Welcome to the Treehouse Community

Want to collaborate on code errors? Have bugs you need feedback on? Looking for an extra set of eyes on your latest project? Get support with fellow developers, designers, and programmers of all backgrounds and skill levels here with the Treehouse Community!

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and join thousands of Treehouse students and alumni in the community today.

Start your free trial

iOS

Alexandra Kaplan
Alexandra Kaplan
1,167 Points
Posted by Alexandra Kaplan
Alexandra Kaplan
1,167 Points

What are you doing to a code when you write index++?

In the code:

numbers = ["1","2","3"]
var index = 0
while index < numbers.count {println (numbers[index]) 
index++ 
}

What is the purpose of the index++ ? My code does not work until I add that.

2 Answers

Michael Hulet
Michael Hulet
47,908 Points
Michael Hulet
Michael Hulet
47,908 Points

Writing "++" after a number variable adds 1 to that number. This is called "incrementing" the variable. In this case, index is part of the condition of the while loop. The loop will run as long as index is less than the count of the numbers array. If you never increment index, the loop will run forever (this is called an "infinite loop"), and will crash your app. Here's line-by-line what your code does:

let numbers = ["1","2","3"]  //Declares an array constant of ["1", "2", "3] and sets it to numbers
var index = 0 //Declares an Int variable of 0 and sets it to index
while index < numbers.count { //Runs the code in the block as long as index is less than the number of items in the numbers array
    println (numbers[index]) //Prints the number at the index of the numbers array at the current value of the index variable to the console 
    index++ //Increments, or adds one, to the index variable
}
Alexandra Kaplan
Alexandra Kaplan
1,167 Points
Alexandra Kaplan
Alexandra Kaplan
1,167 Points

Can you explain why "if you never increment index, the loop will run forever"? This core concept I don't understand-what is making the loop run forever?

Michael Hulet
Michael Hulet
47,908 Points
Michael Hulet
Michael Hulet
47,908 Points

Every time the code in the loop runs, the loop checks the value of index and compares it to the value of numbers.count (which represents the number of items in the numbers array). If index is less than numbers.count, then the code in the loop runs. If index is greater than or equal to numbers.count, then the code in the loop is skipped, and your code moves on to whatever comes after that

Alexandra Kaplan
Alexandra Kaplan
1,167 Points
Alexandra Kaplan
Alexandra Kaplan
1,167 Points

I think I understand! So because the index number is equal to 0, the count of the numbers in the array will always be greater, so the statement will always be true and it will run forever?

akshaykhullar
akshaykhullar
6,943 Points
akshaykhullar
akshaykhullar
6,943 Points

index++ increments the variable index by 1. Initially the variable index is set to 0. When the loop runs for the first time, the output is 1 ( numbers[0] ). The next line adds 1 to the index therefore the next time the loop runs it will output 2 ( numbers[1] ). Hope this helps!