Create a dice chart - c

Question

I am facing a problem right now and can't seem to solve it. Whenever I start my code I get a segmentation fault. My aim is to roll two dice. The value is generated by a random number. I want to roll 10000 times and save the values within an array, so I can create a little chart at the end which will show the diced the values. I appreciate any help and hints on how to solve the problem. Here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define DICE 2
#define DICEEYES (DICE * 6) -DICE +1
#define COUNT 10000
int diceRoll(int dice);
int main(void)
{
srand(time(0));
int valuesOfRoll[DICEEYES] = { 0 };
for(int i = 0; i < COUNT; i++)
{
int index = diceRoll(DICE) - DICE;
valuesOfRoll[index]++;
}
for(int i = 0; i < DICEEYES; i++)
{
if(valuesOfRoll[i] < 1) continue;
printf("The number %2d was rolled %4d times\r\n", i + DICE, valuesOfRoll[i]);
}
return 0;
}
int diceRoll(int dice)
{
int sum;
for(int i = 0; i < dice; i++)
{
sum += rand() % 6 + 1;
}
return sum;
}

Answer 1

You should initialize sum in the function diceRoll(),
/* always return value between [dice..dice*6]*/
int diceRoll(int dice)
{
int sum=0;
for(int i = 0; i < dice; i++)
{
sum += rand() % 6 + 1;
}
return sum;
}
Since sum could have any value (on the stack), you were using an index value that could fall outside the ValuesOfRoll[] array. One way to avoid is to use the modulus operator to limit the returned index to a valid range.
int main(void)
{
srand(time(0));
int valuesOfRoll[DICEEYES] = { 0 };
for(int i = 0; i < COUNT; i++)
{
int index = diceRoll(DICE) - DICE;
/* limit index to [0..DICEEYES] */
index %= DICEEYES;
valuesOfRoll[index]++;
}
for(int i = 0; i < DICEEYES; i++)
{
if(valuesOfRoll[i] < 1) continue;
printf("The number %d was rolled %4d times", i + DICE, valuesOfRoll[i]);
}
return 0;
}

Answer 2

Initialise the value of sum in your function (preferably to 0), otherwise it will hold some garbage value.
int diceRoll(int dice)
{
int sum;
for(int i = 0; i < dice; i++)
{
sum += rand() % 6 + 1;
}
return sum;
}
The garbage value may be a very large number, and so will be the returned value of the function. So, when the returned value is used in this line:
int index = diceRoll(DICE) - DICE;
then the array will have an index of that large number, valuesOfRoll[some_large_integer]++;. This causes an out-of-array-bound situation, and causes seg-fault.

Answer 3

Slightly belatedly, the faults are not initialising sum, and DICEEYES.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define DICE 2
#define DICEEYES (DICE * 6)
#define COUNT 10000
int diceRoll(int dice)
{
int i, sum = 0; // need to initialise!
for(i = 0; i < dice; i++)
sum += rand() % 6 + 1;
return sum;
}
int main(void)
{
int i;
int valuesOfRoll[DICEEYS+1] = { 0 }; // size of array
srand((unsigned)time(0));
for(i = 0; i < COUNT; i++)
valuesOfRoll[diceRoll(DICE)]++;
for(i = DICE; i <= DICEEYES; i++)
printf("The number %2d was rolled %4d times\n", i, valuesOfRoll[i]);
return 0;
}
Program output:
The number 2 was rolled 264 times
The number 3 was rolled 573 times
The number 4 was rolled 798 times
The number 5 was rolled 1145 times
The number 6 was rolled 1373 times
The number 7 was rolled 1697 times
The number 8 was rolled 1385 times
The number 9 was rolled 1143 times
The number 10 was rolled 835 times
The number 11 was rolled 528 times
The number 12 was rolled 259 times

Answer 4

the following code
1) cleanly compiles
2) has corrections to the output formatting
3) properly initializes values (good case for always initialize all values)
4) outputs the proper data
5) does not try to fiddle offset/indexes, etc
6) uses meaningful #define names, function parameter names, etc
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define NUM_DICE (2)
#define NUM_ROLLS (10000)
#define MIN_DICEROLL_VALUE (2)
#define MAX_DICEROLL_VALUE (12)
int diceRoll(int numDice);
int main(void)
{
srand(time(NULL));
int valuesOfRoll[MAX_DICEROLL_VALUE+1] = { 0 }; // notice the +1
for(int i = 0; i < NUM_ROLLS; i++)
{
int index = diceRoll(NUM_DICE);
valuesOfRoll[index]++;
}
for(int i = MIN_DICEROLL_VALUE; i <= MAX_DICEROLL_VALUE; i++)
{
// 2d for values 2...12 5d for values 0...10000 \n so each output on anew line
printf("The number %2d was rolled %5d times\n", i, valuesOfRoll[i]);
}
return 0;
} // end function: main
int diceRoll(int numDice)
{
int sum = 0; // be sure to initialize value
for(int i = 0; i < numDice; i++)
{
sum += (rand() % 6) + 1;
}
return sum;
} // end of function: diceRoll