C: scanf add to total

Question

A quick question, is there a way to perform these 3 operations:

while(...) {
fscanf(input, "%lf %lf", &t, &y);
tTotal += t;
yTotal += y;
}

in one operation where t and y add themselves to tArray and yArray respectively, inside the scanf statement? Something of the form

fscanf(input, "%lf %lf", ...code..., ...code...); 

Thanks, Ash.

Answer 1

In C++ this would be fairly easy, but C it's likely to be pretty challenging (at best).

Answer 2

Pointer arithmetic to the rescue!

while(fscanf(input, "%lf %lf", t, y)) {t++; y++;}

Where t and y point to the start of their arrays initially.

Of course, you'll need to add your own bounds checks.

Answer 3

Doing that would require fscanf() to know the operation that you wanted in addition to where to store the result (and it would be undefined in the case where the variable hadn't yet been initialized).

You could write a wrapper around fscanf() to do it, but that's just a lot of work for a very specific use case, and would ultimately be slower than just doing the += after.

Answer 4

It is not possible. fscanf simply writes the scanned values to the memory locations given by the pointers, and there is no way to make that operation perform any addition.

Answer 5

There is no way to put the addition into the parameters of the fscanf function because fscanf accepts pointers and simply writes to their locations. However, you could still make the addition part of the same statement like this:

while(...) {
    fscanf(input, "%lf %lf", &t, &s), tTotal += t, sTotal += s;
}

Note that the result of this statement is the value of the last expression. So, for example, if you wanted to record the return value of scanf, you should do:

while(...) {
    int res;
    useResult((res = fscanf(input, "%lf %lf", &t, &s), tTotal += t, sTotal += s, res));
}

... which is ugly but works.

(Disclaimer: I haven't compiled this code)

Answer 6

The question is: what are you trying to achieve?
If you just want to reduce clutter, create an inline function something like:

int GetData(FILE *fp, double *ptTotal, double *pyTotal)
{
    double t, y;
    if (2 == fscanf(fp, "%lf %lf", &t, &y))
    {
         *ptTotal += t;
         *pyTotal += y;
         return 1;
    }
    else
    {
        return 0;
    }
}

and call that in a loop.

However if you have hopes of speeding up the loop, then I don't like your chances. fscanf is probably many times slower than the maths, so I don't think you'll speed it up much without switching to a bulk reading style, which will make the program much more complex.