Help with a do...while array

I started working my way through "C All-In-One Desk Reference For Dummies" by Dan Gookin. I've made it to page 240 and got stuck on a do...while statement. The program generates 6 lotto numbers and disregards any duplicates. The snip-it of code I'm stuck on is:

Code:

---

do
{
    ball = random() % RANGE;
}
while(numbers[ball];

---

This segment of the code is apparently checking to make sure the random number generated hasn't already been added to the array but I cannot figure out what calculation/check while(numbers[ball]) is performing to spin the loop. There is no operator present. Any help would be appreciated.

Here is the full code:

Code:

---

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

#define RANGE 50
#define BALLS 6

int main()
{
    int numbers[RANGE]; // 0 to 49
    int c,ball;

    puts("L O T T O  P I C K E R\n");

    srandom((unsigned)time(NULL));

    /* initialize the array */

    for(c=0;c<RANGE;c++) // if c < 50
        numbers[c]=0;

    printf("Press Enter to pick this week's numbers:");
    getchar();

/* draw the numbers */
    puts("Here they come:");
    for(c=0;c<BALLS;c++)
    {
   
/* see if a number has already been drawn */
        do
      {
            ball = random() % RANGE;
      }
        while(numbers[ball]);

/* Number drawn */
        numbers[ball] = 1;
        printf(" %i ",ball+1);
    }

    printf("\n\nGood luck in the drawing!\n");
    return(0);
}

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What do you think this line is doing?

Code:

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numbers[ball] = 1;

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It is recording that the number that matches the value of ball has been picked. The while checks to verify the number has not yet been picked.

Tim S.

Quote:

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what calculation/check while(numbers[ball]) is performing

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Code:

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#include <stdio.h>

int value = 0 ;

void printValue() {
  if (value) printf( "TRUE" );
  if (!value) printf( "FALSE" );
  printf( "\n" );
}

int main( int argc, char *argv[] ) {
 
  value = 0; printValue();
  value = 1; printValue();
  value = 10; printValue();
  value = -10; printValue();
  value = 0; printValue();
  value = 1; printValue();
 
  return 0;
}

---

Code:

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FALSE
TRUE
TRUE
TRUE
FALSE
TRUE

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> There is no operator present.
But in the context of an expression requiring a boolean expression,
while(numbers[ball])
is the same as
while(numbers[ball] != 0)

Since you then go onto do numbers[ball] = 1, the next time you enter that loop, if you pick the same ball again, the while loop will do it's thing.

Thank you, everyone! It makes sense now.