Generate random numbers uniformly over an entire range

Question

I need to generate random numbers within a specified interval, [max;min].

Also, the random numbers should be uniformly distributed over the interval, not located to

Answer 1

[edit] Warning: Do not use rand() for statistics, simulation, cryptography or anything serious.

It's good enough to make numbers look random for a typical human in a hurry, no more.

See @Jefffrey's reply for better options, or this answer for crypto-secure random numbers.

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Generally, the high bits show a better distribution than the low bits, so the recommended way to generate random numbers of a range for simple purposes is:

((double) rand() / (RAND_MAX+1)) * (max-min+1) + min

Note: make sure RAND_MAX+1 does not overflow (thanks Demi)!

The division generates a random number in the interval [0, 1); "stretch" this to the required range. Only when max-min+1 gets close to RAND_MAX you need a "BigRand()" function like posted by Mark Ransom.

This also avoids some slicing problems due to the modulo, which can worsen your numbers even more.

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The built-in random number generator isn't guaranteed to have a the quality required for statistical simulations. It is OK for numbers to "look random" to a human, but for a serious application, you should take something better - or at least check its properties (uniform distribution is usually good, but values tend to correlate, and the sequence is deterministic). Knuth has an excellent (if hard-to-read) treatise on random number generators, and I recently found LFSR to be excellent and darn simple to implement, given its properties are OK for you.