Question

I built a program that is cyclic called. Each time value T changes, and I’d like to compare value T and T from previous cycle, and do it for every cycle.

int T = externalsrc; //some external source
int prevT; //cannot do it because in the first cycle it will have no value when used in comparison
int prevT = 0; //cannot do it, because in every cycle prevT will be zero for my comparison
int comparison = prevT - T;
prevT = T;

How can I do it properly then? I tried also this, but still T is not declared here:

int T;
int prevT;
if (prevT != T)
  prevT = 0;
else
  prevT = externalsrc;
int comparison = prevT - T;
prevT = T;

Answer 1

Use your first answer, but declare prevT as static and init to 0:

while (condition) {
    int T = externalsrc; //some external source
    static int prevT = 0; // declaring static means that it will only be set to 0 once
    int comparison = prevT - T;
    prevT = T;
}

…that way on every subsequent iteration the initialisation of prevT is ignored, and the value is preserved from the last iteration.