Why would rand() return a negative value when min and max values are positive?

Question

I have a simple piece of PHP code which requires a random number to be created. However, even though the input is always positive, it sometimes returns a negative output.

Answer 1

Rand takes as arguments, and returns, integers. Integers (in PHP) usually have a maximum range of 2**32.

Your double arguments are larger than this, causing an integer overflow when they are converted to integers.

Answer 2

Because you're seeing an integer overflow in the arguments.

According to the rand() documentation it takes two int values as arguments. On a 32-bit machine those are 32 bit as well (at least for PHP). So when you pass arguments larger than 2³¹ − 1 they overflow and start at −2³¹ again.

Apparently, if you need larger values you'll have to code it yourself. While simply creating a 64-bit number from two 32-bit numbers works as intended, you can't simply do a modulo operation with your maximum value since that skews the distribution. For a good implementation how to generate a uniformly-distributed random integer between 0 and a certain upper bound you can take a look at java.util.Random.nextInt(int) and adapt accordingly for 64 bit integers.

mt_rand() while usually a nice choice for random numbers because it uses MT19937 instead of a poor LCG, doesn't help here either as its arguments are ints as well.

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Another option you might want to consider if you don't require every possible value to be picked eventually:

• Generate a random floating-point value between 0 and 1 by calling

  $rnd = mt_rand() / (double)mt_getrandmax()
  

• Determine the range of numbers you need:

  $min = $this->gdp * 0.02;
  $max = $this->gdp * 0.17;
  $range = $max - $min;
  

• Multiply this by the previously-obtained random floating-point value and add the minimum:

  $value = $min + $range * $rnd
  

• Now you have a random value between your chosen boundaries. It's approximately uniformly distributed, although there are discrete steps between adjacent random numbers as you are stretching 32 bits of randomness over a larger number of bits. If that's no problem for you, go ahead, though.