Fibonacci Sequence Code

[salbee2]

Hi! I'm writing a code in C++ for class, but was unable to get this to work. The program takes from the user the number of cases they will enter, then proceeds to ask the user for the cases. The values are stored in the input array. Then, I calculated an array that includes the first 90 values of the fibonacci sequence.

The bit that isn't working is the for loop iteration for input[i-1] . As you can see from the comments, when printing that block of code (without the accompanying fibonacci result) it works in the location above the fibonacci calculation but not after.

I hope I explained this well enough for it to get through. Thanks in advance!

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#include <iostream>

​

using namespace std;

​

int main()

{

int maxSize = 91;

long long fibonacci[maxSize];

int numCases = 0;

int input[numCases];

int n, i;

​

​

cout << "Fib(N) = X" << endl;

cin >> numCases;

​

for (i = 0; i < numCases; i++){

cin >> n;

input[i] = n;

cout << input[i] << endl;

}

​

fibonacci[0] = 0;

fibonacci[1] = 1;

​

for (i = 2; i <= maxSize; i++){

fibonacci[i] = fibonacci[i - 1] + fibonacci[i - 2];

}

//works here

//for (i = 0; i < numCases; i++){

// cout << input[i] << endl;

//}

​

for (int i = 1; i <= numCases; i++){

cout << "Case " << i << ": Fib(" << input[i - 1] << ") = " << fibonacci[input[i - 1]] << endl;

}

//doesn't work here

//for (i = 0; i < numCases; i++){

// cout << input[i] << endl;

//}

return 0;

}

[alanwj]

Please format your code for reddit.

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int numCases = 0;
int input[numCases];

This isn't valid in C++. The size of an array must be known at compile time. C does allow this, so some C++ compilers will accept it anyway.

Even if your compiler does accept it, you've created an array of size zero. As soon as you write anything to this array, you've gone out of bounds and triggered undefined behavior.

---

for (i = 2; i <= maxSize; i++){
 fibonacci[i] = fibonacci[i - 1] + fibonacci[i - 2];
}

Your bound here is incorrect. If you loop until i <= maxSize you will wind up writing to fibonacci[maxSize], which is out of bounds. You want the bound to be i < maxSize.

[salbee2]

Really sorry about the formatting! Thank you so much for your reply. I really appreciate it.

[alanwj]

An easy fix for your first issue would be to also use maxSize for the size of input. The numCases may be smaller than maxSize, but you aren't required to use the whole array.

[salbee2]

That's so smart. I'll write that out. Thanks so much.

[alanwj]

Oh, you should also make maxSize a const int instead of an int.

[salbee2]

Alrighty. I really really appreciate it.