Understanding the comma seperator in loop

[andy1978]

If you have something like this,

  do{
    //stuff
  while(a, b, c, d > 100);

So what exactly is happening here. Same for for loops something like,

for(i = 0, j >7, i < 10;i++){

}

[coder777]

All operations in this comma separated list are processed while the result of the last operation is used [by the while/for loop]

[Peter87]

The comma operator only returns the value of the last expression so the first loop is equivalent to:

do{
	//stuff
} while(d > 100);

The second loop is not syntactically correct because you have only one semi-colon.

[andy1978]

Yes that's what I got from the ref, so what is the point of a, b and c if you say the above, do they not act as conditions?

[Peter87]

No, they are pointless.

[andy1978]

So why put them there, why have the option?

[Peter87]

So why put them there

When I see something like this it's often written by a beginner that hasn't learned to use && yet. In that case I would guess this is what he intended to do:

do{
	//stuff
} while(a  > 100 && b > 100 && c > 100 && d > 100);

why have the option?

You can use any expression you want as a loop condition as long as the return type can be converted to a bool.

[andy1978]

I think you are right I think it's a mistake, is this the same for for loops with a comma operator?

If not do you have an example of using commas in a for loop?

[coder777]

for(i = 0, j = 7; i < 10;i++, j++){

}

[andy1978]

Thanks. So I take it something like,

for(i = 0, j = 7; i < 10, j < 8;i++, j++){

} 

Doesn't make sense/work?

[MikeyBoy]

It is legal C++, and and it will run. However, the only condition which will have any effect on whether it keeps looping will be j < 8. It is NOT the same as writing:

for(i = 0, j = 7; i < 10 && j < 8;i++, j++)