pseudo-random number generator


Definition: A deterministic algorithm to generate a sequence of numbers with little or no discernible pattern in the numbers, except for broad statistical properties.

Also known as PRNG, deterministic random bit generator, DRBG.

Generalization (I am a kind of ...)
deterministic algorithm.

Specialization (... is a kind of me.)
linear congruential generator.

See also randomized algorithm.

Note: Any computer program is likely to generate pseudo-random numbers, not actually random numbers. This is important when, say, simulations are sensitive to subtle patterns in the "random" numbers used. Hardware-based random number generators are built from parts with naturally random events, such as noise in a diode.

Pseudorandom number generators are often based on a Deterministic Random Bit Generator (DRBG).

A generator is often "seeded", or initialized, from sources of entropy, such as the current time in microseconds, response time to a disk seek, or other random events to give different sequences every time it is used.

Do NOT use typical "random" number generators for security or cryptographic purposes. Random Numbers from David Wheeler's Secure Programming for Linux and Unix HOWTO, Section 11.3, gives suggestions and guidelines.

comic with the lines 'lets do something random' 'Sorry, living creatures are notoriously terrible at generating true random numbers.'
From Dragon Tails. Used by permission. Copyright © 2004 Tim Dawson.

a C function, named get random number, that always returns    the number 4.  It has comments 'chosen by fair dice roll.' and   'guaranteed to be random.'
From xkcd 221 by Randall Munroe. Creative Commons Attribution-NonCommercial 2.5 License.

Author: PEB


(C++, C, and Fortran). pLab's page on generators including links to software and papers. GAMS (C). Using C libraries to get random numbers in a certain range (C) is C FAQ question 13.16, C FAQ question 12.9 as of 1995 (C), or section 7.8.7 (C) of Steve Summit's Notes to Accompany The C Programming Language.

More information

Random Number Generation with links to reports, standard tests, and on-going research. ent: a program to test the randomness of bytes in a file. Niels Ferguson and Bruce Schneier's Fortuna is a cryptographically strong pseudorandom number generator. The description of Fortuna, Sect. 9.3 of Ferguson, Schneier, and Kohno's "Cryptography Engineering: Design Principles and Practical Applications", explains trade-offs and cautions.

Recommendation for Random Number Generation Using Deterministic Random Bit Generators, NIST SP 800-90A, Revision 1, June 2015 has recommendations for generating (pseudo)random numbers using deterministic random bit generators, for entropy sources, and for implementation. Available at

Go to the Dictionary of Algorithms and Data Structures home page.

If you have suggestions, corrections, or comments, please get in touch with Paul Black.

Entry modified 24 August 2017.
HTML page formatted Tue Jan 16 10:34:44 2018.

Cite this as:
Paul E. Black, "pseudo-random number generator", in Dictionary of Algorithms and Data Structures [online], Vreda Pieterse and Paul E. Black, eds. 24 August 2017. (accessed TODAY) Available from: