Associative Javascript Array

Question

I have an issue and this is not something that I have ever used before.

This is a snippet of the code.

//Defined at top of file
var ships_array = [];

//Function later on

    function refreshJSON(){
    $.getJSON("scripts/getJSON.php", function(json){

      if(json){
        $.each(shiplist,function(i,item){
            $.each(item,function(j,subitem){
                var mmsi = subitem['mmsi'];
                ships_array[mmsi].lat = subitem['lat'];
                ships_array[mmsi].lon = subitem['lon']; 

            });
        });
     } 

    });

    }

The error I am getting is

ships_array[mmsi] is undefined [Break On This Error] ships_array[mmsi].lat = subitem['lat'];

Is this the right way to use an associative array? I have seen an example similar to this but I am going wrong somewhere! At the end of it I want an array of Google Maps markers!

jordancpaul · Accepted Answer · 2011-03-14 07:40:54Z

3

you just need to initialize the value at the hash key first

var mmsi = subitem['mmsi'];
ships_array[mmsi] = ships_array[mmsi] || {};
ships_array[mmsi].lat = subitem['lat'];
ships_array[mmsi].lon = subitem['lon'];

answered Mar 14, 2011 at 7:40

jordancpaul

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8 Comments

Lee Armstrong Over a year ago

Works great,I don't understand the second line, can you point me to a reference or explain please? Will accept when it lets me as it works!

Chris W. Over a year ago

The || symbol is or, when used in assignments it means if the first item evaluates to false assign the next one. So for x = false || 'foo' || 'bar'; x gets the value 'foo'.

vivekian2 Over a year ago

The second line takes a logical OR between the array element - ships_array[mssi] and an empty object literal {}. The former to make sure that an existing object is not overwritten, the latter to ensure that an empty object exists.

jordancpaul Over a year ago

Sure. The problem is that if ships_array @ mmsi is undefined, you can't possibly set .lat or .lon on it - that would be the same as trying to do (undefined).lat = 'test'. The second line is a common javascript pattern for assigning default values. ships_array[mmsi] || {} will return either ships_array[mmsi] if it is defined, or {} if it is not. This is then used to set ships_array[mmsi]. An equivalent alternative second line would be if(!ships_array[mmsi]){ships_array[mmsi]={};}. Does that make it more clear?

Lee Armstrong Over a year ago

Yes spot on, just getting to grips with JavaScript!

|

Khoa Nguyen · Accepted Answer · 2011-03-14 07:57:41Z

1

You can change your code like this:

//Defined at top of file
var ships_array = [];

//Function later on

    function refreshJSON(){
    $.getJSON("scripts/getJSON.php", function(json){

      if(json){
        $.each(shiplist,function(i,item){
            $.each(item,function(j,subitem){
                var mmsi = subitem['mmsi'];
                ships_array[mmsi] = {lat: subitem['lat'], lon:subitem['lon']};
            });
        });
     } 

    });

    }

answered Mar 14, 2011 at 7:57

Khoa Nguyen

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