Effect of using a comma instead of a semi-colon in C and C++

Question

I've noticed on a number of occasions when refactoring various pieces of C and C++ code that a comma is used rather than a semi-colon to seperate statements. Something like this;

int a = 0, b = 0; a = 5, b = 5; 

Where I would have expected

int a = 0, b = 0; a = 5; b = 5; 

I know that C and C++ allow use of commas to seperate statements (notably loop headers), but what is the difference if any between these two pieces of code? My guess is that the comma has been left in as the result of cut & pasting, but is it a bug and does it effect execution?

Answer 1

It doesn't make a difference in the code you posted. In general, the comma separates expressions just like a semicolon, however, if you take the whole as an expression, then the comma operator means that the expression evaluates to the last argument.

Here's an example:

b = (3, 5); 

Will evaluate 3, then 5 and assign the latter to b. So b = 5. Note that the brackets are important here:

b = 3, 5; 

Will evaluate b = 3, then 5 and the result of the whole expression is 5, nevertheless b == 3.

The comma operator is especially helpful in for-loops when your iterator code is not a simple i++, but you need to do multiple commands. In that case a semicolon doesn't work well with the for-loop syntax.

Answer 2

The comma is a operator that returns a value which is always the 2nd (right) argument while a semicolon just ends statements. That allows the comma operator to be used inside other statements or to concatenate multiple statements to appear as one.

Here the function f(x) gets called and then x > y is evaluated for the if statement.

if( y = f(x), x > y ) 

An example when it's used just to avoid a the need for block

if( ... )    x = 2, y = 3;  if( ... ) {    x = 2;    y = 3; }