dictionaries.md

Dictionaries

JavaScript doesn't actually have a data type called dictionary. JavaScript has a data type called object, but one aspect of JavaScript objects is that they work like dictionaries. In this module we're going to use the dictionary aspect of objects, and later we'll cover other aspects of objects.

WTF is a dictionary

A dictionary is a mapping of keys to values. For example you could think of a French to English dictionary as mapping French words to English. For example: chauve-souris => bat. chauve-souris is the key, bat is the value. We could also define an English to French dictionary and use bat as the key and chauve-souris as the value, chauve-souris => bat.

The keys of a dictionary are strings. The values can be any sort of data, including a nested dictionary, or even a function.

The keys act like indexes, like in an array, and we use them the same way. If we had a dictionary called frenchToEnglishDic we'd access its values like so:

frenchToEnglishDic['chauve-souris'] // 'bat'

frenchToEnglishDic['cerf-volant'] // 'kite'

frenchToEnglishDic['porc-épic'] // 'porcupine'

We can declare our dictionary using { } curly brackets, like so:

let frenchToEnglishDic = {
    'chauve-souris': 'bat',
    'cerf-volant': 'kite',
    'porc-épic': 'porcupine'
}

You separate the key and value with a : colon, and multiple key-value pairs can be separated by , commas, like arrays.

As with arrays, the values in a dictionary are mutable. You can change the value of a key or add new keys to the dictionary.

frenchToEnglishDic['vachement'] // undefined

frenchToEnglishDic['vachement'] = 'really'

frenchToEnglishDic['vachement'] // 'really'

frenchToEnglishDic['chauve-souris'] // 'bat'

frenchToEnglishDic['chauve-souris'] = 'bald-mouse'

frenchToEnglishDic['chauve-souris'] // 'bald-mouse'

Examples

List of coordinates

Let's create a list of latitude and longitude coordinates.

let cities = {
    'Montreal': {
        'latitude': 45.3,
        'longitude': 73.34
    },
    'San Francisco': {
        'latitude': 37.47,
        'longitude': 122.25
    },
    'Krakow': {
        'latitude': 50.04,
        'longitude': 19.56
    },
    'Shenzhen': {
        'latitude': 22.33,
        'longitude': 114.06
    }
}

cities['Montreal'] // {'latitude': 45.3, 'longitude': 73.34}

cities['Montreal']['latitude'] // 45.3

cities['Montreal']['longitude'] // 73.34

Social Graph

Let's create a social graph where we can see relationships between people.

let relationships = {
    'Daria': [
        'Jessica',
        'Roberto'
    ],
    'Jessica': [
        'Daria'
    ],
    'Roberto': [
        'Daria'
    ]
}

relationships['Daria'] // ['Jessica', 'Roberto']

relationships['Jessica'][0] // 'Daria'

relationships[relationships['Roberto'][0]][0] // 'Jessica'

Character count

Let's count the number of occurrences of each character in a string.

let string = 'abracadabra'

let occurrences = {
    'a': 5,
    'b': 2,
    'r': 2,
    'c': 1,
    'd': 1
}

let firstCharacter = string[0]

occurrences[firstCharacter] // 5

Exercises

Write code to access the value 'Waldo' from each of the following data structures.

let data = {
    'a': [
        {
            'b': 'Waldo'
        }
    ]
}

Example, you can solve the above with: data['a'][0]['b']. Now try for the below data structures.

data = {
    'a': {
        'b': {
            'c': {
                'd': 'Waldo'
            }
        }
    }
}

data = [
    {
        'a': [
            {
                'b': 'Waldo'
            }
        ]
    }
]

data = {
    'a': [[['Waldo']]]
}

Practice

Write a data structure like the social graph example that describes some of your interpersonal relationships and the fact that certain of your friends know one another.