Question

I am doing some c++ practice and trying to write a program to count the amount of times a dice combination is rolled after 10000 attempts. I have used a 2D array to store every possible dice combination, and I perform 10000 rand()%6+1 and increments the value in the memory allocation it randoms.

This is my attempt.

cout << "\nDice roll analyser" << endl;

const int first = 6;
const int second = 6;
int nRolls[first][second];
int count = 0;

while (count < 10000){
    nRolls[rand()%6+1][rand()%6+1]+=1;
    count++;
}

for (int i=0;i<first;i++){
    for (int j=0;j<second;j++){
        cout << nRolls[i][j] << " ";
    }
}

This is the output that I get;

0 0 0 0 0 0 0 269 303 265 270 264 228 289 272 294 290 269 262 294 303 277 265 294 288 266 313 274 301 245 317 276 292 284 264 260

What I am trying to achieve is the amount of times each combination is rolled e.g. how many times 1, 6 is rolled etc.

Answer 1

You never update your count.

For something where you want to run a code segment n times, where right now n = 10000, this is the general way you wanna do it.

for (int i = 0; i < 10000; ++i)
{
    //loop code.
}

additionally, myVariable+=1 can always be simplified to either ++myVariable or myVariable++ (if you aren’t use the value of myVariable right when you are assigning it, it is better to use the first one. More info on pre/post increment can be found here: http://gd.tuwien.ac.at/languages/c/programming-bbrown/c_015.htm

so instead of nRolls[rand()%6+1][rand()%6+1]+=1;
you can instead do

++(nRolls[rand()%6+1][rand()%6+1]);

Additionally, arrays are zero-indexed, meaning when you do rand()%6+1 you are restricting the values from 1 to 6 and leaving out the 0 position of an array, which is the first one, so consider instead just using

++(nRolls[rand()%6][rand()%6]);

then, to find out how often you roll a (i,j), where i and j are between 1 and 6,

cout << "(" << i << "," << j << "):" << nRolls[i-1][j-1] << endl;