# What does ^= mean in solidity?

I'm looking at a smart contract that has a line of code: `freeIndexSet ^= indexSet;`

I've never seen the `^=` operator before. What does it mean?

It is short for `freeIndexSet = freeIndexSet ^ indexSet;`

The `^` is a bitwise operation, called the XOR (Exclusive Or) operator. It basically turn on the bit only when both bits are different.

For example:

`101010 ^ 111111` returns `010101`:

``````   101010
^ 111111
------
010101
``````

It could be used for different purposes. Like toggling the bits, etc.

• To expand a bit on how XOR relates to toggling: if you calculate `a^b` you can interpret this as conditionally toggling on `a`. If `b=0` it returns the value of `a` unchanged. If `b=1` it will 'toggle' `a`, i.e. it will return `1-a` or `not a`. It is bitwise, so it will do this for each bit individually. Interestingly the XOR operator is symmetric so we can also interpret this as conditionally toggling on `b`. Commented Dec 9, 2022 at 13:50
It is a bitwise operator. It performs bitwise operation and assings the result to `freeIndexSet` variable. Similar to `+=`.