[RESOLVED] What's wrong?

[CodingFreak]

Hi,
I'm coding a program which indicates how many times a number ranging from 0 to max is casted (max is an integer chosen by the user). I'm using the same random number generator as in "C++ without fear". However, when compiled, the program outputs right and wrong results . What's wrong?

#include <iostream>
#include <ctime>
#include <cmath>
#include <stdlib.h>
using namespace std;

int rand_0toN1(int n);

int main() {
    for (;;) {
        int n, i, r, max;

        srand(time(NULL));

        cout << "The highest number to be generated: "; cin >> max;
        int hits[max];
        cout << "Enter how many trials: "; cin >> n;
        for (i = 0; i < n; i++) {
            r = rand_0toN1(max);
            hits[r]++;
        }
        for (i = 0; i < max; i++) {
            cout << i << ": " << hits[i] << " Accuracy: ";
            double results = hits[i];
            cout << results / (n / max) << endl;
        }
    }
    return 0;
}

int rand_0toN1(int n) {
    return rand() % n;
}

[Philip Nicoletti]

You might want to try initializing all elements of the "hits" array to 0.
I don't know if your declaration does that, since it is non-standard.

[CodingFreak]

Is that what you meant?

#include <iostream>
#include <ctime>
#include <cmath>
#include <stdlib.h>
using namespace std;

int rand_0toN1(int n);
int hits[0];

int main() {
    for (;;) {
        int n, i, r, max;

        srand(time(NULL));

        cout << "The highest number to be generated: "; cin >> max;
        cout << "Enter how many trials: "; cin >> n;
        for (i = 0; i < n; i++) {
            r = rand_0toN1(max);
            hits[r]++;
        }
        for (i = 0; i < max; i++) {
            cout << i << ": " << hits[i] << " Accuracy: ";
            double results = hits[i];
            cout << results / (n / max) << endl;
        }
    }
    return 0;
}

int rand_0toN1(int n) {
    return rand() % n;
}

You're right. I needed to initialize the array from outside the main function. Thanks

[GCDEF]

No, the problem is you're calling srand inside your loop. You should only call it once.

[Philip Nicoletti]

this is not correct ...

int hits[0];

that is declaring the array with size 0 (I'm not sure if that is legal or not).

In your "for(;;)" loop, you need to initialize the array to 0 ...

int hits[max];

for (i=0; i<max; ++i)
   hits[i] = 0;

As mentioned, you only need to call srand() once at the start of the program.
But your usage is OK.

[D_Drmmr]

that is declaring the array with size 0 (I'm not sure if that is legal or not).

It's not legal to specify a static array (or dynamic array) of size 0. This is actually used as a way to implement compile-time assertions.

Note however, that the OP's original program uses neither; it uses a variable length array, which is a language extension. IMO, it's better to just use a std::vector instead. In that case the initialization is also very simple.

std::vector<int> hits(max, 0);

[Paul McKenzie]

Is that what you meant?

No, this:

cout << "The highest number to be generated: "; cin >> max;
        int hits[max];

It is illegal in C++ to declare an array using a variable as the number of items in the array.

int main()
{
    int x = 10;
    int hits[x];  // error
}

This code should not compile if compiled using ANSI C++. More than likely, you're using gcc or an IDE that uses the gcc compiler underneath. If so, you must change the compiler settings:

-Wall -pedantic

Once you do that, then you will see that your code will not compile. If this is a C++ course or homework, then you must change it to compile ANSI C++ and not use language extensions, otherwise you may get points taken off for using non-C++ code.

Once you see that your code doesn't compile, the real alternative is to use std::vector<> as a dynamic array of hits.

#include <iostream>
#include <ctime>
#include <cmath>
#include <stdlib.h>
#include <vector>

using namespace std;

int rand_0toN1(int n);

int main() {
    for (;;) {
        int n, i, r, max;

        srand(time(NULL));

        cout << "The highest number to be generated: "; cin >> max;
        std::vector<int> hits(max, 0);
        cout << "Enter how many trials: "; cin >> n;
//...

Regards,

Paul McKenzie