[RESOLVED] What's wrong?

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?

Code:

---

#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;
}

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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.

Is that what you meant?

Code:

---

#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

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

this is not correct ...

Code:

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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 ...

Code:

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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.

Quote:

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Originally Posted by Philip Nicoletti

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.

Code:

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

---

Quote:

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Originally Posted by CodingFreak

Is that what you meant?

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No, this:

Code:

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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.

Code:

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

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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:

Quote:

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-Wall -pedantic

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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.

Code:

---

#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;
//...

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Regards,

Paul McKenzie