# Careful with the subtraction assignment operator in C

This might be obvious to someone who majored in computer science (and does make sense if you think about it), but a little care is required when using the subtraction assignment operator (-=) in C and related languages.

For example, if I wanted to shorten a = a - b - c by using the subtraction assignment operator, I might (carelessly) replace it with: a -= b - c

Unfortunately, this is wrong. The operator is not a simple replacement of a = a - with a -=. Instead, whatever is on the right-hand side of -= is subtracted from the left-hand side altogether. In other words, a -= b - c is equivalent to a = a - (b - c) = a - b + c, which is not the desired operation.

Instead, the correct operation is: a -= b + c.