looping through object literal and array

Question

I believe I am having an issue with my looping of object literals and arrays. I am trying to use an object literal for the 'map' where I had used an array before. The reason I want to use an object literal is because I want a variable to display the name of the position it is in. As of right now I always get an issue with loc[2][1] = player;.

var room0 = [
        [blank, blank, blank, blank],
        [blank, blank, blank, blank],
        [blank, player, blank, doorRight],
        [blank, blank, blank, blank]
];
var room1 = [
        [blank, blank, blank, blank],
        [blank, blank, blank, blank],
        [blank, blank, blank, blank],
        [blank, blank, blank, blank]
];

var loc = room0;

var map = {
    "living room": room0,
    "Bedroom": room1,
    "Outside": room2
};

function moveRight() {
    for (var y = 0; y < loc.length; y++) {
        var posX = loc[y].indexOf(player);
        if (posX <= -1) continue;

        // if going to the next room to the right
        if (loc[y][posX+1] == doorRight) {

            for (var name in map) {
                if (loc == map[name]) {
                    var rightLoc = map[name+1];
                }
            }


            loc[y][posX] = blank;
            loc = rightLoc;
            loc[2][1] = player;
            break;
        }

        if (loc[y][posX+1]  != blank) break;
        if (posX < loc[y].length - 1) {
            loc[y][posX] = blank;
            loc[y][posX + 1] = player; 
        }
        break;
    }
}

This has nothing to do with the actual question, but: this is already the kind of complexity where I'd suggest using classes. It would make naming the rooms easier, too. — Ingo Bürk
– Ingo Bürk, Commented May 2, 2013 at 5:55
@IngoBürk: What would that help here? Then all those variables would be object properties, and… ? — Bergi
– Bergi, Commented May 2, 2013 at 6:00
Perhaps I did a poor job at explaining what is going on.... I'll try editing it to be more clear. — Alpha Centauri A B
– Alpha Centauri A B, Commented May 2, 2013 at 6:22
I didn't think it was necessary to show you where the player is defined or room2. Sorry, I just didn't think you would want to look through a lot of code.... The definition of the player/room2/blank obviously isn't the issue here. — Alpha Centauri A B
– Alpha Centauri A B, Commented May 2, 2013 at 6:41

Ja͢ck · Accepted Answer · 2013-05-02 07:35:31Z

3

You just need to hold different things in your map, because map[name+1] doesn't magically point to the next room identified by name.

var map = {
    "living room": 0,
    "Bedroom": 1,
    "Outside": 2
};

var rooms = [room0, room1, room2];

Now, map['living room'] will hold the index in rooms, so you can still use numeric addition to "move" into the next room. Your code will then become:

var rightLoc;

for (var name in map) {
    if (loc == map[name]) {
        rightLoc = map[name] + 1;
    }
}

loc = rooms[rightLoc];

This code still lacks some checks to make sure you always have a room to go into, but I'm sure you can figure that out yourself :)

answered May 2, 2013 at 7:35

Ja͢ck

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Comments

jahroy · Accepted Answer · 2013-05-02 07:35:45Z

It looks like rightLoc will be undefined when that line of code is reached.

The variable rightLoc gets set to the value of map[name + 1]

Because the properties of map are strings, name + 1 will be a string that ends with 1.

Therefore, map[name + 1] is undefined and cannot be de-referenced.

The key point is that your for loop is iterating over the properties of your object.

For each iteration of the loop, name will be a string.

Here's a fiddle that demonstrates the problem:

var map = {
    "living room": 'room0',
        "Bedroom": 'room1',
        "Outside": 'room2'
};

for (var name in map) {
    if (name === 'Bedroom') {
        var badKey = name + 1;
        var theType = typeof map[badKey];
        alert('key: ' + badKey + ' type: ' + theType);
    }
}

output:

key: Bedroom1 type: undefined

If you read the MDN documentation, you'll see that the for in loop iterates over object properties in arbitrary order, which means this technique will not work.

Two important tools that can help you find errors like this quicker:

read the documentation (google "javascript mdn for in loop")
learn to use a debugger (inspecting variables is SUPER easy with Firebug)

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