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 (std::isalpha(letter)) {
  std::cout << "The character " << letter << " is a letter.\n";
}

You might expect the return value from isalpha to be a bool, but it is actually an integer. It returns 0 if the argument is not a letter, and some non-zero value if it is. C++ inherits this odd behavior from the C language, which C++ was originally based on and it has kept these oddities mostly for compatibility with C.

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.

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.

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

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

        void stringToUpper (string &input) {
---
void stringToUpper (string input) {                         #paired
---
   int i = 0;
---
   while (i < input.length()) {
---
   while (i < input.length() - 1) {  #paired
---
      if (isalpha(input[i]) && islower(input[i])) {
---
      if (isalpha(input[i]) && isupper(input[i])) {                        #paired
---
         input[i] = toupper(input[i]);
      }
---
         toupper(input[i]);                        #paired
      }
---
      i++;
   }
}
        

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

        void stringToLower (string &input) {
---
void stringToLower (string input) {                         #paired
---
   int 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]);
      }
---
         input[i] = tolower(input[0]);                        #paired
      }
---
      i++;
   }
}
        

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