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?

2 Answers 2


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:

 ^ 111111

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

To learn more about bitwise operations, read this article I recently wrote:


  • 1
    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 +=.

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