Characters, Strings, and the cstring library

Recap

• Recall that a C-style string is a character array that ends with the null character
• Character literals in single quotes
  • 'a', '\n', '$'
• string literals in double quotes
  • "Hello World\n"
  • Remember that the null-character is implicitly a part of any string literal
• The name of an array acts as a pointer to the first element of an array (i.e. it stores the address of where the array starts)
  • Recall that this means when an array is passed into a function, the function has access to the original array contents

Recap: the cctype library

• Conversion functions: These return the ascii value of a character
  • int toupper(int c) - returns the uppercase version of c if it's a lowercase letter, otherwise returns c as is
  • int tolower(int c) - returns the lowercase version of c if it's an uppercase letter, otherwise returns c as is
• Query Functions: These all return true (non-zero) or false (0), in answer to the question posed by the function's name. They all take in the ascii value of a character as a parameter.
  • int isdigit(int c) - decides whether the parameter is a digit (0-9)
  • int isalpha(int c) - decides whether the character is a letter (a-z, A-Z)
  • int isalnum(int c) - digit or a letter?
  • int islower(int c) - lowercase digit? (a-z)
  • int isupper(int c) - uppercase digit? (A-Z)
  • int isxdigit(int c) - hex digit character? (0-9, a-f)
  • int isspace(int c) - white space character?
  • int iscntrl(int c) - control character?
  • int ispunct(int c) - printing character other than space, letter, digit?
  • int isprint(int c) - printing character (including ' ')?
  • int isgraph(int c) - printing character other than ' ' (space)?

String I/O:

• Recall that in the special case of arrays of type char, which are used to implement c-style strings, we can use these special cases with the insertion and extraction operators:

    char greeting[20] = "Hello, World";
    cout << greeting;		// prints "Hello, World"
  
    char lastname[20];
    cin >> lastname;		// reads a string into the array 'lastname'
  				//  adds the null character automatically
  

• Also remember the following:
  • Using a char array with the insertion operator << will print the contents of the character array, up to the first null character encountered
  • The extraction operator >> used with a char array will read in a string, and will stop at white space.
  • These examples only apply to the special case of the character array.
• Clearly, the above cin example is only good for reading one word at a time. What if we want to read in a whole sentence into a string? Well, there are some other library functions worth knowing about

Reading strings: get and getline

• There are two more member functions in class istream (in the iostream library), for reading and storing C-style strings into arrays of type char. Here are the prototypes:

    char* get(char str[], int length, char delimiter = '\n');
    char* getline(char str[], int length, char delimiter = '\n');
  

• Note that this get function is different than the two versions of get we've already seen, which were for reading single characters from an input stream:

    char ch;
    ch = cin.get();		// extracts one character, returns it
    cin.get(ch);			// extracts one character, stores in ch
  

• The functions get and getline (with the three parameters) will read and store a c-style string. The parameters:
  • First parameter (str) is the char array where the data will be stored. Note that this is an array passed into a function, so the function has access to modify the original array
  • Second parameter (length) should always be the size of the array -- i.e. how much storage available.
  • Third parameter (delimiter) is an optional parameter, with the newline as the default. This is the character at which to stop reading
• Both of these functions will extract characters from the input stream, but they don't stop at any white space -- they stop at the specified delimiter. They also automatically append the null character, which must (as always) fit into the size of the array.
• Sample calls:

     char buffer[80];
  
     cin >> buffer;		// reads one word into buffer
     cin.get(buffer, 80, ',');	// reads up to the first comma, stores in buffer
     cin.getline(buffer, 80);	// reads an entire line (up to newline)
  

• So what is the difference between get and getline?
  • get will leave the delimiter character on the input stream, and it will be seen by the next input statement
  • getline will extract and discard the delimiter character

Example

 char greeting[15], name[10], other[20];

 cin.getline(greeting,15);    // gets input into the greeting array
 cin.get(name,10,'.');        // gets input into the name array
 cin.getline(other,20);       // gets input into the other array

Suppose that the data on the input stream (i.e. typed onto the keyboard, for instance) is:

 Hello, World
 Joe Smith.  He says hello.

 greeting:  "Hello, World"
 name:      "Joe Smith"
 other:     ".  He says hello."

The standard C string library:

 #include <cstring>

This string library contains many useful string manipulation functions. These are all for use with C-style strings. A few of the more commonly used ones are mentioned here. (The textbook contains more detail in chapter 10)

• strlen
  • takes one string argument, returns its length (not counting the null character)
  • Prototype:

     int strlen(const char str[]);
    

  • Sample calls:

     char phrase[30] = "Hello, World";
    
     cout << strlen("Greetings, Earthling!");	// prints 21
     int length =  strlen(phrase);			// stores 12
    

• strcpy
  • Takes two string arguments, copies the contents of the second string into the first string. The first parameter is non-constant, the second is constant
  • Prototype:

     char* strcpy(char str1[], const char str2[]);   // copies str2 into str 1
    

  • Sample calls:

      char buffer[80], firstname[30], lastname[30] = "Smith";
    
      strcpy(firstname, "Billy Joe Bob");	// copies name into firstname array
      strcpy(buffer, lastname);		// copies "Smith" into buffer array
      cout << firstname;			// prints "Billy Joe Bob"
      cout << buffer;			// prints "Smith"
    

• strcat
  • Takes two string arguments (first non-constant, second is const), and concatenates the second one onto the first
  • Prototype:

     char* strcat(char str1[], const char str2[]);	
    			// concatenates str2 onto the end of str1
    

  • Sample calls:

      char buffer[80] = "Dog";
      char word[] = "food";
    
      strcat(buffer, word);		// buffer is now "Dogfood"
      strcat(buffer, " breath");	// buffer is now "Dogfood breath"
    

• strcmp
  • Takes two string arguments (both passed as const arrays), and returns an integer that indicates their lexicographic order
  • Prototype:

     int strcmp(const char str1[], const char str2[]);
    
     // returns:  a negative number, if str1 comes before str2
     //           a positive number, if str2 comes before str1
     //           0                , if they are equal
     //
     // Note:  Lexicographic order is by ascii codes.  It's NOT the same
     //        as alphabetic order.
    

  • Sample calls:

      char word1[30] = "apple";
      char word2[30] = "apply";
    
      if (strcmp(word1, word2) != 0)		
        cout << "The words are different\n";
    
      strcmp(word1, word2)		// returns a negative, means word1 comes first
      strcmp(word1, "apple")	// returns a 0.  strings are the same
      strcmp("apple", "Zebra")	// returns a positive.  "Zebra" comes first!  
    				//  (all uppercase before lowercase in ascii)
    

• Note that the above calls rely on the null character as the terminator of C-style strings. Remember, there is no built-in bounds checking in C++
• strncpy, strncat, strncmp - these do the same as the three listed above, but they take one extra argument (an integer N), and they go up to the null character or up to N characters, whichever is first. Examples:

    char buffer[80];
    char word[11] = "applesauce";
    char bigword[] = "antidisestablishmentarianism";
  
    strncpy(buffer, word, 5);		// buffer now stores "apple"
    strncat(buffer, " piecemeal", 4);	// buffer now stores "apple pie"
  
    strncmp(buffer, "apple", 5);		// returns 0, as first 5 characters
  					//  of the strings are equal
  
    strncpy(word, bigword, 10);		// word is now "antidisest"
  					//   word only had 11 slots!
  

  These functions can be used to help do safer string operations. The extra parameter can be included to guarantee that array boundaries are not exceeded, as seen in the last strncpy example