map Method

Keys

The Scala map method transforms one collection to another collection using the algorithm you supply
It’s like a basic for/yield expression (with one generator and no definitions or guards)
You supply an algorithm that works on an individual element that’s in the collection. so if it contains Ints, your algorithm works on one Int at a time.
It may help to think of “map” as “transform”

Examples:

val xs = List(1, 2, 3)
val ys = xs.map(x => x * 2)
val ys = xs.map(_ * 2)

val ys = xs.map { x =>
    // can be multiple lines
    x * 2
}

map + your algorithm works like this:

    x    y
    ------
    1 => 2
    2 => 4
    3 => 6

Another:

val xs = List("big", "belly", "burger")
val ys = xs.map(x => x.length)
val ys = xs.map(_.length)

map + your algorithm works like this:

    x           y
    -------------
    "big"    => 3
    "belly"  => 5
    "burger" => 6

Ways to think about map

When you type this x, say to yourself, “for EACH x in xs, apply this algorithm to yield a new result”:

val ys = xs.map(x => x * 2)
                ----

map is the same as a basic for expression:

// [1]
val ys = xs.map(x => x * 2)

// [2]
val ys = for x <- xs yield x *2

// [3]
val ys = for
    x <- xs
yield
    x *2

Passing functions into map

def double(i: Int): Int = i * 2
val ys = xs.map(x => double(x))

// shortcuts
val ys = xs.map(double(_))
val ys = xs.map(double)

The function you pass in must match the data type that’s in the collection:

// collection contains Ints, so your algorithm needs to
// accept an Int (needs to be a function that takes an Int):
val nums = List(1, 2, 3)
val doubles = nums.map(_ * 2)
def double(i: Int): Int = i * 2

// collection contains Strings, so your algorithm needs to
// accept an String:
val names = List("Bert", "Ernie")
val nameLengths = names.map(name => name.length)
val nameLengths = names.map(_.length)
def strlen(s: String): Int = s.length

Your algorithm can also return whatever it needs to return:

val nameLengths = names.map(name => (name, name.length))

map method with the Map class

val states = Map(
    "ak" -> "alaska",
    "al" -> "alabama"
)

val newMap = states.map((k,v) => (k.toUpperCase, v.toUpperCase))

val a = Map(
    1 -> 'a',
    2 -> 'b'
)

// return whatever value/type you need:
val b = a.map( (k,v) => (k, v.toUpper) )
val c = a.map( (k,v) => (k.toDouble, v.toUpper) )

// a multiline example
val d = a.map { (k, v) =>
    // can be multiple lines
    (k, v.capitalize)
}

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