Map

Part of the Logic & Flow section of Coddy's Swift journey — lesson 23 of 56.

map takes a closure and applies it to every element, returning a new array of the results. The new array has the same length as the source.

let nums = [1, 2, 3, 4]
let doubled = nums.map { $0 * 2 }
print(doubled)               // [2, 4, 6, 8]

$0 is shorthand for the closure's first parameter. The longer form spells out the parameter name and the return type:

let doubled = nums.map { (n: Int) -> Int in
    return n * 2
}

map can also change the element type. Mapping an [Int] through a closure that returns String gives you a [String]:

let labels = nums.map { "#\($0)" }
print(labels)                // ["#1", "#2", "#3", "#4"]

Challenge

Easy

Read a single line of input: a comma-separated list of integers (temperatures in Fahrenheit).

Convert each one to Celsius using the formula C = (F - 32) * 5 / 9 in integer arithmetic (truncate toward zero, no decimals). Print, on separate lines:

The Celsius temperatures, joined with ,
One label per temperature with the prefix F=<f> C=<c>, joined with ;

Use map for both lines.

For input 32,212,98,0, the output is:

0,100,36,-17
F=32 C=0;F=212 C=100;F=98 C=36;F=0 C=-17

Cheat sheet

map applies a closure to every element and returns a new array of the same length:

let nums = [1, 2, 3, 4]
let doubled = nums.map { $0 * 2 }  // [2, 4, 6, 8]

$0 is shorthand for the closure's first parameter. Explicit form:

let doubled = nums.map { (n: Int) -> Int in
    return n * 2
}

map can change the element type (e.g. [Int] → [String]):

let labels = nums.map { "#\($0)" }  // ["#1", "#2", "#3", "#4"]

Try it yourself

let f = readLine()!.components(separatedBy: ",").map { Int($0)! }

// TODO: line 1 = celsius joined with ","
// TODO: line 2 = labels "F=<f> C=<c>" joined with ";"