Always watch the program flow

I just might have saved myself 5 hours of searching for inefficiency problems in the code I'm working on now.

The problem I was tasked with was, that a function recalculating values for a large number of items (patients with a certain amount of medication data) was leading to memory problems, crashing, and/or taking unbelievably long. So I relentlessly searched the function in question for inefficient bits, fixed and rewrote, only to never seem to make any progress.

When I started work freshly this morning, I started to debug and look up how long one call of the function in question was taking. Tracing the steps the program took, I noticed something... (code is not the actual program code, but simplified to make the problem clearer)

// Function that is called when the menu item is clicked
void menuFunction() {
    while(patient != NULL) {
        recalculateValues();
        patient = nextPatient;
    }
}

void recalculateValues() {
    while(patient != NULL) {
        // ... do the stuff that it's supposed to do
        patient = nextPatient;
    }
}

Classic case of a double while loop. A problem I could have fixed in less than 5 minutes, but ended up taking 5 hours.

Always watch the program flow, people!