I have been working on HackerRank challenges recently. Most of questions require me to read input at the beginning. Input is usually read from standard input. It’s not necessary to use standard input with the usual iOS development, so some people might not know how to do it. I want to share how to do it this time.

Reading String

The key function is readLine(strippingNewline:) global function. This function has a default parameter for strippingNewline, so you can use it just like readLine(). This function returns String? type object that is the input just written.

let input = readLine()
if let input = input {
    print(input) // -> the input will be printed out.
}

When readLine() function is run on Xcode, the debug console waits for input. The rest of the code will be resumed after input is done. By the way, print(_:separator:terminator:) aka print(_:) function writes the passed argument to standard output. When I do HackerRank challenges, I usually put ! at the end to do forced unwrapping for convenient, like readLine()!.

Reading Int

When you want to input an integer value, you can use Int’s init(_:radix:) initializer. This initializer is a failable initializer, so the return value is of type Int?.

let inputNumber = Int(readLine()!)
if let inputNumber = inputNumber {
    print(inputNumber)
}

Reading an Array of Int

When you want to get input as an array of Int, things start getting tricky. After readLine() function, you get space-separated numbers. Swift Standard Library has a cool method called split(separator:maxSplits:omittingEmptySubsequences:) in AnyBidirectionalCollection protocol. You can use this method with an input string like this:

let input = readLine()
if let input = input {
    let inputNumberCharacters = input.characters.split(separator: " ")
    let numbers = inputNumberCharacters.map { Int(String($0))! }
    print(numbers)
}

One gotcha point here is you can call split(separator: " ") method on input.characters, not input. You need to convert String type to kind of Array of Character type. And the returned value is also kind of an array of Character. You need to map it to an array of Int. Since Int type doesn’t have an initializer with Character, you need to convert it to String type beforehand. With the above example, if you input 1 2 3 4 5 to the debug console, the return value is [1, 2, 3, 4, 5] which is of type Array<Int>.

This post is written with this environment:

Apple Swift version 3.0.2 (swiftlang-800.0.63 clang-800.0.42.1)
Target: x86_64-apple-macosx10.9