📓1.5: Compound Assignment Operators

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 1.5

Compound Assignment Operators

Compound assignment operators are shortcuts that do a math operation and assignment in one step. For example, x += 1 adds 1 to the current value of x and assigns the result back to x. It is the same as x = x + 1. This pattern is possible with any operator put in front of the = sign, as seen below. If you need a mnemonic to remember whether the compound operators are written like += or =+, just remember that the operation (+) is done first to produce the new value which is then assigned (=) back to the variable. So it’s operator then equal sign: +=.

Since changing the value of a variable by one is especially common, there are two extra concise operators ++ and --, also called the plus-plus or increment operator and minus-minus or decrement operator that set a variable to one greater or less than its current value.

Thus x++ is even more concise way to write x = x + 1 than the compound operator x += 1. You’ll see this shortcut used a lot in loops when we get to them in Unit 4.

Similarly, y-- is a more concise way to write y = y - 1.

These shortcuts only exist for + and - as they don’t really make sense for other operators.

Compound Operator Cheatsheet

Here’s a table of all the compound arithmetic operators and the extra concise increment and decrement operators and how they relate to fully written out assignment expressions.

Operator + - * / %
Written out x = x + 1 x = x - 1 x = x * 2 x = x / 2 x = x % 2
Compound x += 1 x -= 1 x *= 2 x /= 2 x %= 2
Extra concise x++ x--      

⭐️ Summary

  • Compound assignment operators (+=, -=, *=, /=, %=) can be used in place of the assignment operator.

  • The increment operator (++) and decrement operator (--) are used to add 1 or subtract 1 from the stored value of a variable. The new value is assigned to the variable.

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.