Question

I have an issue understanding the “do – while” statement in C.

Here is the complete code: http://pastebin.com/uPRvRscd

The program generates 6 numbers ranged from 0 to 50.
None of the 6 numbers repeats.

This is the do-while loop:

for(c=0;c<BALLS;c++)
{
    /* See if a number has been allready been drawn */
    do
    {
        ball = rand() % RANGE; /* Generate the random ball */
    }
    while(numbers[ball]); /* How is this compare made ? */
    /* Number drawn */
    numbers[ball] = 1; /* What is this for ?!?  */
    printf("%2d ", ball+1); /* add 1 to ball so ball won't be zero */
}

• How does the logical compare work ?

I know that the DO depends if the WHILE is true or false.

numbers[ball] = 1; What is this supposed to do ? ( if i remove it the result is the same )

Thank you

Answer 1

C has a rule that “anything that is zero” is false, everything else is true. So when you write if(x) it is the same as if (x != 0) and if(!x) means if (x == 0).

Same wit conditions in for, while and do – while.

So your code does:

do
{
    ball = rand() % RANGE; /* Generate the random ball */
}
while(numbers[ball] != 0);

I assume that numbers is an array of 50, that is filled with zero [if it’s of static storage duration, then it’s automatically set to zero if nothing else is stated.]

When a number has been drawn numbers[ball] = 1; sets that number to non-zero, so if we draw the same number again, the do-while loop will loop again and pick another number [we hope – if the random number generator is really rubbish, it may turn into an infinite loop]