C++ long integer oddities? - c++

Question

I'm brushing up on some beginner's algorithms as I familiarize myself with C++. There are already some bugs I have no idea where to start fixing.
1) The following gives me seg faults around the time when the fib() function returns its results. EDIT: with inputs >= 9
#include <iostream>
using namespace std;
int fib(int n) {
int fibs[] = {1, 1}; // dynamic array
int i = 2; // start at 3rd element
while(i < n) {
fibs[i] = fibs[i-2] + fibs[i-1];
cout << "DEBUG: fibs[" << i << "] = " << fibs[i] << endl;
i = i+1;
}
cout << "about to return to main()" << endl;
return fibs[n-1];
}
int main() {
cout << "\n================= Hello cruel world =================" << endl;
cout << "Enter a number: ";
int x;
cin >> x;
cout << "fib(" << x << ") = " << fib(x) << endl;
cout << "================ Goodbye cruel world ================\n" << endl;
return 0;
}
Otherwise, the code works just fine, the numbers are found correctly. But 2) when I change the function to support long integers, it starts acting weird:
#include <iostream>
using namespace std;
long fib(int n) {
long fibs[] = {1L, 1L}; // dynamic array
int i = 2; // start at 3rd element
while(i < n) {
fibs[i] = fibs[i-2] + fibs[i-1];
cout << "DEBUG: fibs[" << i << "] = " << fibs[i] << endl;
i = i+1;
}
cout << "about to return to main()" << endl;
return fibs[n-1];
}
int main() {
cout << "\n================= Hello cruel world =================" << endl;
cout << "Enter a number: ";
int x;
cin >> x;
cout << "fib(" << x << ") = " << fib(x) << endl;
cout << "================ Goodbye cruel world ================\n" << endl;
return 0;
}
Output:
================= Hello cruel world =================
Enter a number: 7
DEBUG: fibs[2] = 2
DEBUG: fibs[0] = 1
DEBUG: fibs[1] = 30071067265
DEBUG: fibs[2] = 30071067266
DEBUG: fibs[14] = 0
about to return to main()
fib(7) = 140733637791872
================ Goodbye cruel world ================
It doesn't make any sense to me. Thanks for any help.

Answer 1

int fibs[] = {1, 1};
is equivalent to:
int fibs[2] = {1, 1};
In another word, the array fibs(in both programs) has only two elements, it's illegal to access fibs[n] if n is bigger than 1.

Answer 2

In
long fibs[] = {1L, 1L};
the [] does not mean "as big as it needs to be", it means "count the number of initializers". It's the same as
long fibs[2] = { 1L, 1L };
And the comment
// dynamic array
is just plain WRONG.

Answer 3

The other answers explain what's wrong. To fix it, you can declare fibs as a vector instead:
vector<int> fibs(n, 1);
which will construct a vector of n integers and initialize them all to 1. Replacing that single line of code should be all you need!

Answer 4

Yes it will fail because fibs[2] doesn't exist!
This is not dynamic array by any mean. Its array of 2 long
long fibs[] = {1L, 1L}; // dynamic array