While Loops

Question

I'm getting this error message when I give the following response to this challenge:
You need to use 'println()'
let index = 0
while numbers.count < 9 {
  println(numbers[index])
} 
I am using the 'println()' command so I'm puzzled by this error message.
Can anyone help?
Thanks in advance!
```while_loops.swift
let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
let index = 0
while numbers.count < 9 {
  println(numbers[index])
} // You need to use 'println()'
```

Gunhoo Yoon · Accepted Answer

I haven't had a chance to start Swift course but I think there is some logic error in your code. But I think I know where you made mistake.
So let me first correct your mistakes but from second.

//Second mistake
while numbers.count < 9 {
   println(numbers[index])
}

1 Your loop won't actually run because numbers.count has 10 elements. Probably that's why the compiler didn't recognize your println() since it never reached that state.
2 Even adjusting your loop condition to number.count < 11, 12 .... whatever it will only result infinite loop because number.count never changes. So it is either broken loop or indefinite loop.
3 Even if your loop did run, it will only printout numbers[0] because you've never increased the index in your code.

//First mistake.
let index = 0

4 From what I've searched on Stackoverflow let is a constant which can't be changed. source. (http://stackoverflow.com/questions/24002092/what-is-the-difference-between-let-and-var-in-swift) So even if you increased the index you would get error because it cannot be changed. So you need to use var like normal variable.
I went from second to first because that's what was actually causing You need to use 'println()'
This is the working version.
```
let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
//Use variable that is change-able.
var index = 0
//The index will start from 0, will increase every iteration, so it will eventually end.
while index < numbers.count {
  //print first because you start from index 0
  println(numbers[index])
  //this does this incrementing
  index++
}
```

Michael Southon · Answer

Thanks to you both! Your responses really helped me understand what was going on. Thanks for your patience with a beginner :-)

Unsubscribed User · Answer

I have never done Swift before, but I will try to explain the problems I see with your code.
1) You are using the keyword let for your index, which means you are declaring index as a constant variable. This will mean that further down in your code, you will not be able to modify index.
2) Your while loop will never iterate. The condition for your while is saying 'loop while numbers.count is less than 9'. However, numbers.count has a total of 10 items, but the count will return '9' (because the array index starts at 0). This is the same as checking if 9 is less than 9, which will always be false.
3) Inside of your loop, you are only calling println. In order for a while loop to iterate through your array, you would need to add a variable which is either incremented or decremented on each loop. It looks like you were going to use index for this, but it wasn't implemented correctly.
I came up with the following solution which passes the code challenge:
```swift
let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
var index = 0
while index < numbers.count {
  println(numbers[index])
  ++index
}
```
This will loop through each item in the array and print the value. At the end of each loop, ++index is adding 1 to the index variable, which is helping us with our iteration through the array. For this to work correctly, I had to declare index with var and not with let.
Hope this helps!

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Michael Southon

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While Loops

3 Answers

Gunhoo Yoon

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Unsubscribed User

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Michael Southon

Michael Southon