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iOS Swift Basics (retired) Operators Unary Operators

Lawrence Yeo
Lawrence Yeo
3,236 Points
Posted by Lawrence Yeo
Lawrence Yeo
3,236 Points

Postfix -- Decrements Confusing Me

var levelScore = 0 // 0

levelScore-- // 0

levelScore // -1

var totalScore = levelScore-- // -1

totalScore // -1 -> Shouldn't this value be -2? In the previous line of code, it looks like levelScore, which was previously assigned a value of -1, was decremented by 1. Shouldn't that make it -2, or the same value that is assigned to levelScore?

levelScore // -2

Thanks!

bryan tan
bryan tan
1,178 Points

*disclaimer: I'm a newbie but I found apple's way of describing it pretty clear. I've cut and paste their explanation for the increment operator but you can use the same reasoning for the decrement operator:

var a = 0 let b = ++a // a and b are now both equal to 1 let c = a++ // a is now equal to 2, but c has been set to the pre-increment value of 1

In the example above, let b = ++a increments a before returning its value. This is why both a and b are equal to the new value of 1.

However, let c = a++ increments a after returning its value. This means that c gets the old value of 1, and a is then updated to equal 2.

Unless you need the specific behavior of i++, it is recommended that you use ++i and --i in all cases, because they have the typical expected behavior of modifying i and returning the result.

Jorge Solana
Jorge Solana
6,064 Points
Jorge Solana
Jorge Solana
6,064 Points

What you are doing in this line is: var totalScore = levelScore-- // -1

Assign the variable totalScore the value of the variable levelScore, which is -1 at that point. Then you decrease the value of levelScore. That way totalScore equals -1 and levelScore equals -2.

This works like precedent operators. If the operand "++"/"--" is located in front of a variable, that math comes first, but if it is located behind, you first assign the value and then increment/decrease the variable value.

Hope it helps.