Question

I’d like your help with understanding why this function does not stop when I enter any other number than 1.

int main(void) {

double sum,v;

while (scanf("%lf",&v)==1) {
    printf("\t%.2f\n", sum += v);

}

It looks like it’s suppose to stop whenever the input would be different from 1. I believe that it is has to do with the condition, maybe it checks something before what I think it does.

Answer 1

The function scanf returns the number of items matched and filled, not the actual value it read.

Upon successful completion, these functions shall return the number of
successfully matched and assigned input items; this number can be zero
in the event of an early matching failure. If the input ends before
the first matching failure or conversion, EOF shall be returned.

So in your code scanf will always return 1 for a successful read. You should be testing v instead (but not with ==).