A char variable is actually an 8-bit integral value. It will have values from 0 to 255. These are almost always ASCII codes, but other encodings are allowed. 0 stands for the C-null character, and 255 stands for an empty symbol.

So, when you write the following assignment:

char a = 'a'; 

It is the same thing as this on an ASCII system.

char a = 97;

So, you can compare two char variables using the >, <, ==, <=, >= operators:

char a = 'a';
char b = 'b';

if( a < b ) printf("%c is smaller than %c", a, b);
if( a > b ) printf("%c is smaller than %c", a, b);
if( a == b ) printf("%c is equal to %c", a, b);

Note that even if ASCII is not required, this function will work because C requires that the digits are in consecutive order:

int isdigit(char c) {
    if(c >= '0' && c <= '9') 
        return 1;
    return 0;
} 

First of, in C single quotes are char literals, and double quotes are string literals. Thus, 'C' and "C" are not the same thing.

To do string comparisons, use strcmp.

const char* str = "abc";
if (strcmp ("abc", str) == 0) {
   printf("strings match\n");
}

To do char comparisons, use the equality operator.

char c = 'a';
if ('a' == c) {
   printf("characters match\n");
}

char charTime[] = "TIME"; char buf[] = "SOMETHINGELSE";

C++ and C (remove std:: for C):

bool equal = (std::strcmp(charTime, buf) == 0);

But the true C++ way:

std::string charTime = "TIME", buf = "SOMETHINGELSE";
bool equal = (charTime == buf);

Using == does not work because it tries to compare the addresses of the first character of each array (obviously, they do not equal). It won't compare the content of both arrays.