• Learn how characters are stored and manipulated in C++ programs
• Introduce several standard character-oriented functions
• Introduce C-style character strings (aka C strings)
• Introduce and discuss some commonly used C string functions

1. The char Data Type for Storing Characters
2. Character Expressions and Comparisons
3. Character Functions
4. Character Input
5. Other Character Sets
6. C-style Character Strings (aka C strings)
7. Input of C Strings
8. Output of C Strings
9. Comparison of C Strings
10. Copying (assigning) a C String
11. C String Length

    const char ASTERISK = '*';
    char ch;
    char letter;
    letter = 'a';
    cin >> ch;
    cout << letter;

The char data type sets aside one byte to store a character value. Recall that one byte equals 8 bits, where each bit contains either a zero or a one. We will number the bits of a byte as shown in this diagram:

What values do the 8 bits have when they represent various characters? For example, what pattern of zeroes and ones is used to represent, say, the letter A? A common encoding scheme for character data is ASCII, which stands for American Standard Code for Information Interchange. This standard describes what bit patterns represent various symbols, digits, letters, and so forth. By and large, all the computer systems you use encode characters using ASCII. (We will discuss why this is a bit of a simplification later in this lab.)

Before we examine an ASCII code chart, we must realize that any pattern of zeroes and ones in a byte is also a binary number and thus has a numeric value! For example, the bit pattern of all zeroes

In the ASCII code, bit b₇ is always a zero. Since this leaves us with 7 bits (b₆ through b₀) for our code, this means there are 128 possible patterns.   (2⁷ = 128.)   An ASCII code chart will thus have 128 entries; because we start numbering from zero, the entries will be numbered 0 through 127.

Please click on the ASCII Code Chart link to open a new window that will display the chart.

Examine the entries in the ASCII Code Chart. Notice that the 128 entries are divided into four sections of 32 characters. The first section, in green, consists of the control characters. Many of these characters (for example, the horizontal tab, HT) cannot be displayed directly on a screen. The next section, in light brown, consists of various symbols, punctuation marks, and numerals. The third section, in blue, consists mainly of uppercase letters of the alphabet. The fourth and final section, in light blue, consists mainly of the lowercase letters of the alphabet. Each entry has a (very small red) number identifying it. For example, the uppercase letter A (in the blue third section) has the number 65. This number represents the value of the bit pattern that represents the letter A.   Here is the exciting part: in C++ the number 65 and the character 'A' are considered to be the same thing!   Thus, in the following IF statement:     if ('A'==65)     the boolean expression (aka predicate) would evaluate to true.

Note in particular the very first ASCII character: the NULL character (abbreviated NUL). The bit pattern for the NULL character is all zeroes and thus the NULL character has a value of zero. Note also that the blank character is called space (abbreviated SP) in the ASCII code and has a value of 32.

In ASCII, the uppercase characters 'A' to 'Z' have the codes 65 through 90. Thus the boolean expression   'A'<'B'   will evaluate to true. In ASCII, the lowercase letters have the codes 97 through 122. Thus the boolean expression   'a'<'b'   will evaluate to true. Because of the ASCII encoding scheme, an uppercase letter is less than a lowercase letter. Thus the boolean expression   'A'<'a'   will evaluate to true. Note also that digits (numerals) are less than letters. Thus the boolean expression   '9'<'A'   will evaluate to true.

Because char values are just a form of integer data, it is possible to do arithmetic with them. For example, 'C'-'A' evaluates to 2, because 67-65 equals two. Can you predict what the following would print?

    char ch;
    ch = 'a' + 10;
    cout << ch;

Usually the C++ compiler can determine from context if we are using a char item as an ASCII character or as a numeric quantity. Sometimes, however, we need to be explicit in a program that we want to use an ASCII character as a numeric quantity. In those instances, we can use the C++ type cast operator int. It is used much like a function. Thus int('0') is the value 48, int('A') is the value 65, and int('a') is the value 97. Similarly, the C++ type cast operator char will produce a character whose code is the given integer. Hence char(97) is 'a', char(48) is '0', etc. Thus the following code would first print the letter A on one line, the number 65 on the second line, the number 37 on the third line, and the symbol % (percent sign) on the fourth line.

    int  ac = 37;
    char ch = 'A';
    cout << ch       << endl;   // prints A
    cout << int(ch)  << endl;   // prints 65
    cout << ac       << endl;   // prints 37
    cout << char(ac) << endl;   // prints %

We can ask C++ not to skip whitespace on input by setting a format state flag. The following C++ statement would cause the flag skipws to be turned off (it defaults to on, which means that whitespace is skipped over):

After the flag is turned off, if the input is

Turning on and off a format state flag is a hassle. C++ provides an easier mechanism to allow one to input one character at a time with whitespace treated the same way as any other character. The get() function reads the very next character in the input stream without skipping any whitespace. Using get() instead of cin in the above example, if the input is again

            $ script lab13ex4.log
            $ pr -n -t -e4 cla13a.cpp
            $ c++ cla13a.cpp -o recase
            $ recase < $CLA/cla13a.cpp
            $ exit

Note: this section is optional reading.
If you are pressed for time, you may skip to the next section (C-style Character Strings ) .

