`rand()%n` is wrong

For a first post, we’ll start with an idea that’s come up twice for me in the last few weeks.

Lately, I’ve been exploring writing more software in C & C++, in an attempt to start solving problems in new domains. During that effort, I’ve been reading Accelerated C++¹. In that book, I came across a brief discussion² of the issues that arise when trying to map the uniform random numbers generated by rand().

What’s wrong with `rand()%n`?

If somebody had asked me to generate a uniform random number from $1$ – $N$ , I would have said sure, no problem:

#include <stdlib.h>
int naive_rand(int n) {
    return (rand() % n) + 1;
}

And this will definitely generate a pseudo-random number, assuming you seed rand(). But that number won’t be uniform.

Consider rand()’s summary:

Returns a pseudo-random integer value between 0 and RAND_MAX (0 and RAND_MAX included).

From the rand() docs

Ok, so let’s see. Assume RAND_MAX is 16, and we want a random number between 1 & 7. Let’s see how values of RAND_MAX map to output values.

`rand()` output	`naive_rand()` output
0	1
1	2
2	3
3	4
4	5
5	6
6	7
7	1
8	2
9	3
10	4
11	5
12	6
13	7
14	1
15	2
16	3

This generation method results in 1, 2, and 3 having a better chance of being generated than the other numbers! In fact, the chance of getting a 1–3 is about 50% more likely than getting a 4–7.

The problem here is in the mapping. The numbers 0–16 are generated uniformly, but they don’t uniformly map to 0–6, using the modulo operator.

At the time, I didn’t realize that this phenomenon has a name – modulo bias.

A simple approach to fix modulo bias: rejection sampling

To account for this bias, we can think of our output values $1 - N$ as “buckets” to choose from, and select one of the buckets by distributing all the values from $[0, R A N D_M A X]$ evenly into our $N$ buckets.

Once we have fewer than $N$ elements left to distribute (so the buckets would have uneven numbers), we simply throw the rest of the values away.

With this strategy, output values that fall outside the range that can be evenly distributed have an equal number of potential inputs that will cause them to be selected.

The following implementation satisfies this strategy:

int uniform_rand(int n)
{
  // n is the "number of buckets" we want to use
  const int bucket_size = RAND_MAX / n;
  int result;
  do {
    result = rand() / bucket_size;
  } while (result >= n);

  return result + 1;
}

That gives us the following result:

`rand()` output	`unfiorm_rand()` output
0	1
1	2
2	3
3	4
4	5
5	6
6	7
7	1
8	2
9	3
10	4
11	5
12	6
13	7
14	NO MAPPING – Try again
15	NO MAPPING – Try again
16	NO MAPPING – Try again

How slow is this?

At first glance, this seems obviously slow. If nothing else, there’s a loop in there now. The question then, is: “How many times does the loop execute?”

To understand how many times we’d expect the loop to run, we need to know the probability that a given rand() call will give us a number outside of the appropriate range (a “discarded” number). That probability is: $\frac{# d i s c a r d e d v a l u e s}{R A N D_M A X}$

To count this set, we first need to identify it. Can we determine the worst-case scenario of “discarded” values?

We’ll do this in two steps:

Given some bucket count, $N$ , how many elements are discarded in the worst case?
How does that value compare to RAND_MAX?

Finding the worst-case number of discarded elements

Let’s try examples.

First, assume $N = 2$ . When we have two buckets, we’ll distribute the elements from 0 – RAND_MAX into each bucket alternatingly. If there are an even number of elements in the range, we’ll distribute all of them. (Nothing thrown away). If there’s an odd number, we’ll have to keep 1 element not distributed. So, for N=2, we throw away, at most, 1 value, no matter what RAND_MAX is.

Next, let’s try $N = 3$ . In this case, we do the same thing, adding one element to each bucket, until we are out of elements. Now, if RAND_MAX was a multiple of 3, then each bucket would get the same number. Otherwise it’s either one or two away from a multiple of 3, so either one or two elements are left over. Thus, the worst case for $N = 3$ is 2.

… And so it goes. Here’s the worst-case for $N = 7$ :

Continuing on, we find a pattern: the “worst-case” scenario is throwing away $N - 1$ values.

Comparing discarded elements to `RAND_MAX`

We’ve found that $N - 1$ is the worst-case number of elements we will discard. Let’s see what value of RAND_MAX will mean that those invalid $N - 1$ elements are chosen as frequently as possible.

Now, obviously we have to distribute at least one element into every bucket, because otherwise there would be values in $[1, N]$ we could not choose!

The “key insight” here is that, to get the worst-case scenario, we shouldn’t distribute more than one value per bucket, since distributing more values than that just makes it more likely that we’ll select a “good” value!

And thus we’ve found our worst-case scenario:

After distributing $N$ elements (one per bucket) from RAND_MAX, there are $N - 1$ elements remaining.

Or, algebraically: $\begin{aligned} R A N D_M A X - N & = N - 1 \\ ⟹ 2 N & = R A N D_M A X + 1 \\ ⟹ N & = ⌊ \frac{R A N D_M A X}{2} ⌋ \end{aligned}$

So, the worst case number of thrown-away values occurs when $N \approx \frac{R A N D_M A X}{2}$

Plugging that back into our original expression for probability of “looping again”: $\frac{# d i s c a r d e d v a l u e s}{R A N D_M A X} \approx \frac{R A N D_M A X / 2}{R A N D_M A X} = \frac{1}{2}$

So the probability of generating a usable number is $\approx \frac{1}{2}$ , And thus, we expect to run the loop about twice per call. Not too shabby; that’s still constant time!

Practical application

In general, this isn’t often all that practically useful, considering most languages give you a function with a uniform distribution from $0 - 1$ , which maps very easily to $N$ . But modulo bias as a concept is generally useful.

Further Research

There are more sophisticated techniques for working around modulo bias, so at some point I’d love to learn more of them! If you already know some and you’d like to explain them to me, please feel free to drop me a line.

You may ask why someone would be reading a 23-year-old book on C++ in the year 2022. I like books, and I find that I learn technologies best by following their historical progression. Obviously your mileage may vary.↩︎
Section §7.2.2, for those interested↩︎

What’s wrong with rand()%n?