Quick Tip — Increment(++) and Decrement ( — )operators

Understanding POST and PRE increment operators

I was recently helping someone on a project and there was an issue with the incorrect item in an array being accessed. It all came down to using the increment ++ operator.

I’m sure many beginners and experienced coders alike have experienced an issue with these at some points so I thought a quick explanation would be useful.

The examples are in javascript but they *should* translate to most languages!

So what is the increment operator. Well it normally looks like this ++ and comes after (or before) some kind of numerical value. The idea being that the value is incremented by 1. So 1 would become 2 for example.

The confusion often arises around when this is applied. Take the following example:

let x = 1;
let y = 0;
y = x++;
console.log("x = " , x);
console.log("y = ", y);

You might expect this to output the following:

x = 2
y = 2

But in actual fact it will output x = 2, y=1 . As the ++ operator comes after x the increment happens post evaluation. So y is assigned the value of x and then x is incremented. Similarly:

let x = 1;
let y = 1;
y = x--;

y would still be equal to 1 in this example and then x would be equal to 0. Again as this is a post-decrement value.

Pre-increment and pre-decrement are the opposite. The value is incremented or decremented and then the value of the expression is returned.

let x = 1;
let y = 0;
y = ++x;

So in this example y and x would now both be equal to 2. x is incremented by 1 making it 2 and is then assigned to y

let x = 1;
let y = 1;
y = --x

And finally for the decrement example, both the values of x and y will now be equal to 0. e.g 1–1 = 0, then the resulting value is assigned to y .

I hope you find this tip useful and it helps you avoid any “gotchas” in the future. If you did please consider giving this story a clap and following Room Y. If you’d like to see more of this kind of content then please leave a comment below.