Generate random numbers with rand() except number zero

Question

I would like to generate random numbers between 1 and 25 with the method rand().
But I only know how to generate random numbers this way, which includes the number zero by default:

int r = rand() % 26 /* random int between 0 and 25 */

Anyone? Thank you.

Iharob Al Asimi · Accepted Answer · 2014-12-30T21:54:15.003

4

Very simple

int r = 1 + rand() % 25 /* random int between 1 and 25 */

but you should use this

int r = (int)(1.0 + 25.0 * rand() / RAND_MAX)

as mentioned in the comments, the second is the more robust way to generate random numbers see this link

edited Dec 30 '14 at 21:54

answered Dec 30 '14 at 21:32

Iharob Al Asimi

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This method may not work, depending on how strict requirements for uniformity are. – void_ptr Dec 30 '14 at 21:35
@void_ptr can you elaborate please. – Iharob Al Asimi Dec 30 '14 at 21:39
http://c-faq.com/lib/randrange.html – Alok-- Dec 30 '14 at 21:40
The rand() generator and the % 25 does not align. This is an important technicality if you want fair number generation. – Captain Giraffe Dec 30 '14 at 21:40
It worked for me though! Thank you. I'll use the 2nd method. – Julia Dec 30 '14 at 21:44
Over time you'll learn that "It worked" doesn't mean it's right. void_ptr is right that using the simple mod method (and the advanced bit of math you missed--adding 1) isn't quite uniform, though it's probably OK for 90% of uses (if you're just doing a simple game, selecting a random piece of music, etc.). – Lee Daniel Crocker Dec 30 '14 at 21:49
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Why `(1.0 + RAND_MAX)`? – dtech Dec 30 '14 at 21:53

Generate random numbers with rand() except number zero

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