Just how random is std::random_shuffle?

Question

I'd like to generate a random number of reasonably arbitrary length in C++. By "reasonably arbitary" I mean limited by speed and memory of the host computer.

Let's assume:

• I want to sample a decimal number (base 10) of length ceil(log10(MY_CUSTOM_RAND_MAX)) from 0 to 10^(ceil(log10(MY_CUSTOM_RAND_MAX))+1)-1

• I have a vector<char>

• The length of vector<char> is ceil(log10(MY_CUSTOM_RAND_MAX))

• Each char is really an integer, a random number between 0 and 9, picked with rand() or similar methods

If I use std::random_shuffle to shuffle the vector, I could iterate through each element from the end, multiplying by incremented powers of ten to convert it to unsigned long long or whatever that gets mapped to my final range.

I don't know if there are problems with std::random_shuffle in terms of how random it is or isn't, particularly when also picking a sequence of rand() results to populate the vector<char> .

How sketchy is std::random_shuffle for generating a random number of arbitrary length in this manner, in a quantifiable sense?

(I realize that there is a library in Boost for making random int numbers. It's not clear what the range limitations are, but it looks like MAX_INT . That said, I realize that said library exists. This is more of a general question about this part of the STL in the generation of an arbitrarily large random number. Thanks in advance for focusing your answers on this part.)

Answer 1

I'm slightly unclear as to the focus of this question, but I'll try to answer it from a few different angles:

• The quality of the standard library rand() function is typically poor. However, it is very easy to find replacement random number generators which are of a higher quality (you mentioned Boost.Random yourself, so clearly you're aware of other RNGs). It is also possible to boost (no pun intended) the quality of rand() output by combining the results of multiple calls, as long as you're careful about it: http://www.azillionmonkeys.com/qed/random.html
• If you don't want the decimal representation in the end, there's little to no point in generating it and then converting to binary. You can just as easily stick multiple 32-bit random numbers (from rand() or elsewhere) together to make an arbitrary bit-width random number.
• If you're generating the individual digits (binary or decimal) randomly, there is little to no point in shuffling them afterwards.