The C++Course provides a general introduction to programming in C++. It is based on A.B. Downey's book, How to Think Like a Computer Scientist. Click here for details.

Random Seeds

If you have run the code in this chapter a few times, you might have noticed that you are getting the same "random" values every time. That's not very random!

One of the properties of pseudorandom number generators is that if they start from the same place they will generate the same sequence of values. The starting place is called a seed; by default, C++ uses the same seed every time you run the program.

While you are debugging, it is often helpful to see the same sequence over and over. That way, when you make a change to the program you can compare the output before and after the change.

If you want to choose a different seed for the random number generator, you can use the srand function. It takes a single argument, which is an integer between 0 and RAND_MAX.

For many applications, like games, you want to see a different random sequence every time the program runs. A common way to do that is to use a library function like gettimeofday to generate something reasonably unpredictable and unrepeatable, like the number of milliseconds since the last second tick, and use that number as a seed. The details of how to do that depend on your development environment.

Last Update: 2005-11-21