7.14. Character classification

It is often useful to examine a character and test whether it is upper or lower case, or whether it is a character or a digit. C++ provides a library of functions that perform this kind of character classification. In order to use these functions, you have to include the header file ctype.h.

char letter = 'a';
if (isalpha(letter)) {
  cout << "The character " << letter << " is a letter." << endl;
}

You might expect the return value from isalpha to be a bool, but for reasons I don’t even want to think about, it is actually an integer that is 0 if the argument is not a letter, and some non-zero value if it is.

This oddity is not as inconvenient as it seems, because it is legal to use this kind of integer in a conditional, as shown in the example. The value 0 is treated as false, and all non-zero values are treated as true.

Technically, this sort of thing should not be allowed—integers are not the same thing as boolean values. Nevertheless, the C++ habit of converting automatically between types can be useful.

Other character classification functions include isdigit, which identifies the digits 0 through 9, and isspace, which identifies all kinds of “white” space, including spaces, tabs, newlines, and a few others. There are also isupper and islower, which distinguish upper and lower case letters.

Finally, there are two functions that convert letters from one case to the other, called toupper and tolower. Both take a single character as a parameter and return a (possibly converted) character.

char letter = 'a';
letter = toupper (letter);
cout << letter << endl;

The output of this code is A.

As an exercise, use the character classification and conversion library to write functions named stringToUpper and stringToLower that take a single string as a parameter, and that modify the string by converting all the letters to upper or lower case. The return type should be void.

Try writing the stringToUpper and stringToLower functions in the commented sections of the active code below. Both functions take a single string as a parameter and have return type void. stringToUpper should convert the string to uppercase, and stringToLower should convert the string to lowercase. Some functions that you might find useful include isalpha, isupper, islower, toupper, and tolower. If you get stuck, you can reveal the extra problems at the end for help.

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