# 4.2: Random Numbers

- Page ID
- 15433

Given the same inputs, most computer programs generate the same outputs every time, so they are said to be **deterministic**. Determinism is usually a good thing, since we expect the same calculation to yield the same result. For some applications, though, we want the computer to be unpredictable. Games are an obvious example, but there are more.

Making a program truly nondeterministic turns out to be not so easy, but there are ways to make it at least seem nondeterministic. One of them is to use algorithms that generate **pseudorandom** numbers. Pseudorandom numbers are not truly random because they are generated by a deterministic computation, but just by looking at the numbers it is all but impossible to distinguish them from random.

The `random`

module provides functions that generate pseudorandom numbers (which I will simply call “random” from here on).

The function `random`

returns a random float between 0.0 and 1.0 (including 0.0 but not 1.0). Each time you call `random`

, you get the next number in a long series. To see a sample, run this loop:

import random for i in range(10): x = random.random() print x

The function `randint`

takes parameters `low`

and `high`

and returns an integer between `low`

and `high`

(including both).

>>> random.randint(5, 10) 5 >>> random.randint(5, 10) 9

To choose an element from a sequence at random, you can use `choice`

:

>>> t = [1, 2, 3] >>> random.choice(t) 2 >>> random.choice(t) 3

The `random`

module also provides functions to generate random values from continuous distributions including Gaussian, exponential, gamma, and a few more.

Exercise \(\PageIndex{1}\)

*Write a function named choose_from_hist that takes a histogram as defined in Section 11.1 and returns a random value from the histogram, chosen with probability in proportion to frequency. For example, for this histogram:*

>>> t = ['a', 'a', 'b'] >>> hist = histogram(t) >>> print hist {'a': 2, 'b': 1}

*your function should return *`’a’`

* with probability 2/3 and *`’b’`

* with probability 1/3.*