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Ask the Wizard Questions

Random number behavior question

The Question is:

I have noticed that the rand function in the C run-time
library returns a sequence of odd number followed by even
number. Can you tell me why this is. It happens with VAXC
on VAX and DEC C on Alpha. This is segment of code which
shows the behaviour.


int main()

    int i;
    int seed;
    char buffer[80];

    printf("enter a seed :");
    seed = atoi(buffer);

    if (seed != 0)

    /* just print low order bit to evaluate odd or even */

    for (i = 0; i < 100; i++)
        printf("random number = %d\n", (rand() & 0x1));


The Answer is:

The algorithm used is the one specified by the ANSI C standard.  It is not
very good, as you have discovered.  For a better routine, see the description
of "random" in the DEC C Run-Time Library Reference Manual.