rand() and RAND_MAX giving different values depending upon the datatype

Question

Here when I print the value of x its giving zero as output.Whereas when I print y, I am getting correct value(a random number between 0 and 1),the typecasting is the problem it seems.Why do i need to typecast it?

double x,y;
x=rand()/RAND_MAX;
printf("X=%f\n",x);
y=(double)rand()/RAND_MAX;
printf("Y=%f",y);

Output

X=0.000000
Y=0.546745

Close voters: This should be closed as a duplicate. It's not "asking us to recommend or find a book, tool, software library, tutorial or other off-site resource". — Millie Smith, Mar 19 '16 at 19:22
Possible duplicate of [Does "n \* (rand() / RAND\_MAX)" make a skewed random number distribution?](http://stackoverflow.com/questions/10219355/does-n-rand-rand-max-make-a-skewed-random-number-distribution) — Evan Carslake, Mar 19 '16 at 20:36
Why would you ever use `rand()`? Its almost as bad as the [xkcd rng](https://xkcd.com/221/). — CodesInChaos, Mar 19 '16 at 22:41
@CodesInChaos random numbers have applications in areas that don't require cryptographically unpredictable sequences. — Millie Smith, Mar 19 '16 at 23:50

score 1 · Answer 1 · answered Mar 19 '16 at 19:06

1

When you divide an integer by an integer, you get truncating integer division. So using

y = (double)rand() / RAND_MAX;

is absolutely the right way to get the result you want.

answered Mar 19 '16 at 19:06

Steve Summit

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score 0 · Answer 2 · answered Mar 19 '16 at 20:55

Different types of division yielded different responses.

// Integer division
x=rand()/RAND_MAX;

// floating-point division
y=(double)rand()/RAND_MAX;

It isn't the a cast is needed, but it is one of the ways to insure floating-point division. I like the last one as it insures the division will be done at least to the precision of x, be it float, double or long double without changing code.

x = (double)rand()/RAND_MAX;

x = 1.0*rand()/RAND_MAX;

x = rand()/(1.0*RAND_MAX);

x = rand();
x /= RAND_MAX;

BTW, often code needs to generate a number [0.0 ... 1.0): from 0.0 to almost 1.0.

x = rand();
x /= RAND_MAX + 1.0;

rand() and RAND_MAX giving different values depending upon the datatype

2 Answers2