Charles Wilson's Blog

Update 2015-12-21: Apple released Swift source code. I’ve updated my sample again to reflect what I learned.

Update 2015-09-27: It’s been a year and much has happened with Swift. Please see my latest post on Swift command line input for current code.

Recently I’ve been watching Stanford intro CS classes. I like to see how they present the fundamental concepts and techniques of programming. This got me thinking about those missing bits of Swift that would allow me to actually write a command line-based application. [see my previous post Swift: Second Things First] Having these bits would be to allow me to teach Swift as a first language without having to teach the abstractions and interfaces required to properly develop for a graphical interface. I’m not much into attempting to teach in a way that breaks the “go from strength to strength” methodology. If you’re going to teach me to sing, it’s a whole lot easier if I don’t have to learn how to spin plates at the same time.

So, I spent some time and created a simple set of routines that when added to a Swift command line application allow you to get and put strings, integers and floats. Not exactly rocket science, which begs the question, “Why didn’t Apple do it?” Well, since I’m not Apple, I have no idea.

Here without further comment is the content of the file I wrote. That it is not the best Swift code, I have no doubt. If you can make it better, cool. And Apple, it you read this, please make something sensible of it.

Update: I’ve manually wrapped a few lines as WordPress is clipping.

//
//  swift_intput_routines.swift
//  swift input test
//
//  Created by Charles Wilson on 9/27/14.
//  Copyright (c) 2014 Charles Wilson.
// Permission is granted to use and modify so long as attribution is made.
//

import Foundation

func putString (_ outputString : NSString = "")
{
  if outputString.length >= 1
  {
    NSFileHandle.fileHandleWithStandardOutput().writeData(
               outputString.dataUsingEncoding(NSUTF8StringEncoding)!)
  }
}

func getString (_ prompt : NSString = "") -> NSString
{
  if prompt.length >= 1
  {
    putString(prompt)
  }

  var inputString : NSString = ""
  let data        : NSData?  = NSFileHandle.fileHandleWithStandardInput().availableData

  if ( data != nil )
  {
    inputString = NSString(data: data!, encoding: NSUTF8StringEncoding)!
    inputString = inputString.substringToIndex(inputString.length - 1)
  }

  return inputString
}

func getInteger (_ prompt : NSString = "") -> Int
{
  if prompt.length >= 1
  {
    putString(prompt)
  }

  var inputValue : Int = 0
  let inputString = getString()

  inputValue = inputString.integerValue

  return inputValue
}

func getFloat (_ prompt : NSString = "") -> Float
{
  if prompt.length >= 1
  {
    putString(prompt)
  }

  var inputValue : Float = 0.0
  let inputString = getString()

  inputValue = inputString.floatValue

  return inputValue
}

And here’s a little test program that uses it.

//
//  main.swift
//  swift input test
//
//  Created by Charles Wilson on 9/27/14.
//  Copyright (c) 2014 Charles Wilson. All rights reserved.
//

import Foundation

var name = getString("What is your name? ")

if name.length == 0
{
  name = "George"

  putString("That's not much of a name. I'll call you '\(name)'\n")
}
else
{
  putString("Your name is '\(name)'\n")
}

let age = getInteger("How old are you \(name)? ")

putString("You are \(age) years old\n")

let number = getFloat("Enter a number with a decimal point in it: ")

putString("\(number) is a nice number")

putString("\n\n")
putString("bye\n")

You’re probably wondering why I don’t use print(). Well, print() doesn’t flush stdout’s buffer. And, I really like to enter data on the same line as the prompt. And for those of you who say that you can use print() in XCode’s output window, I’ll remind you that a simulator isn’t the target device.

“But, wait. You didn’t comment the code.” No, I didn’t. By the time a student has enough understanding of Cocoa to compose the I/O routines provided above, the comments would be unnecessary.

So, there you have it. The typical second program that you’d ask a student to write.

Read Full Post »