random number generation how?

I was wondering how the random number functions work. I mean is the server time u开发者_如何学Gosed or which other methods are used to generate random numbers? Are they really random numbers or do they lean to a certain pattern? Let's say in python:

import random
number = random.randint(1,10)

Random number generators vary (of course) by different platform, but in general, they're only "pseudo-random" numbers. That is, the "random" numbers are generated by an algorithm that is chosen to provide a distribution of numbers that's reasonably even and with a statistical distribution similar to what one would expect of true randomness. These random number generators typically take a "seed" value, which is used to initiate the "sequence"; usually, the same "seed" value will return the same "random" number (indicating that it's clearly not actually "random").

One can obtain reasonable pseudorandom results, however, by seeding the "random" number function with a rapidly changing number, such as the time (in ticks) from the machine, or other varying seed values. That doesn't change the fact, however, that these "random" numbers aren't really random; however, for most purposes, they can be considered "good enough".

One note as an addendum: there are actual random number generators that are hardware based that can be purchased and used that actually are random. These typically depend on the measurement of a varying quantity, such as the number of photons received by a detector, and biased such that they return truly random values. These are relatively rare, however.

Yes, time is typically used to seed a random number generator when it's not important that the numbers be unpredictable. For example, if you are displaying random images in a slideshow then the time is a good value to use so that the sequence of images isn't the same the next time you run the slideshow.

However, since the time is known by everyone to a high degree of accuracy, this would be a terrible seed for crypto purposes. Netscape used to use this method and it was shown to be vulnerable to attack. Nowadays secure random numbers are generated using entropy gathered by devices like mouse movement and microphone input. "Headless" network devices use characteristics of its network traffic as a more-or-less unpredictable entropy source. For really special applications sometimes hardware randomness sources are used like cameras and Geiger counters. On unix systems you can get secure random numbers from /dev/random and it'll block if there's not "enough entropy" (estimated through a counter) to guarantee secure randomness.

Its pseudo random number generator, exact working depends on implementation, but I assume it's some kind of c implementation of Mersen-Twister: http://docs.python.org/library/random.html (Third paragraph)

Oh, and exact function randint is built upon base random function. Random returns real number from range (0,1], and randint(a,b) returns integer from range [a,b] and can be implemented as lambda a,b: int(a + random.random()*(b+1-a))

Depending on your background you might like Numerical Recipes. I am a physicist and I really like this book (even though mathematicians occasionally write bad things about it, it gives nice overviews on a lot of topics).

See chapter 7 for a nice introduction into random numbers.