random between variables in C++?

First of all, I am still a beginner at learning programming. So please pardon me!

If i were to have 4 boolean values, let's say

a[0] = true
a[1] = false
a[2] = false
a[3] = true

and I were to randomize ONLY true values, how am I suppose to random them?

For example, since a[0] and a[3] is true, I can only random number 0 and 3. but random goes something like this

int random = rand() % 4 + 0;

So it will random from range 0-3. How am I suppose to do that with only true values? Please guide me along, thank you!