Cannot return bool from function

Question

This is the piece of code in base C:

static bool is_ascii_value_of_digit(char ascii_value){

    if((ascii_value<40)&&(ascii_value >= 30)){
        return true;
    }
    return false;
}

The avr studio gcc compiler is giving me error :

../calculator.h:4: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'is_ascii_value_of_digit'

Similar errors are also evident in other functions. I do not know why this is happening. Spent an hour figuring out why and finally gave up. As far as I have seen online, my syntax is not the problem. Probably something that I am overlooking.

Question RESOLVED !

Thank you very much for your quick help as it saved me a lot of time. I was under the assumption that bool is a keyword in c.

Solution 1

You probably have just forgotten to #include <stdbool.h>.

However, you have a mismatch between name and behaviour of your function,

if((ascii_value<40)&&(ascii_value >= 30))

the decimal digits, 0-9, occupy the places 48-57 in the ASCII table, that's hexadecimal 0x30-0x39, so to match the name, you should test

if (ascii_value < 0x3A && ascii_value >= 0x30)

Solution 2

C does not have bool type, neither true or false. Use int and 0/1 values instead.

Solution 3

If your compiler supports, at least partially, the C99 standard (or C11, but that's not yet likely), add

#include <stdbool.h>

to the top of your source file to make bool, false, and true visible.

If it doesn't (Microsoft's support for C99 is not good), a workaround is:

typedef enum { false, true } bool;

This doesn't quite match the semantics of C99's bool (actually _Bool) type, but it's probably close enough.

Incidentally, you don't need an if/else statement in your function:

static bool is_ascii_value_of_digit(char ascii_value) {
    return ascii_value >= '0' && ascii_value <= '9';
}

bool values are values, and they can be stored and returned from functions just like any other values.

Another guideline: Don't compare boolean values to true or false, just test them directly. In a condition (such as in an if statement), any non-zero value is considered true, so this:

if (cond == true) ...

can fail if cond has non-zero value other than 1. Just write:

if (cond) ...

or, to test whether it's false:

if (!cond) ...

Recommended reading: section 9 of the comp.lang.c FAQ. (Further recommended reading: all the rest of the sections.)

Comments

[Daniel Fischer]

It does since 1999, all you need is <stdbool.h>.

[Timothy Jones]

or just return isdigit(ascii_value) :)

[Daniel Fischer]

Yeah :D But I was under the impression do-it-yourself was the point here.

[tomlogic]

Clearer: if (ascii_value >= '0' && ascii_value <= '9') (assuming you're trying to do this without isdigit()).

[Keith Thompson]

void main() is wrong; it should be int main(void). b==False is better written as !b. Read section 9 of the comp.lang.c FAQ.

[Keith Thompson]

@tomlogic: That's not only clearer, it's more portable (in EBCDIC, '0 .. '9' are 0xF0 .. 0xF9. (C does guarantee that the values are contiguous, a guarantee it doesn't make for letters.)

[remmy]

I wouldn't say that it's "wrong", it's just not what you want most of the time. Sometimes you have to have main be of type void, for example many compilers for embedded systems require it.

[Admin]

@Demizey: Many compilers and libraries for embedded systems are not standards conforming and in the context of ANSI C they are wrong.

[remmy]

@oscode: False. In hosted implementations you need to return a value, but freestanding implementations, which are very common in embedded environments, have no such requirements and void main() is just fine.