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Why does Swift have dictionaries as well as arrays? - a free tutorial

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Why does Swift have dictionaries as well as arrays?

Paul Hudson    @twostraws   

Dictionaries and arrays are both ways of storing lots of data in one variable, but they store them in different ways: dictionaries let us choose a “key” that identifies the item we want to add, whereas arrays just add each item sequentially.

So, rather than trying to remember that array index 7 means a user’s country, we could just write user["country"] – it’s much more convenient.

Like sets, dictionaries don’t store our items with a specific order, so they optimize the way they store items for fast retrieval. So, when we say user["country"] it will send back the item at that key (or nil) instantly, even if we have a dictionary with 100,000 items inside.

Unlike tuples, you can’t be guaranteed that a key in a dictionary exists. This is why reading a value from a dictionary might send back nothing – you might have requested a key that doesn’t exist!

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