comma operator in if condition

Here is an example, provided by wikipedia, which shows another use case:

The comma can be used within a condition (of an if, while, do while, or for) to allow auxiliary computations, particularly calling a function and using the result, with block scoping:

if (y = f(x), y > x) { ... // statements involving x and y }

// See this Wikipedia discussion

Many C programmers have encountered the comma in the initializer part of a for statement, but not as many have seen it used in an if statement. In the case above it allows you to initialize y before the if statement tests the condition y>x.

http://en.wikipedia.org/wiki/Comma_operator:

In the C and C++ programming languages, the comma operator (represented by the token ,) is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type).

In your first if:

if (a, b)

a is evaluated first and discarded, b is evaluated second and returned as 0. So this condition is false.

In your second if:

if (b, a)

b is evaluated first and discarded, a is evaluated second and returned as 1. So this condition is true.

If there are more than two operands, the last expression will be returned.

If you want both conditions to be true, you should use the && operator:

if (a && b)