Verilog comparator - verilog

Question

I'm newbie to a verilog.
I did a lot of research, and finally wrote this code, but it seems to not work.
Can anyone fix it for me?
module comparator();
reg[3:0] a, b;
wire[1:0] equal, lower, greater;
if (a<b) begin
equal = 0;
lower = 1;
greater = 0;
end
else if (a==b) begin
equal = 1;
lower = 0;
greater = 0;
end
else begin
equal = 0;
lower = 0;
greater = 1;
end;
initial begin
$monitor($time,
"a=%b, b=%b, greater=%b, equals=%b, lower=%b",
a, b, greater, equal, lower);
a=9; b=10;
#100 $display ("\n", $time, "\n");
end
endmodule

Answer 1

Behavioural procedures must be enclosed within an always block, like this:
Also, your module needs inputs and outputs. A more correct version would be like this:
module comparator (
input wire [3:0] a,
input wire [3:0] b,
output reg equal,
output reg lower,
output reg greater
);
always #* begin
if (a<b) begin
equal = 0;
lower = 1;
greater = 0;
end
else if (a==b) begin
equal = 1;
lower = 0;
greater = 0;
end
else begin
equal = 0;
lower = 0;
greater = 1;
end
end
endmodule
I suggest reading some tutorial about behavioral modelling with Verilog, because you missed a lot of points:
How to correctly define inputs and outputs in a module
What things can be wires and what things should be regs
The use of always #* to model combinational logic
And most important: how to write a test bench. Test benches are written as module with no inputs and outputs) that instantiates your UUT (unit under test), provides inputs, read outputs and check whether they are valid.
module testcomp;
reg [3:0] a, b;
wire eq, lw, gr;
comparator uut (
.a(a),
.b(b),
.equal(eq),
.lower(lw),
.greater(gr)
);
initial begin
a = 0;
repeat (16) begin
b = 0;
repeat (16) begin
#10;
$display ("TESTING %d and %d yields eq=%d lw=%d gr=%d", a, b, eq, lw, gr);
if (a==b && eq!=1'b1 && gr!=1'b0 && lw!=1'b0) begin
$display ("ERROR!");
$finish;
end
if (a>b && eq!=1'b0 && gr!=1'b1 && lw!=1'b0) begin
$display ("ERROR!");
$finish;
end
if (a<b && eq!=1'b1 && gr!=1'b0 && lw!=1'b1) begin
$display ("ERROR!");
$finish;
end
b = b + 1;
end
a = a + 1;
end
$display ("PASSED!");
$finish;
end
endmodule
You can play with this example at EDAPlayGround using this link:
http://www.edaplayground.com/x/CPq

Answer 2

Without always block:
module comparator (
input wire [3:0] a,
input wire [3:0] b,
output reg equal,
output reg lower,
output reg greater
);
assign equal = (a===b);
assign lower = (a<b)?1'b1:1'b0;
assign greater = (a>b)1'b1:1'b0;
end
Be careful, you need to consider 'X' and 'Z', use "===" instead of "=="