CODEWITHSUNDEEP
javascriptarrayssyntax
Chris22
Chris22

Reputation: 2043

Using multiple sets of brackets in Javascript to create a type of array?

Just when I think I'm getting an understanding of the syntax of Javascript, I run into something like this...

I saw this code on the jqPlot plugin page.

what kind of array is [[]] in javascript? A nested array? I thought I understood the double brackets, but in the commented text, I don't understand why there is an extra set (question below).

$(document).ready(function(){
// Our data renderer function, returns an array of the form:
// [[[x1, sin(x1)], [x2, sin(x2)], ...]]
var sineRenderer = function() {
var data = [[]];
for (var i=0; i<13; i+=0.5) {
  data[0].push([i, Math.sin(i)]);
}
return data;
};

Why is there an extra set of brackets in this commented statement (the outermost set)?

[[[x1, sin(x1)], [x2, sin(x2)], ...]]

Thanks

Upvotes: 0

Views: 1548

Answers (4)

Tina Bell Vance
Tina Bell Vance

Reputation: 81

Hunter is correct -- it's the syntax for an array containing other arrays. You could also look at doing this as an object, as illustrated here: http://www.quirksmode.org/js/associative.html.

Upvotes: 1

Bergi
Bergi

Reputation: 664464

Yes, it's a nested array. The data array contains an array to which other arrays with value and sine of the value are pushed.

AFAIK, the jQPlot plugin would use every array in data to print a new graph. You could for example do

function graph(fn, from, to, gap) {
    // creates an array with points
    var res = [];
    for (var i=from; i<to; i+=gap) {
        res.push([i, fn(i)]);
    }
    return res;
}

var data = [
   graph(Math.sin, 0, 13, 0.5),
   graph(Math.cos, 0, 13, 1)
];
...

Upvotes: 1

Sarfraz
Sarfraz

Reputation: 382696

Since you are putting in arrays in your data array (which creates arrays inside an array), doing this would avoid that:

var data = [];
for (var i=0; i<13; i+=0.5) {
  data.push([i, Math.sin(i)]);
}

When you use [[]] and then do data[0], you are not utilizing outer array there which jumps in when you output it.

Upvotes: 1

Amadan
Amadan

Reputation: 198314

It would be a single set of brackets if they said data.push. Since the 0-th element of the array is an array, they use data[0].push to stuff stuff in it.

Why they need it double-wrapped, no idea without more context.

Upvotes: 1

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