Counting the position of ones in a binary vector

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only as guide, not tested

process(data_in)
begin
for i in 0 to 15 loop
    if data_in(i) = '0' then
          data_out(i*4:i*4+3) <= "0000";
     else
          data_out(i*4:i*4+3) <= std_logic_vector(to_unsigned(i,4));
     end if;
end loop;
end process;

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That doesn't do what ganeshmirajkar put forward, as e.g. bitposition 3 will always fill something into output nibble 3. So we need at least a variable. Steeling your code snippet:

process(data_in)
variable j : natural ;
begin
j := 0 ;
data_out <= (others => '0') ;
for i in 0 to 15 loop
    if data_in(i) = '1' then
          data_out(j*4:j*4+3) <= std_logic_vector(to_unsigned(i,4));
          j := j + 1 ;
     end if;
end loop;
end process;

I wonder how this compiles.