Start learning free

LESSON 272 OF 678 · TUPLES, SORTING & BUILT-INS

Shout the Words in Swift: Transforming an Array with map

Practice Read the full explanation

You get an array called words. Each word in it is in lowercase letters.

Return a new array with every word shouted.

To shout a word, write it in capital letters and put ! after it. So "hey" becomes "HEY!".

Keep the words in the same order.

 

Example 1:

Input: words = ["hey", "you"]
Output: ["HEY!", "YOU!"]
Explanation: Both words are in capital letters now.
Each one has a ! at the end.

Example 2:

Input: words = ["go"]
Output: ["GO!"]
Explanation: One word goes in, one word comes out.

Example 3:

Input: words = ["no", "no", "no"]
Output: ["NO!", "NO!", "NO!"]
Explanation: A word that repeats is shouted each time.

 

Constraints:

  • words has at least one word.
  • Every word has only the letters a to z.
STEP 1 OF 2
    Ctrl + Enter to check

    SWIFT · TUPLES, SORTING & BUILT-INS

    Shout The Words in Swift, explained

    Problem overview

    The function shout receives an array of lowercase words and returns a new array in which every word is in capital letters with ! added at the end. The order stays the same, and a repeated word is shouted each time. So ["hey", "you"] becomes ["HEY!", "YOU!"], and ["no", "no", "no"] becomes ["NO!", "NO!", "NO!"].

    Approach

    This is a "change every item" job, and that is what map does. It runs a closure on each item of an array and collects the results in a new array of the same length and order. The closure here receives one word, written $0, and returns the shouted version.

    To shout a word, call uppercased() on it, which returns a new string in capital letters, and add "!" with +. In Swift, strings are values, so the original word and the original array are not changed.

    With ["hey", "you"], the closure runs twice. The first call receives "hey" and returns "HEY!". The second receives "you" and returns "YOU!". map puts the two results in a new array, in the same order as the words that produced them. Because every word produces exactly one result, the new array always has as many items as words.

    Step by step

    1. Build shouted with words.map. The closure { $0.uppercased() + "!" } turns each word into its shouted form. One word goes in and one comes out, in the same place.
    2. Return shouted. The new array is the answer, and words itself didn't change.

    The solution in Swift

    class Solution {
    func shout(_ words: [String]) -> [String] {
        let shouted = words.map { $0.uppercased() + "!" }
        return shouted
    }
}

    Swift details:

    • $0 is the current item. It is a shorthand for a closure's first parameter. The same closure can be written with a name: words.map { word in word.uppercased() + "!" }.
    • The result type is inferred. The closure returns a String, so shouted is a [String], which matches the function's return type.
    • uppercased() is a method call. The parentheses are required. Without them, the compiler reports "function produces expected type 'String'; did you mean to call it with '()'?".
    • Plain + joins strings. "HEY" + "!" is "HEY!".

    A loop does the same job with more lines: create an empty [String], append word.uppercased() + "!" for each word, and return it. map says the same thing in one line, and it is the usual choice when each item becomes exactly one new item.

    When you do want a loop, the result array has to be created first with its type, as var out = [String](). With map there is nothing to declare or append to, which removes a common source of mistakes such as forgetting to return the array.

    Edge cases and common mistakes

    • One word. ["go"] gives ["GO!"].
    • Repeated words. ["no", "no", "no"] gives three "NO!". map never merges or drops items.
    • Putting the ! before the conversion. ($0 + "!").uppercased() gives the same result here, because ! has no capital form. The order matters for other characters, but not in this lesson.
    • Expecting words to change. map returns a new array. After the call, words still holds the lowercase words, so return shouted, not words.

    Testing

    • ["hey", "you"] gives ["HEY!", "YOU!"].
    • ["go"] gives ["GO!"].
    • ["no", "no", "no"] gives ["NO!", "NO!", "NO!"].
    Keep going with Swift500+ step-by-step lessons, from your first line of code to coding interviews. Free to start.
    Start learning free