Strange behaviour in a Dice Throwing simulation program-CodePudding

I have written this piece of code which is supposed to simulate throwing dice many many times and counting that how many times each face is up. I have attached the output down there and as you can see it looks kind of strange. For example face 5 comes up exactly 10 times, faces 2, 3, 4 are about the same and face 6 comes zero in two rounds. The only face which acts about normal is 1. Can anyone explain this to me? Is this normal? Am I doing something wrong or is it something related to my system?

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void) {

    unsigned long int freq1, freq2, freq3, freq4, freq5, freq6;
    unsigned long int L00p = 1;
    unsigned short int DF;

    while (L00p <= 6e7){
        srand (time(NULL));
        DF = 1   (rand ()%6);
        switch (DF)
        {
        case 1:
              freq1;
            break;
        case 2:
              freq2;
            break;
        case 3:
              freq3;
            break;
        case 4:
              freq4;
            break;
        case 5:
              freq5;
            break;
        case 6:
              freq6;
            break;
        default:
            break;}
          L00p;
    }

    printf ("%s%s\n", "Dice's Face", "Face Frequency");
    printf ("1%lu\n", freq1);
    printf ("2%lu\n", freq2);
    printf ("3%lu\n", freq3);
    printf ("4%lu\n", freq4);
    printf ("5%lu\n", freq5);
    printf ("6%lu\n", freq6);


    return 0;
}

and here is the program's output after four times running it:

CodePudding user response：

You don't initialize the frequency counters, so they'll likely contain garbage from the stack. (So yes, you were getting randomness, but not the randomness you want.)
You don't want to call srand() in the loop, but only once before it. Calling srand() with the same number (and time(NULL) will quite inevitably return the same second in a tight loop) will reset the rand() generator to return the same sequence of numbers, and since you only ever call rand() once before calling srand() again, you'll get a whole bunch of the same number.

The following version works fine, but you'd have a better time with an array.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void) {
  unsigned long int freq1 = 0, freq2 = 0, freq3 = 0, freq4 = 0, freq5 = 0,
                    freq6 = 0;
  srand(time(NULL));

  for (int loop = 0; loop < 1000; loop  ) {
    int DF = 1   (rand() % 6);
    switch (DF) {
    case 1:
        freq1;
      break;
    case 2:
        freq2;
      break;
    case 3:
        freq3;
      break;
    case 4:
        freq4;
      break;
    case 5:
        freq5;
      break;
    case 6:
        freq6;
      break;
    default:
      break;
    }
  }

  printf("%s%s\n", "Dice's Face", "Face Frequency");
  printf("1%lu\n", freq1);
  printf("2%lu\n", freq2);
  printf("3%lu\n", freq3);
  printf("4%lu\n", freq4);
  printf("5%lu\n", freq5);
  printf("6%lu\n", freq6);

  return 0;
}

Example output:

Dice's Face           Face Frequency
1                      177
2                      160
3                      166
4                      169
5                      155
6                      173

For reference, a version using an array of 6 ints:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void) {
  unsigned long int freqs[6] = {0};
  srand(time(NULL));

  for (int loop = 0; loop < 1000; loop  ) {
    freqs[rand() % 6]   ;
  }

  printf("%s%s\n", "Dice's Face", "Face Frequency");
  for(int face = 0; face < 6; face  ) {
    printf("%d%lu\n", face   1, freqs[face]);
  }

  return 0;
}

CodePudding user response：

Here is an annotated adaptation of your code for educational purposes. You've learned about "loops", so here is an application for a do/while() loop.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

// Global variables are frowned upon because as the code grows more complex
// it is difficult or impossible to see where a value may be changed (inappropriately)

// For a tiny program like this that is unlikely to grow
// this proves the global variables are, by default, initialised to zero.

unsigned long int freq1, freq2, freq3, freq4, freq5, freq6;

int main() {
    unsigned long int L00p = 0; // "local" var initialised. Good!

    srand( time( NULL ) ); // called once at start of program.

    do {
        switch( ( rand() % 6 ) ) { // braces consistent with your main()
            case 1:   freq1; break; // get used to base-0 counting
            case 2:   freq2; break;
            case 3:   freq3; break;
            case 4:   freq4; break;
            case 5:   freq5; break;
            case 0:   freq6; break; // Ha-ha!! modulo!!!
            default: printf( "A miracle has happened!!\n" );
                break;
        } // DO NOT hide that closing brace as you did. Prominent!
    } while(   L00p < 6e3 ); // increment counter after each loop done

    // Swapped your output columns
    // Using one format specifier for header and one for counts
    // Notice how easy to modify only one instance?
    char *tFmt = "%9s : %s   Loops = %d\n";
    char *oFmt = "%9lu : %d\n";

    printf( tFmt, "Frequency", "Face", L00p );

    // and... why not???
    for( L00p = 0; L00p < 6; L00p   ) {
        int n; // not init'd because used immediately
        switch( L00p ) {
            case 1: n = freq1; break;
            case 2: n = freq2; break;
            case 3: n = freq3; break;
            case 4: n = freq4; break;
            case 5: n = freq5; break;
            case 0: n = freq6; break;
        }
        printf( oFmt, n, L00p   1 );
    }

    return 0;
}

Output

Frequency : Face   Loops = 6000
      958 : 1
     1038 : 2
     1018 : 3
     1031 : 4
      956 : 5
      999 : 6

Again, for a simple, small piece of code like this, being able to see the entire switch block and compare values at a glance, concatenating statements can AID in writing bug-free code.