Generate bit vector with t random ones

Question

I want to generate an array/vector v of size p with t ones and p-t zeros. The position of the t ones must be random.

This is the solution that I've written so far, but I'm not sure if it's the most efficient one. In addition I never used random_device or mt19937 (found them here) before, so I don't know what are the possible drawbacks.

#include <algorithm>
#include <random>
#include <vector>
...
int p=10, t=3;
std::vector<int> v(p,0);
for(int i=0;i<t;i++)  //better way?
    v[i] = 1;
std::random_device rd;
std::mt19937 g(rd());
std::shuffle(std::begin(v), std::end(v), g);

This is the matlab code that I'm trying to reproduce:

rp = randperm(p);
I_s(i,:) = rp(1:t);
v = zeros(p,1);
v(I_s(i,:)) = 1;

Demo here!

Btw, http://codereview.stackexchange.com/ may be a better place. — Jarod42, Jul 12 '16 at 17:28
This don't look as "bit-vector". It is simple vector of integers! — nosbor, Jul 12 '16 at 17:29
@Jarod42 Thanks for the tip, I think std::fill_n is even a better solution. And yes, maybe codereview.stackexchange.com is a better place, hoping some admin move it — justHelloWorld, Jul 12 '16 at 17:43
@justHelloWorld You can mod flag this and ask for them to move it. — NathanOliver, Jul 12 '16 at 17:44
@Jarod42 I disagree this would be a valid question for SE Code Review, because obviously the code doesn't work as intended (required). — πάντα ῥεῖ, Jul 12 '16 at 17:54
@πάνταῥεῖ: As I understand OP want a random permutation of a vector with `t` `1`, and `p-t` `0`. so code seems to work. — Jarod42, Jul 12 '16 at 18:05

πάντα ῥεῖ · Answer 1 · 2016-07-12T17:56:40.393

1

std::shuffle(std::begin(v), std::end(v), g);

Doesn't make the bit position random, it just changes the positions of the existing items (which all have the zero position bit set) in the vector randomly.

If I understood your requirement correctly, What you actually should have is something like:

std::random_device rd;
std::mt19937 g(rd());
for(int i=0;i<t;i++)
     v[i] = 1 << rd(); // set a random bit position

edited Jul 12 '16 at 17:56

answered Jul 12 '16 at 17:49

πάντα ῥεῖ

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My logic is a little bit different: initialize v with 0, fill the first t positions with ones, randomly permutate v through std::shuffle. Trust me, it works ;) – justHelloWorld Jul 12 '16 at 17:54
@justHelloWorld I doubt _it works_. _"The position of the t ones must be random."_ There's no code in your sample to manipulate the bit positions. – πάντα ῥεῖ Jul 12 '16 at 17:58
@justHelloWorld So your `std::vector` is meant to represent a single bit in each element? That looks extremely inefficient for me. You're probably better of with a `std::bitset`or even the infamous `std::vector` specialzaiton. – πάντα ῥεῖ Jul 12 '16 at 18:10
You're right, but this is going to be used in `Eigen`, where you cannot declare a `bool` or `std::bitset` vecotr, unfortunately. – justHelloWorld Jul 13 '16 at 06:30

Generate bit vector with t random ones

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