Writing error checking code is a waste of time.
You should assume you'll never mess up the code and that the user will never mess up input.
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I'm actually being half serious here.
The user will always mess up input.
The user will always find the most creative way to work around your error checking.
It's a waste, just let it all crash and burn and have the user learn to use the program.
It's a better idea to actually write your program rather than write nonstop error checking.