February 16, 2015

Lookup - Javascript

Javascript Objects - Part 2

Continued from Part 1 - Objects and JSON

Following from the previous example, let’s say we are only interested in the actions a user can take

var actions = [
    {text: "Move", x: 0, y: 0},
    {text: "Attack", x: 50, y: 0},
    {text: "Skill", x: 0, y: 20},
    {text: "Item", x: 50, y: 20}
];

What if we wanted to find the property x of the Move action? We could try defining actions as an object instead of an array like so

var actions = {
    move: {text: "Move", x: 0, y: 0},
    attack: {text: "Attack", x: 50, y: 0},
    skill: {text: "Skill", x: 0, y: 20},
    item: {text: "Item", x: 50, y: 20}
};

and you would then be able to use the actions object as a lookup to find x using

var x = actions.move;

Although this works, there is a duplication of text which may lead to bugs if the code is modified later on. Another limitation is that you would be restricted to only looking up based on 1 variable.

A more flexible and extendable approach would be to define a second object to use as a lookup. Using the actions array from the first code example,

var lookup = {};
for(var i=0; i<actions.length; i++) {
    lookup[actions[i].text] = actions[i].x;
}

// lookup = {
//     Move: 0,
//     Attack: 50,
//     Skill: 0,
//     Item: 50
// }

You can then use the lookup object to find x using

var x = lookup.Move;
//or
var x = lookup["Move"];

The lookup object can also be modified to look up based on both the x and y properties to find the text property

var lookup = {};
for(var i=0; i<actions.length; i++) {
    // if the property does not exist yet
    if(!lookup.hasOwnProperty(actions[i].x)) {
        lookup[actions[i].x] = {};
    }
    lookup[actions[i].x][actions[i].y] = actions[i].text;
}

// lookup = {
//     0: {
//         0: "Move",
//         20: "Skill"
//     },
//     50: {
//         0: "Attack",
//         20: "Item"
//     }
// }

You can then use the lookup object like so

var text = lookup[x][y];

This method of creating another object for looking up can also be applied to simple encoding/decoding schemes such as

var encoder = {
    e: "a",
    h: "c",
    l: "r",
    o: "y"
}

var decoder = {};
for(var key in encoder) {
    decoder[encoder[key]] = key;
}

var plainText = ["h", "e", "l", "l", "o"];

var encryptedString = [];
for(var i=0; i<plainText.length; i++) {
    encryptedString[i] = encoder[plainText[i]];
}
// encryptedString = ["c", "a", "r", "r", "y"];

var decryptedString = [];
for(var i=0; i<encryptedString.length; i++) {
    decryptedString[i] = decoder[encryptedString[i]];
}
// decryptedString = ["h", "e", "l", "l", "o"];

- ksami

Tags: programming, Javascript