number of ones in array

Question

I am trying to count the number of ones in a 4-bit binary number in Verilog, but my output is unexpected. I\'ve tried several approaches; this is the one I think should work

Answer 1

$countones can be used for this purpose (refer to IEEE Std 1800-2012, 20.9 Bit vector system functions):

module tb;

reg [3:0] in;
wire [2:0] one = $countones(in);
initial begin
    $monitor("in=%b one=%d", in, one);
    #1 in = 4'b0000;
    #1 in = 4'b0001;
    #1 in = 4'b1101;
end

endmodule

Output:

in=xxxx one=0
in=0000 one=0
in=0001 one=1
in=1101 one=3