c++ - Why is the use of rand() considered bad?

Question

I heard some guys telling that the use of rand() is bad EVEN AFTER USING srand() to get a seed. Why is that so? I want to know how the stuff happens... And sorry for another question.. but what is an alternative to this then?

Answer 1

There are two parts to this story.

First, rand is a pseudorandom number generator. This means it depends on a seed. For a given seed it will always give the same sequence (assuming the same implementation). This makes it not suitable for certain applications where security is of a great concern. But this is not specific to rand. It's an issue with any pseudo-random generator. And there are most certainly a lot of classes of problems where a pseudo-random generator is acceptable. A true random generator has its own issues (efficiency, implementation, entropy) so for problems that are not security related most often a pseudo-random generator is used.

So you analyzed your problem and you conclude a pseudo-random generator is the solution. And here we arrive to the real troubles with the C random library (which includes rand and srand) that are specific to it and make it obsolete (a.k.a.: the reasons you should never use rand and the C random library).

• One issue is that it has a global state (set by srand). This makes it impossible to use multiple random engines at the same time. It also greatly complicates multithreaded tasks.

• The most visible problem of it is that it lacks a distribution engine: rand gives you a number in interval [0 RAND_MAX]. It is uniform in this interval, which means that each number in this interval has the same probability to appear. But most often you need a random number in a specific interval. Let's say [0, 1017]. A commonly (and naive) used formula is rand() % 1018. But the issue with this is that unless RAND_MAX is an exact multiple of 1018 you won't get an uniform distribution.

• Another issue is the Quality of Implementation of rand. There are other answers here detailing this better than I could, so please read them.

In modern C++ you should definitely use the C++ library from <random> which comes with multiple random well-defined engines and various distributions for integer and floating point types.