fatal error with a simple swift program - swift

Question

import UIKit
import Foundation
func randomnumber (low:Int,high:Int )->Int
{
let range = high - (low-1)
return (Int (arc4random()) % range ) + ( low - 1)
}
let answer = randomnumber(low: 0, high: 100)
var turn = 1
while (true)
{
print ("Guess #\(turn): enter a number between 0 and 100")
let userinput = readLine();
if let guess:Int = Int(userinput!)
{
if( guess<answer )
{
print("choose a higher number")
}
if ( guess>answer )
{
print ("choose a smaller number")
}
if( guess==answer)
{
print("wohoo you won")
break;
}
}
}
this code is about a simple game using swift , by having a random number and then putting an input and if this input is bigger than the random number we have to choose smaller number and the opposite if we choose a smaller number and its still not equal to the random number and if it's equal to the random number you win.
this error appears :
fatal error: unexpectedly found nil while unwrapping an Optional value

Answer 1

As Tristan Beaton pointed out, readLine() doesn't work on playground hence your userInput is always nil.
CREATE A COMMAND LINE TOOL APPLICATION
CHOSE SWIFT WHEN YOU SAVE
COPY AN PASTE YOUR CODE
RUN AND PLAY
Also be really careful when you force unwrapping. That's always a crash waiting to happen ;) You can read this tutorial

Answer 2

I have tested this in a Command Line Tool and it worked. Also don't force unwrap optionals since you can just check if they have data without crashing the app.
I have added continue statements within your other if statements. Although it isn't really needed in this case, it is good practice to have it so that any code after the continue doesn't get executed. It just saves a bit of computing power.
import Foundation
func randomnumber(low: Int, high: Int) - >Int {
let range = high - (low-1)
return (Int(arc4random()) % range) + (low - 1)
}
let answer = randomnumber(low: 0, high: 100)
var turn = 1
while (true) {
print("Guess #\(turn): enter a number between 0 and 100")
if let userinput = readLine() {
if let guess:Int = Int(userinput) {
// Putting this here will only increase the guess count if their input in a number.
turn += 1
if guess < answer {
print("choose a higher number")
continue
}
if guess > answer {
print ("choose a smaller number")
continue
}
if guess == answer {
print("wohoo you won")
break
}
}
}
}
This is the console output