I am recording acceleration data for the x-axis, so I have two values coming in - the x value, and the epoch time of the acceleration event. I want to store these as a hash, where the epoch time is the key and x-value is the value.
I created an array:
var arrX = new Array();
How can I add to it? Like this?
arrX[acceleration.timestamp] = acceleration.x
You should use an object, which can serve as a sort of "associative array" for this application. An object will provide support for the arbitrary, non-sequential keys that you mentioned, where an array would not.
var arrX = {};
arrX[acceleration.timestamp] = acceleration.x;
More information:
Array.prototype methods will only work with numeric indices. Also an Array doesn't require that the indices are sequential.
You can do it but without the array. Just use an object like this
var values = {};
values[acceleration.timestamp] = acceleration.x;
because if you do something like this
var x = [];
x[1555] = 500;
you will create an array of 1556 elemenets, with all elements but the 1555 set to undefined
x[1555] example sets x.length to 1556 because .length is always 1 higher than the highest assigned index, but it doesn't actually create any of the other elements. It's a subtle difference, but using the in operator or .hasOwnProperty() you can tell whether a particular index has been explicitly assigned undefined or just never assigned a value. The Array.forEach() method also skips over array indexes never assigned a value...In Javascript if you need an associative array, you use an object:
var hash1 = {}; // declare
hash1.first = 'abc'; // set property first to a string
hash1['second'] = 'def'; // set property second to a string
var t = 'third';
hash1[t] = 'ghi'; // set property third to a string
hash1.forth = {a:1, b: 'abc'}; // set property forth to be an object / hash
// with 2 properties
alert (hash1.forth.a); // will alert with 1