10.7. Random numbers


Most computer programs do the same thing every time they are executed, so they are said to be deterministic. Usually, determinism is a good thing, since we expect the same calculation to yield the same result. For some applications, though, we would like the computer to be unpredictable. Games are an obvious example.

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 generate pseudorandom numbers and use them to determine the outcome of the program. Pseudorandom numbers are not truly random in the mathematical sense, but for our purposes, they will do.

C++ provides a function called random that generates pseudorandom numbers. It is declared in the header file cstdlib, which contains a variety of “standard library” functions, hence the name.

The return value from random is an integer between 0 and RAND_MAX, where RAND_MAX is a large number (about 2 billion on my computer) also defined in the header file. Each time you call random you get a different randomly-generated number. To see a sample, run this loop:

Take a look at the active code below, which generates 4 random numbers.

On my machine I got the following output:


You will probably get something similar, but different, on yours.

Of course, we don’t always want to work with gigantic integers. More often we want to generate integers between 0 and some upper bound. A simple way to do that is with the modulus operator. For example:

int x = random ();
int y = x % upperBound;

Since y is the remainder when x is divided by upperBound, the only possible values for y are between 0 and upperBound - 1, including both end points.


Keep in mind, though, that y will never be equal to upperBound.

The active code below generates random numbers between 1 and 7.

It is also frequently useful to generate random floating-point values. A common way to do that is by dividing by RAND_MAX. For example:

int x = random ();
double y = double(x) / RAND_MAX;

This code sets y to a random value between 0.0 and 1.0, including both end points. As an exercise, you might want to think about how to generate a random floating-point value in a given range; for example, between 100.0 and 200.0.

The active code below generates random numbers between 0 and 1. Can you modify it to generate random numbers between 100.0 and 200.0? If you’re stuck you can reveal the hint below!

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