randomizing number of 1's in an array in UVM without using $countones?

Question

In UVM , I want to constraint an array such that I can fix the number of ones in an array to 3, I have written the following code using constraint which uses $countones, but how to do it without using $countones ??

class class_1;
rand bit[31:0] array;

constraint three_ones {
$countones(array) == 3;
}
endclass

Answer 1

What you wrote is equivalent to

constraint three_ones {
int'(array[0]) + int'(array[1]) + int'(array[2]) + ... + int'(array[31]) )
   == 3;
}
endclass

Update

To do this programmatically without any functions, you can create another array with a list of indexes that should be set to one. The size of the array is the number of one you want set.

bit [0:255] array;
int countones = 5;
rand int unsigned bitset[]; 
constraint c_bits { bitset.size == countones;
                    foreach(bitset[i]) bitset[i] inside {[0;$bits(array)-1];
                    unique {bitset};
                  }
function void post_randomize();
   array = 0;
   foreach(bitset[i]) array[bitset[i]] = 1'b1;
endfunction

Now there is a way to do this without the unique constraint, and without post_randomize, but that's too much work for me.