C：字符逻辑运算符不起作用

Question

Using the code

    sscanf(argv[1], "%d", &num1);
    sscanf(argv[2], "%c", &op);
    sscanf(argv[3], "%d", &num2);

        if ((op != '-')||(op != '*')||(op != '/')||(op != '+'))
    {
        puts("Error:");
        printf("'%c' is not a valid operator", op);
        return 0;
    }

Compiles fine.

However, does not work for any input. For instance when the input is 4 + 7

Console prints:

Error:

'+' is not a valid operator

Edit: without this error checking code, the rest of the programme operates correctly, including a switch statement based on op!

Answer 1

Makes sense, since your condition is always true.

 (op != '-')||(op != '*')||(op != '/')||(op != '+')

or even

 (op != '-')||(op != '*')

will always evaluate to true .

You're basically saying "If op is not - OR op is not * ... do whatever".

You probably meant to use && instead.

Answer 2

You have a logic problem. You should change the OR (||) operator with AND (&&)

Answer 3

你要

   if ((op != '-')&&(op != '*')&&(op != '/')&&(op != '+'))