What is the most appropriate way to assign and test a variable in an if statement?

Question

Of the following, which is the preferred way of doing things, and why? Are there any specific situations in which it makes any difference, assuming that the function bar() does not take the value zero at any time?

Case 1: Test the truth value of both conditions

if ((foo = bar()) && foo < 0)
    error();

Case 2: Test only the assigned variable

if ((foo = bar()) < 0)
    error();

Answer 1

The preferred way is to separate them:

foo = bar();
if (foo < 0)
    error(); 

Edit: This is better way for both readability and avoiding bugs, such as in your first case:

if (foo = bar() && foo < 0)
    error();

That should probably be:

if ((foo = bar()) && foo < 0)
    error();

Answer 2

The first one is plain wrong. Because of precedence rules, you get:

if (foo = (bar() && foo < 0))
    error();

which is usually not what you expect.