My array has the wrong values

Question

I'm working my way through C++ Primer Plus by Stephen Prata. Please Help! My program reads 10 or less int values( golf scores) into an array then calls a function to print out the values and the average. I have used the code::blocks debugger as best I can. I get the proper value into the area by prompting via the input function but when I return a *int ptr to the display function the values are different when I print them out. I suspect my use of pointers is incorrect since I am just learning about them. If anyone sees any obvious errors on my part, I would be most grateful if you point them out. I've been at this for too long and I'm lost. Here is all the code:

#include <iostream> using namespace std; const int SIZE = 10; int* input() { int scores[10] = {0}; int score = 0; for(int i = 0; i < SIZE; i++) { cout << "Enter golf score: " << endl; if(cin >> score) { scores[i] = score; } else break; } return scores; } float average(int* p_ints) { float sum = 0; int i = 0; while(p_ints[i] != 0 && i < SIZE) { sum += float(p_ints[i]); i++; } return sum/i; } void display(int* p_ints) { cout << "scores: "; int i = 0; while(p_ints[i] != 0 && i < SIZE) { cout << p_ints[i] << " "; i++; } cout << "average: " << average(p_ints) << endl; } int main() { int* p_ints = input(); display(p_ints); return 0; }

Community · Accepted Answer · 2017-05-23 11:56:55Z

Your problem is that input() returns a pointer to a locally constructed object

int scores[10] = {0}; // ... return scores; // PROBLEM HERE, scores goes out of scope and its memory re-claimed by the OS

When you exit from the function input, scores is out of scope and you end up with a so-called dangling pointer.

So, either allocate memory dynamically for scores,

int* scores = new int[10]{0}; // this memory is not being automatically reused by the OS at exit from the function

and don't forget to delete[] the returned pointer after usage, or (better) pass the pointer scores (of which memory you want to fill) to the function input as a parameter

void input(int* scores) // pass a pointer to the output array

Or, better yet, use standard C++ containers like std::vector.

I wouldn't go for the new just yet. Pass the function an array, or return a vector or something. With that SIZE variable there are a lot of options. Also, +1.
@keyser agree, I would pass the array directly (actually, I would return a std::vector ;) )
It makes sense now. I see the error pertaining to the dangling pointer. Thanks.
@user3713343 something to remember: never return a pointer that refer to an object that is stack-allocated like Foo foo; return &foo; //oops. Such stack-allocated objects cease to exist at the exit from the function.

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