why is this javascript array undefined?

Question

I have the following code, which attempts to generate a 2 dimensional array of random numbers:

var block_size = 32;
var can_width = can.width;
var color_depth = 12;
var passes = can_width / block_size;
var map_store = new Array(passes);

for(i=0;i<passes;i++) {
  for(j=0;j<passes;j++) {
    map_store[i] = new Array(passes);
    color = Math.random() * color_depth;
    map_store[i][j] = Math.round(color);
  }
}

which seems to work fine if i put console.log statements within the loop, however if I try to access the map_store array outside of the loops. all of it's elements are undefined. why is this?

looks like the last element in the array should have a value — DHall
– DHall, Commented Aug 23, 2011 at 18:58

Halcyon · Accepted Answer · 2011-08-23 18:59:01Z

9

map_store[i] = new Array(passes); should be above the 2nd for loop. You're clearing your previous j values.

for(i=0;i<passes;i++) {
  map_store[i] = new Array(passes); // <--
  for(j=0;j<passes;j++) {
    color = Math.random() * color_depth;
    map_store[i][j] = Math.round(color);
  }
}

answered Aug 23, 2011 at 18:59

Halcyon

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4 Comments

gen_Eric Over a year ago

You could also just do map_store[i] = [];

Halcyon Over a year ago

There is no real benefit in this case as you know the length of the Array before hand. Why is [] generally better than new Array? (I know it is, but I don't know why)

user166390 Over a year ago

@Frits van Campen Style really -- since the language has a built-in feature, might as well take advantage of it (even though new Array(len) and [] are slightly different when len is not 0). It should also be noted that Array constructor is somewhat special in how it treats arguments. That is, [x] and new Array(x) are different while [x,y] and new Array(x,y) are the same. Awesome design :)

JSPP Over a year ago

What Frits van Campen has is fine if you care for speed. Declaring new Array(500) and then overwriting each element is generally faster than var foo = []; and 500 pushes barring VM-specific optimizations. See my post on stackoverflow.com/questions/7375120/… for a detailed explanation from a performance perspective. You'll find in high-performance JS libs, new Array(x) is still used. Of course, we don't want pre-mature optimization either :); however, the point is that [] - like all best practices - is not a "law."

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