Why am I getting NaN here when I'm just accessing an array?

Question

  var inc = .001;
  var z = new Array(1.0/inc);
  for (var x = 0.0; x < 1.0; x += inc) {
    z.push(Math.cos(x));
  }
  var y = new Array(1.0/inc);
  for (x = 0.0; x < 1.0; x += inc) {
    y.push(1 - ((x * x) / 2) + ((x * x * x * x) / 24));
  }
  var sum = 0;
  for (var i = 0; i < (1.0/inc); i++) {
    sum += y[i] - z[i];
  }
  console.log(sum);
  console.log(sum/(1.0/inc));

I'm pretty new to Javascript, but the arrays here are filled with floats and when I take the difference and try to print them it returns NaN. I'm stumped here. Here's a fiddle with the code (http://jsfiddle.net/2v7wu/). Thanks!

explain what you expect var z = new Array(1.0/inc); to generate....it's creating aarray with 1000 eleemtns that are all undefined. Tru console.log(z) right after you declare it. doubt it's what you wanted to do — charlietfl
– charlietfl, Commented Nov 23, 2013 at 15:24

Gareth · Accepted Answer · 2013-11-23 15:22:37Z

3

You're creating arrays that consist of 1000 empty values, and then pushing extra elements onto those. Your arrays end up 2000 elements long, of which you iterate over the first (empty) 1000.

You don't need to declare the length of arrays in Javascript, so just using

var z = []
var y = []

will be fine.

Finally, you need to change your array index in the last loop to

sum += y[i] - z[i];

answered Nov 23, 2013 at 15:22

Gareth

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6 Comments

dbarnes Over a year ago

I think there is also an issue here as to how javascript does floating point numbers as well. I'm betting your sums won't be what you want

Gareth Over a year ago

Maybe, but they won't be NaN at least

T.J. Crowder Over a year ago

@Gareth: I don't know what I was thinking, of course a.push(value) is basically a[a.length] = value. But it's important to understand that it's not that the other entries are "empty", it's that they aren't there at all (because these aren't really arrays). var a = new Array(3); a.push("x"); gives you an array with one entry and a length of 4.

Gareth Over a year ago

@T.J.Crowder absolutely, and if OP were iterating using forEach then there wouldn't be a problem (other than having a messy array in the first place)

user3025173 Over a year ago

I was actually looking for a way to iterate over two arrays with forEach in Javascript. Is there a way? Also, @dbarnes, what would the floating point error be?

|

Community · Accepted Answer · 2017-05-23 12:19:13Z

2

Because you're using x where you mean to be using i:

for (var i = 0; i < (1.0/inc); i++) {
  sum += y[x] - z[x];
  //       ^------^--------- these should be i, not x
}

See also Gareth's point below, which relates to my side note #2 below. By starting out with an initial length and then using push, you're not putting things where you think you are. :-)

Two side notes:

You're constantly re-evaluating 1.0/inc. I recommend doing that once and storing it in a variable.
In JavaScript, there's rarely any reason to write new Array(length). Just use var z = []; and var y = []; These arrays aren't really arrays at all and they'll "grow" as needed (but again, without lots of memory reallocations, because they're not really arrays).

edited May 23, 2017 at 12:19

CommunityBot

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answered Nov 23, 2013 at 15:22

T.J. Crowder

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Comments

Jose Torres · Accepted Answer · 2013-11-23 15:23:53Z

0

In your third loop, you're trying to use the floating point value x as an index into y and z. This produces the result undefined, and undefined - undefined evaluates to NaN.

Presumably, you meant to use i as the index.

answered Nov 23, 2013 at 15:23

Jose Torres

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