Sunday, December 14, 2014

Some useful Scala I learned this week

1. Indexes of a collection

If you want to  traverse a list of elements keeping the index in mind, you could do this with a fold but a more elegant way is something like:

    val words = List("the", "quick", "brown", "fox", "jumped", "over", "the", "lazy", "dog")

    words.zipWithIndex // returns tuples like (word, index)

2. Choosing only some elements of a collection

Now, let's say we want the tuples of those words that begin with 't'. We do this with the collect function in TraversableLike.

    words.zipWithIndex collect {
        case (word, index) if word.startsWith("t") => (word, index)

(You will note that we pass collect a partial function). This will return the list:

List((the,0), (the,6))

That is, the 0th and 6th words begin with 't', and they are both 'the'.

3. Counting the elements that meet a criteria

is very simple:

    words count (_ startsWith("t"))

Will, unsurprisingly, return 2 for the above collection.

What I learned this week is that you could do all these with folds etc but Scala (I am told) has methods that do pretty much anything you could want. It's just a matter of learning them.

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