Barbara Ericson, Allen B. Downey, Jason L. Wright (Editor)
Section10.7Random 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 (pseudo- meaning "fake"). It is declared in the header file cstdlib, which contains a variety of “standard library” functions, hence the name.
random is not a perfect random number generator, but it is good enough for the simple purposes we are using it for. If you are doing serious work that requires high-quality random numbers, like cryptography, you should use a different library.
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:
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:
Notice that we cast x to a double before dividing by RAND_MAX. This is because RAND_MAX is an integer, and if we divided two integers, the result would also be an integer, and we would lose the fractional part.
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.
This active code 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!
If we wanted to generate a random number between 0 and 12, and we have previously declared int int x = random();, what should be our next line of code?