Using boolean flag to detect input error, C

Question

I am attempting to create a boolean flag that will output an error message when an invalid input code is entered. This is for a class assignment and I am just learning C so please forgive the formatting and notify me if it could be improved. I have the code doing everything successful except for outputting the error message if the input or output variables are incorrect. I believe this is an error with the boolean check part of the code. A diagnosis would be great.

This code will:

request and source currency, destination currency, and source amount
output the destination currency amount or error message if the input variables are incorrect

Parameters:

the source and destination currency must be Canadian dollars ($), Yen (Y), or Euros (E)
a boolean flag must be used

Thanks in advanced.

# include 

int main() {

    char $ = $; 
    char Y = Y;
    char E = E;

    char in_currency, out_currency;

    char flag = 1;
    char new_line;

    float in_value;
    float out_value;

    //Calculation variables

    printf("Enter the source currency: ");
    scanf("%c%c", &in_currency, &new_line);

    printf("Enter the destination currency: ");
    scanf("%c%c", &out_currency, &new_line);

    if ( ( in_currency || out_currency ) != ( '$' || 'Y' || 'E' ) )
        flag = 0;

    printf("Enter the value: ");
    scanf("%f%c", &in_value, &new_line);

    //Calculations

    if (flag) {
        printf("%c%.2f = %c%.2f 
", in_currency, in_value, out_currency, out_value);
    }
    else {
        printf("There was an error with the input. 
");
    }

    return 0;
}

Jonathan Leffler · Accepted Answer

Converting some comments and other notes into an answer

These statements: char $ = $; char Y = Y; char E = E; invoke undefined behaviour (or, at the least, achieve nothing useful) by initializing the variable with their own uninitialized values. The variable $ isn't valid in standard C; you can probably find a C compiler that allows it (especially if you work on VMS). That, sadly, isn't a good start. The fact that the rest of the code doesn't use the variables is an additional problem. You can simply delete them.

Note that if the user enters 'source currency' as GPB, the new_line variable isn't going to contain a newline. You'd probably do better with using " %c" (space, percent, 'c') as the format string; it'll skip (optional) leading white space (blanks, tabs, newlines), and then one character. Repeat for the destination currency; it will skip the newline you expect and then read the next non-white space character. There's very little point in the "%f%c" format; it doesn't guarantee that you'll read a newline (and you don't check that it does).

Additionally, as others also pointed out, the condition:

if ( ( in_currency || out_currency ) != ( '$' || 'Y' || 'E' ) )

does not do what you want it to do. Because of the way C works, the RHS of the != evaluates to true (because '$' is not zero), and the LHS of it evaluates to true unless both in_currency and out_currency are zero, which is very unlikely (many people don't know how to type an ASCII NUL — aka the zero byte or '\0' — at a keyboard).

As I noted in a comment:

The original is likely to be intended to mean: _If in_currency is not one of '$' or 'Y' or 'E', or if out_currency is not one of '$' or 'Y' or 'E', then set flag to 0.

You need something more like

if (( in_currency != '$' &&  in_currency != 'Y' &&  in_currency != 'E') ||
    (out_currency != '$' && out_currency != 'Y' && out_currency != 'E'))
    flag = 0;

I got to see a version of the condition written as:

if (( in_currency != '$' ||  in_currency != 'Y' ||  in_currency != 'E' ) ||
    (out_currency != '$' || out_currency != 'Y' || out_currency != 'E' ))
    flag = 0;

This does not work properly. Think about it: suppose in_currency is Y (yen). The term in_currency != '$' will be true (because the value is Y), so the overall condition will be true (without any further terms being evaluated) and flag will be set to zero. If you had && separating the in_currency terms, then in_currency != '$' would be true, but in_currency != 'Y' would be false, so the (in_currency != '$' && in_currency != 'Y' && in_currency != 'E') part of the condition would be false — which is good because the currency code is valid. It would then go on to evaluate the out_currency terms (also with &&) and provided that the out_currency value was correct, would also pass.

Note that a condition of the form (a != v1 || a != v2) where the same variable a is compared to two distinct values v1 and v2 is always tautologously true. Beware, common English usage is a little slippery in this area; you have to be very careful and fastidious getting conditions correct. You also need to know De Morgan's Theorems (q.v.).

There are other issues one can pick with your code. For example, your error message is not very informative. You require the user to continue entering data after you've got enough information to know that it is invalid. You'll need to make sure your code handles the case where the user elects to convert from Euros to Euros (or, more generally, enters the same currency code twice).

Writing good code is, in part, an exercise in understanding how the user could subvert your best intentions, and heading off trouble before it can happen.

Using boolean flag to detect input error, C

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