Random double between min and max

Question

double get_random(double min, double max) {
  /* Returns a random double between min and max */

  return min * ((double) rand() / (double) RAND_MAX) - max;
}

That's my function to generate random doubles between a min and a max. However, when I call get_random(-1.0, 1.0);, I get values between -2.0 and -1.0.

Any idea of what I'm doing wrong and how I can fix it?

(rand/RAND_MAX) generates a number between 0.0 and 1.0. So put those two extremes into the rest of the equation, and you should see why it doesn't work. — Oliver Charlesworth, May 27 '12 at 18:00
see: http://stackoverflow.com/questions/686353/c-random-float — Trevor Hickey, May 27 '12 at 18:05

score 21 · Accepted Answer · answered May 27 '12 at 18:01

21

Shouldn't the formula be

(max - min) * ( (double)rand() / (double)RAND_MAX ) + min

(double)rand() / (double)RAND_MAX returns a random number between 0 and 1
(max - min) * ( (double)rand() / (double)RAND_MAX ) returns a random number between 0 and max - min.
the whole expression will return a random number between 0 + min and min + (max-min) - i.e. min and max.

answered May 27 '12 at 18:01

Luchian Grigore

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Just as a warning: this method will probably generate a small fraction, well under 1%, of possible random numbers in your selected range. Use `` (if available) for higher-quality random doubles. – Robert Cooper May 27 '12 at 19:13

score 1 · Answer 2 · edited Jun 24 '13 at 03:20

1

You can use this for generating random double or floating numbers:

((double) rand()*(max-min)/(double)RAND_MAX-min);

edited Jun 24 '13 at 03:20

jogojapan

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answered Jun 24 '13 at 03:05

user2514807

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Is there any specific advantage of this compared to the accepted answer? Could you explain more? – jogojapan Jun 24 '13 at 03:20
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I agree with @jogojapan, and the final `-min` is the only difference from the accepted answer of `+min` (as far as I can see), thus this answer is wrong. – Ken Y-N Jun 24 '13 at 03:27

Random double between min and max

2 Answers2