verilog number of ones in array

Question

I am trying to know the number of ones in a 4-bit binary number in verilog but no output happens. I've tried several approaches this is the one I think should work but it doesn't.

module ones(one,in);
input [3:0]in;
output [1:0]one;

assign one = 2'b00; 
assign one = one+in[3]+in[2]+in[1]+in[0] ;

endmodule

Answer 1

First, you can't assign the variable twice.

Second, your range is off, 2 bits can only go from 0 to 3. You need a 3 bit output to count up to 4.

This is more like what you need:

module ones(
  output wire [2:0] one,
  input wire [3:0] in
);

assign one = in[3]+in[2]+in[1]+in[0] ;

endmodule

Answer 2

$countones is useful in testbenches, but it is not synthesizable (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

/*

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

*/