C++: Multiple exit conditions in for loop (multiple variables): AND -ed or OR -ed?

For loops and multiple variables and conditions.

I am using a for loop to set source and destination indexes to copy items in an array.

for(int src = 0, dst = 8;
    src < 8, dst >= 0;
    src ++, dst --)
{
    arr2[dst] = arr1[src];
}

Something like that anyway.

(AND) || (||)

My question is about the exit conditions. There are two here. src < 8 and dst >= 0. Are these conditions AND-ed (&&) or OR-ed (||).

To further explain, are the conditions evaluated like this:

(src < 8) && (dst >= 0)

Or are they evaluated like this?

(src < 8) || (dst >= 0)

Or is it something else entirely different? I imagine the logical thing to do would be to evaluate one of the two ways I specified above, and not something else.

Answer

The comma operator will return the value of the right expression, so writing this:

 src < 8, dst >= 0;

As a condition will be the same as just writing dst >= 0. The src < 8 will be completely ignored in this case, as it’s evaluated separately from the second condition, and then the second condition is returned. This doesn’t evalute to AND or to OR, but in fact just has the effect of “throwing away” the first check entirely.

If you want to evaluate this correctly, you should use one of your two options (explicitly specifying the behavior via || or &&).

For details, see Comma Operator:

When the set of expressions has to be evaluated for a value, only the rightmost expression is considered.

Leave a Reply

Your email address will not be published. Required fields are marked *