CODEWITHSUNDEEP
c++randomtimesrand
ldehart
ldehart

Reputation: 5

Issue with C++ Srand Values

So I'm creating a lottery ticket with random values and then sorting it. I'm not worried about the sorting technique since the teacher isn't looking for classes etc. on this assignment and it works, however, my ticket values that I produce - despite using srand (time(0)) and then later the rand() % 40 + 1 - which I think should make my randoms between 1-40... but mainTicket[0] always equals 0. Any ideas? Sorry about the formatting, made me add extra spaces and mess up my indention.

#include <iostream>
#include <cstdlib>
#include <time.h>

using namespace std;

void mainLotto (int main[]);
void lottoSort (int ticketArr[]);

int main()
{
int mainTicket[5];

srand (time(0));

mainLotto(mainTicket);

do
{
    lottoSort(mainTicket);
} while (mainTicket[0] > mainTicket[1] || mainTicket[1] > mainTicket[2] || mainTicket[2] > mainTicket[3] || mainTicket[3] > mainTicket[4] || mainTicket[4] > mainTicket[5]);

for (int i = 0; i < 5; i++)
{
    cout << mainTicket[i] << "\n\n";
}

return 0;
}

///
/// <> Creates the actual lottery ticket
///
void mainLotto (int main[])
{
// Creating the ticket
for (int i = 0; i < 5; i++)
{
    main[i] = rand() % 40 + 1;
}
}

///
/// <> Sorts the actual lottery ticket
///
void lottoSort (int ticketArr[])
{
 // Sorting the ticket
for (int j = 0; j < 5; j++)
{
    if (ticketArr[j] > ticketArr[j+1])
    {
        int temp;

        temp = ticketArr[j+1];

        ticketArr[j+1] = ticketArr[j];

        ticketArr[j] = temp;
    }
}
}

Upvotes: 0

Views: 74

Answers (2)

gsamaras
gsamaras

Reputation: 73444

I just printed the item you claim to be always zero, after the mainLotto() and it yielded 30.

I suspect the problem lies where you tell us not to look. :)

In the sorting function you do:

for (int j = 0; j < 5; j++) {
    if (ticketArr[j] > ticketArr[j + 1]) {

The array is of size 5. Your j will take eventually a value equal to 4.

Then you do ticketArr[j + 1], which is actually an out of bound access.

The fix is to go until 4 in your loop, not 5.

As Galik said, mainTicket[4] > mainTicket[5] is also an out of bounds access and after reading my answer, you should be able to understand why. :)

Upvotes: 2

Galik
Galik

Reputation: 48665

I see two problems with your arrays being accessed out of bounds:

Here:

int main()
{
    int mainTicket[5];

    srand(time(0));

    mainLotto(mainTicket);

    do
    {
        lottoSort(mainTicket);
    }
    while(mainTicket[0] > mainTicket[1] || mainTicket[1] > mainTicket[2]
        || mainTicket[2] > mainTicket[3] || mainTicket[3] > mainTicket[4]);
//      || mainTicket[4] > mainTicket[5]); // OUT OF BOUNDS!!!

    for(int i = 0; i < 5; i++)
    {
        cout << mainTicket[i] << "\n\n";
    }

    return 0;
}

And here:

void lottoSort(int ticketArr[])
{
    // Sorting the ticket
    for(int j = 0; j < 4; j++) // j < 4 NOT 5!!! <== WAS OUT OF BOUNDS
    {
        if(ticketArr[j] > ticketArr[j + 1])
        {
            int temp;

            temp = ticketArr[j + 1];

            ticketArr[j + 1] = ticketArr[j];

            ticketArr[j] = temp;
        }
    }
}

Likely the sort routine was dragging in a zero from out side the array bounds.

Upvotes: 2

Related Questions