题目：
A common source of errors: How to avoid making latches
When designing circuits, you must think first in terms of circuits:

I want this logic gate
I want a combinational blob of logic that has these inputs and produces these outputs
I want a combinational blob of logic followed by a set of flip-flops
What you must not do is write the code first, then hope it generates a proper circuit.

If (cpu_overheated) then shut_off_computer = 1;
If (~arrived) then keep_driving = ~gas_tank_empty;
Syntactically-correct code does not necessarily result in a reasonable circuit (combinational logic + flip-flops). The usual reason is: “What happens in the cases other than those you specified?”. Verilog’s answer is: Keep the outputs unchanged.

This behaviour of “keep outputs unchanged” means the current state needs to be remembered, and thus produces a latch. Combinational logic (e.g., logic gates) cannot remember any state. Watch out for Warning (10240): … inferring latch(es)" messages. Unless the latch was intentional, it almost always indicates a bug. Combinational circuits must have a value assigned to all outputs under all conditions. This usually means you always need else clauses or a default value assigned to the outputs.

解题：

module top_module (input      cpu_overheated,output reg shut_off_computer,input      arrived,input      gas_tank_empty,output reg keep_driving  ); //always @(*) beginif (cpu_overheated)shut_off_computer = 1;elseshut_off_computer=0;endalways @(*) beginif (~arrived)keep_driving = ~gas_tank_empty;elsekeep_driving =0;endendmodule

结果正确：

本题注意点：
组合逻辑中，如果if缺少else语句，或是自己赋值给自己的情况就会产生latch。为了避免latch产生,if需要加上else语句，覆盖所有可能。