How do I create a sparse array in javascript through iteration?

Question

I wrote this code to scrape a sparse array from a series of dom elements. when done in one dimension the code works but in 2 dimensions it fails. Is there something i'm missing?

23         function initCellHover(){
24                 $cells.each(function(){
25                         var arrayX = $(this).position().left/cellWidth;
26                         var arrayY = $(this).position().top/cellHeight;
27                         var arrayValue = $(this);
28                         cellLookup[arrayX][arrayY] = arrayValue;
29                 });     
30         }

Your code, here, looks fine. Did you check whether and cellHeight, cellWidth are correctly defined? How, exactly, does it fail? — Pierre
– Pierre, Commented May 27, 2012 at 8:33
Uncaught TypeError: Cannot set property '0' of undefined is the error I get. I get the same error if I try a = [[]]; a[1][22] = "test"; however b = [];b[22] = "test"; yields [,,,,,,,,,,,,,,,,,,,,,,"test"]. the other two properties all work fine. I think it's an issue with how javascript handles nested arrays. If that's the case I'm more interested in a workaround than a solution to the problem. — Adam Siegel
– Adam Siegel, Commented May 27, 2012 at 8:52

Li0liQ · Accepted Answer · 2012-05-27 08:33:29Z

5

In line 28 you may be referring to a property of undefined. It makes sense to check, if there already is a property in the array and add it, if needed:

cellLookup[arrayX] = cellLookup[arrayX] || [];
cellLookup[arrayX][arrayY] = arrayValue;

answered May 27, 2012 at 8:33

Li0liQ

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Adam Siegel Over a year ago

thanks, that set me on the right track. I ended up using if(!cellLookup[arrayX]){ cellLookup[arrayX] = []}; if(!cellLookup[arrayX][arrayY]){ cellLookup[arrayX][arrayY] = []};cellLookup[arrayX][arrayY] = arrayValue; to fix it.

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