7.7 Write Assignments in Separate Statements

It is often convenient to write an assignment inside an if-condition, but that can reduce the readability of the program. Here’s an example of what to avoid:

if (x = advance (x))
  …

The idea here is to advance x and test if the value is nonzero. However, readers might miss the fact that it uses ‘=’ and not ‘==’. In fact, writing ‘=’ where ‘==’ was intended inside a condition is a common error, so GNU C can give warnings when ‘=’ appears in a way that suggests it’s an error.

It is much clearer to write the assignment as a separate statement, like this:

x = advance (x);
if (x != 0)
  …

This makes it unmistakably clear that x is assigned a new value.

Another method is to use the comma operator (see Comma Operator), like this:

if (x = advance (x), x != 0)
  …

However, putting the assignment in a separate statement is usually clearer unless the assignment is very short, because it reduces nesting.