If you are French and your name is Thévenod, or you are German and your name is Günter, or you are Italian and your name is Pezzè (note the special characters in each name), clearly ASCII is an inadequate encoding scheme---it simply does not have the characters or symbols necessary to write many foreign words and names. Extra characters and symbols can be grafted atop the ASCII code by relaxing the assumption that bit b₇ always be zero. A discussion of other character sets is beyond the scope of this closed lab. However, if you are interested in the subject, you are encouraged to follow the following link: Character sets & Encodings.

But what if you need to write in Chinese or Japanese? Languages such as these have an enormous alphabet of characters. UNICODE is a code that can accommodate even such enormous alphabets. UNICODE is a 16-bit encoding scheme with 65536 characters.   (2¹⁶ = 65536.)   The use of UNICODE is becoming more common. (For example, strings in the languages Python 3 and Java are stored as UNICODE characters.) If you are interested in the subject, you are encouraged to visit http://www.unicode.org for more information.

    cout << "Enter the test score:";

    cout << "x=";

Thus a list of characters contained in double quotes is a C string, also sometimes called a C-style character string. Because C strings are stored as an array, the C string "x=" is stored as

A C string is terminated by the special character '\0' called the NULL character. The NULL character acts as a sentinel and marks the end of a C-style character string.

Note the difference between a C string and a character. For example, the C string "x" is stored as an array

whereas the character 'x' is stored as a single character

Character arrays are used to store C strings. For example:

    char lastName[10] = "Jackson";

If instead, we had initialized lastName with a string bigger than can be stored in the array, for example:

    char lastName[10] = "Washington";

    char line[80];
    char ch;
    int charCount = 0;

    // Read a line of text into the character
    // array "line".  If the line is too long,
    // only read the first 79 characters

    cin.get(ch);
    while ( ch!='\n' && charCount<79 )
    {
        line[charCount] = ch;
        charCount++;
        cin.get(ch);
    }
    // Place the NULL terminating character in the last position
    line[charCount] = '\0';

Alternately, the extraction (input) operator >> can be used to read in a C string. As when reading numeric and character data, beginning whitespace characters (blanks, newline, tab, etc.) are ignored; next, all characters up to the first whitespace character are read and placed in the variable. When the first whitespace character is encountered, reading terminates and the NULL character is placed in the string. For example, if we have:

    char name[20];
    cout << "Please enter the name: "
    cin >> name;

The same input will occur no matter how many blanks, tabs, etc preceded Mouse. As you can see, the first whitespace terminates the string. We must remember to use the whitespace as a separator of data items. For example, if we read

    cin >> name >> test;

then the name will contain:

and the variable test will not have any value read into it.



Output of C Strings
A C string can be printed one character at a time, for example using the put() function. Alternately, the C string can be printed all at once using cout. Assume name is a character string initialized as follows:

    char name[15] = "Washington";

    cout << endl << name;

    cout << setw(15) << name;

    cout << left << setw(15) << name;

You can not turn on right justification, instead we must resume right justification by turning off left justification! The following statement will do this:

    cout.unsetf(ios::left);

            $ script lab13ex6.log
            $ pr -n -t -e4 cla13b.cpp
            $ c++ cla13b.cpp
            $ a.out
            ...the data you enter...
            $ exit

Comparison of C Strings
The function strcmp() has been provided to compare two C strings, say str1 and str2. If str1 < str2 (based on a character by character comparison), a value less than 0 is returned. If str1 and str2 are the same, a value of 0 is returned. If str1 > str2, a value greater than 0 is returned. The function makes character comparisons of the elements in str1 and str2 starting with the 0th character. It stops when it finds characters that are not equal or when it reaches the end of one of the strings.

Examples:

    strcmp("A","B")         returns < 0 (negative)
    strcmp("James","Jami")  returns < 0 (negative) because 'e' < 'i'
    strcmp("135", "24")     returns < 0 (negative) because '1' < '2'
    strcmp("ABCD","ABC")    returns > 0 (positive)
    strcmp("ABC","ABCD")    returns < 0 (negative)
    strcmp("89", "89")      returns 0   (zero)

Example:

    char name[15];
    strcpy(name, "Mr. Mouse");

If the source string is shorter than the destination string, the remaining characters in the destination string remain unchanged. If the source string is longer than the destination string, storage locations following those allocated to the destination string will be overwritten. This may cause surprising (and hard to debug) results.

For example, strlen("Mr. Mouse") returns the value 9.

    char str1[5];
    char str2[5]="mom";
    int length;

            $ script lab13ex9.log
            $ pr -n -t -e4 cla13c.cpp
            $ c++ cla13c.cpp -o length
            $ length
            ...the data you enter...
            $ exit

On the answer sheet, give the names of five additional C string functions not described above.

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