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L64,72: You're missing
break statements unless you intended to fall through to the next case. That won't give you an error, but will definitely give you the wrong answer.
As Furry Guy said, we need to know the exact error message.
The error might be that "Numbersin.txt" and/or "Revised Numbers.txt" cannot be opened. If so the reason is that they are not where the program expect them. Try an absolute path in that case.