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 cctype.

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 return a string with all the characters converted to upper or lower case. The return type should be string.

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.

#include <iostream> #include <cctype> using namespace std; string stringToUpper(string input) { // ``stringToUpper`` should convert a string to uppercase and // and return it. Write your implementation here. } string stringToLower(string input) { // ``stringToLower`` should convert a string to lowercase and // and return it. Write your implementation here. } int main() { string upper = "This String Should Be Converted To Uppercase!"; upper = stringToUpper (upper); cout << upper << endl; string lower = "This String Should Be Converted To Lowercase!"; stringToLower (lower); cout << lower << endl; }

Let’s write the code for the stringToUpper function. stringToUpper should convert a string to uppercase.

        string stringToUpper(string input) {
---
void stringToUpper(string input) {                          #paired
---
   size_t i = 0;
---
   while (i < input.length()) {
---
   while (i > input.length()) {                             #paired
---
       if (isalpha(input[i]) && islower(input[i])) {
---
       if (isalpha(input[i]) || islower(input[i])) {        #paired
---
           input[i] = toupper(input[i]));
---
       }
       i++;
   }
---
   return result;
---
}
        

Let’s write the code for the stringToLower function. stringToLower should convert a string to lowercase.

        string stringToLower(string input) {
---
void stringToLower(string input) {                          #paired
---
   size_t i = 0;
---
   while (i < input.length()) {
---
   while (i > input.length()) {                             #paired
---
       if (isalpha(input[i]) && isupper(input[i])) {
---
       if (isalpha(input[i]) && isupper(input[i])) {        #paired
---
           input[i] = tolower(input[i]);
---
       }
       i++;
   }
---
   return input;
---
}
        

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