What does the ',' operator do in C?

Question

What does the ',' operator do in C?

Answer 1

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

Answer 2

The expression:

(expression1,  expression2)

First expression1 is evaluated, then expression2 is evaluated, and the value of expression2 is returned for the whole expression.

Answer 3

It causes the evaluation of multiple statements, but uses only the last one as a resulting value (rvalue, I think).

So...

int f() { return 7; }
int g() { return 8; }

int x = (printf("assigning x"), f(), g() );

should result in x being set to 8.

Answer 4

As earlier answers have stated it evaluates all statements but uses the last one as the value of the expression. Personally I've only found it useful in loop expressions:

for (tmp=0, i = MAX; i > 0; i--)

Answer 5

The only place I've seen it being useful is when you write a funky loop where you want to do multiple things in one of the expressions (probably the init expression or loop expression. Something like:

bool arraysAreMirrored(int a1[], int a2[], size_t size)
{
  size_t i1, i2;
  for(i1 = 0, i2 = size - 1; i1 < size; i1++, i2--)
  {
    if(a1[i1] != a2[i2])
    {
      return false;
    }
  }

  return true;
}

Pardon me if there are any syntax errors or if I mixed in anything that's not strict C. I'm not arguing that the , operator is good form, but that's what you could use it for. In the case above I'd probably use a while loop instead so the multiple expressions on init and loop would be more obvious. (And I'd initialize i1 and i2 inline instead of declaring and then initializing.... blah blah blah.)

Answer 6

I've seen used most in while loops:

string s;
while(read_string(s), s.len() > 5)
{
   //do something
}

It will do the operation, then do a test based on a side-effect. The other way would be to do it like this:

string s;
read_string(s);
while(s.len() > 5)
{
   //do something
   read_string(s);
}

Answer 7

The comma operator combines the two expressions either side of it into one, evaluating them both in left-to-right order. The value of the right-hand side is returned as the value of the whole expression.

• (expr1, expr2) is like (expr1 && expr2) but both expr1 and expr2 will always be evaluated, whatever expr1 returns.
• (expr1, expr2) is like { expr1; expr2; } but it can be used as an expression in a function call or assignment.

It is often seen in for loops to initialise or maintain multiple variables like this:

for (low = 0, high = MAXSIZE; low < high; low = newlow, high = newhigh)
{
    /* do something with low and high and put new values
       in newlow and newhigh */
}

Apart from this, I've only used it "in anger" in one other case, when wrapping up two operations that should always go together in a macro. We had code that copied various binary values into a byte buffer for sending on a network, and a pointer maintained where we had got up to:

unsigned char outbuff[BUFFSIZE];
unsigned char *ptr = outbuff;

*ptr++ = first_byte_value;
*ptr++ = second_byte_value;

send_buff(outbuff, (int)(ptr - outbuff));

Where the values were shorts or ints we did this:

*((short *)ptr)++ = short_value;
*((int *)ptr)++ = int_value;

Later we read that this was not really valid C, because (short *)ptr is no longer an l-value and can't be incremented, although our compiler at the time didn't mind. To fix this, we split the expression in two:

*(short *)ptr = short_value;
ptr += sizeof(short);

However, this approach relied on all developers remembering to put both statements in all the time. We wanted a function where you could pass in the output pointer, the value and and the value's type. This being C, not C++ with templates, we couldn't have a function take an arbitrary type, so we settled on a macro:

#define ASSIGN_INCR(p, val, type)  ((*((type) *)(p) = (val)), (p) += sizeof(type))

By using the comma operator we were able to use this in expressions or as statements as we wished:

if (need_to_output_short)
    ASSIGN_INCR(ptr, short_value, short);

latest_pos = ASSIGN_INCR(ptr, int_value, int);

send_buff(outbuff, (int)(ASSIGN_INCR(ptr, last_value, int) - outbuff));

I'm not suggesting any of these examples are good style! Indeed, I seem to remember Steve McConnell's Code Complete advising against even using comma operators in a for loop: for readability and maintainability, the loop should be controlled by only one variable, and the expressions in the for line itself should only contain loop-control code, not other extra bits of initialisation or loop maintenance.