Why does incrementing one array, increment another? - c++

Question

I am having a problem with incrementing a single item in an array. It ends up incrementing another array.. How does it do this? this is what I have:
string simulateFIFO(darray<int> requests, int frameSize) {
string results;
int currentPages[frameSize];
int timer[frameSize];
int hits = 0;
int faults = 0;
cout << "STARTING FIFO" << endl;
for (int i = 0; i < requests.size(); i++) {
cout << requests[i] << " ";
}
cout << endl;
cout << endl;
for (int i = 0; i < frameSize; i++) {
currentPages[i] = -1;
timer[i] = 0;
}
// Start Calculations
for (int i = 0; i < requests.size(); i++) {
bool requestSet = false;
for (int j = 0; j < frameSize; j++) {
if (currentPages[j] < 0) {
// Current frame does not have a page
cout << "FRAME IS EMPTY" << endl;
currentPages[j] = requests[i];
requestSet = true;
faults++;
j = frameSize;
}
else if (currentPages[j] == requests[i]) {
cout << "FRAME IS SAME AS REQUEST" << endl;
// Current Frame is set the the page being requested
timer[j] = 0;
requestSet = true;
hits++;
j = frameSize;
}
cout << currentPages[j] << endl;
timer[j]++;
cout << currentPages[j] << endl;
}
// Check to see if a request was set or not
if (requestSet == false) {
cout << "FRAME NEEDS REPLACED" << endl;
// The request wasnt set. Determine which frame to replace with the new request
int lastUsed = 0;
int lastUsedIndex = 0;
for (int j = 0; j < frameSize; j++) {
if (timer[j] > lastUsed) {
lastUsed = timer[j];
lastUsedIndex = j;
}
}
currentPages[lastUsedIndex] = requests[i];
//timer[lastUsedIndex] = 0;
faults++;
}
cout << "HEY 3: " << currentPages[0] << endl;
cout << "NEW FRAME: ";
for (int j = 0; j < frameSize; j++) {
cout << currentPages[j] << " ";
}
cout << endl;
cout << endl;
}
cout << "FIFO" << endl;
cout << faults << endl;
cout << hits << endl;
cout << endl;
return results;
}
My Output ends up being
0
1
Why does increasing one array actually increase the other as well?

Answer 1

Your code includes a possible path of execution:
j = frameSize;
followed by
timer[j]++;
This accesses out of bounds: for an array of dimension frameSize, the valid indices are 0 through frameSize-1.
I guess you actually want to exit the loop; if so then replace j = frameSize; with break; .
NB. int timer[frameSize]; is not permitted in ISO C++; array bounds must be known at compile-time. Your code is relying on a compiler extension.