The Difference Between = and ==

One of the biggest mistakes new C programmers make is to use = when they mean ==. It is very important to understand that these are two completely different operators.

  • "=" is used to assign a value to a variable.
  • "==" is used to compare two values for equivalence.

Be careful not to confuse = and ==. They are not interchangeable!

Example

A very common mistake new C programmers will make is to do the following:

if (x = 5) { }

This statement will always be true because x is being assigned the value of 5 (a non-zero value) rather than being compared to 5. An assignment used where a conditional expression is expected will always be evaluated as true unless the expression equals 0. If you do fall into this trap, it can be a very hard bug to catch since the compiler won't flag it as an error. The only way you will know the bug is there is that the if block will always run, even in what should be a false condition.

In the example above, the string "Hi!" will be printed to the output window because x is non-zero. If we changed the statement to say:

if (x = 0) { printf("Hi!"); }

Then the printf() would never execute because x is always 0.

© 2016 Microchip Technology, Inc.
Information contained on this site regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. Microchip disclaims all liability arising from this information and its use. Use of Microchip devices in life support and/or safety applications is entirely at the buyer's risk, and the buyer agrees to defend, indemnify and hold harmless Microchip from any and all damages, claims, suits, or expenses resulting from such use. No licenses are conveyed, implicitly or otherwise, under any Microchip intellectual property rights.