y:0},{x:1,y:1},{x:1,y:-1},
                    {x:-1,y:0},{x:-1,y:1},{x:-1,y:-1}]
   for (var index in controls) {
      var newX = x+controls[index].x;
      var newY = y+controls[index].y;
      if(newX >= 0 && newY >= 0 && newX < seq.length && newY < seq[newX].length
        && seq[newX][newY]) {
        cnt++;
      }
   }       
   return cnt;
}

console.log(neighbors(seq, 0,1))
console.log(neighbors(seq, 1,0))
console.log(neighbors(seq, 2,2))

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