30 Strings and Characters

A character is simply an object in GAP that represents an arbitrary character from the character set of the operating system. Character literals can be entered in GAP by enclosing the character in singlequotes '.

    gap> 'a';
    'a'
    gap> '*';
    '*' 

A string is simply a dense list of characters. Strings are used mainly in filenames and error messages. A string literal can either be entered simply as the list of characters or by writing the characters between doublequotes ". GAP will always output strings in the latter format.

    gap> s1 := ['H','a','l','l','o',' ','w','o','r','l','d','.'];
    "Hallo world."
    gap> s2 := "Hallo world.";
    "Hallo world."
    gap> s1 = s2;
    true
    gap> s3 := "";
    ""    # the empty string
    gap> s3 = [];
    true 

Note that a string is just a special case of a list. So everything that is possible for lists (see Lists) is also possible for strings. Thus you can access the characters in such a string (see List Elements), test for membership (see In), etc. You can even assign to such a string (see List Assignment). Of course unless you assign a character in such a way that the list stays dense, the resulting list will no longer be a string.

    gap> Length( s2 );
    12
    gap> s2[2];
    'a'
    gap> 'e' in s2;
    false
    gap> s2[2] := 'e';;  s2;
    "Hello world." 

If a string is displayed as result of an evaluation (see Main Loop), it is displayed with enclosing doublequotes. However, if a string is displayed by Print, PrintTo, or AppendTo (see Print, PrintTo, AppendTo) the enclosing doublequotes are dropped.

    gap> s2;
    "Hello world."
    gap> Print( s2 );
    Hello world.gap> 

There are a number of special character sequences that can be used between the single quote of a character literal or between the doublequotes of a string literal to specify characters, which may otherwise be inaccessible. They consist of two characters. The first is a backslash \ . The second may be any character. The meaning is given in the following list

n:

newline character. This is the character that, at least on UNIX systems, separates lines in a text file. Printing of this character in a string has the effect of moving the cursor down one line and back to the beginning of the line.

":

doublequote character. Inside a string a doublequote must be escaped by the backslash, because it is otherwise interpreted as end of the string.

':

singlequote character. Inside a character a singlequote must escaped by the backslash, because it is otherwise interpreted as end of the character.

\ :

backslash character. Inside a string a backslash must be escaped by another backslash, because it is otherwise interpreted as first character of an escape sequence.

b:

backspace character. Printing this character should have the effect of moving the cursor back one character. Whether it works or not is system dependent and should not be relied upon.

r:

carriage return character. Printing this character should have the effect of moving the cursor back to the beginning of the same line. Whether this works or not is again system dependent.

c:

flush character. This character is not printed. Its purpose is to flush the output queue. Usually GAP waits until it sees a newline before it prints a string. If you want to display a string that does not include this character use \ c.

other:

For any other character the backslash is simply ignored.

Again, if the line is displayed as result of an evaluation, those escape sequences are displayed in the same way that they are input. They are displayed in their special way only by Print, PrintTo, or AppendTo.

    gap> "This is one line.\nThis is another line.\n";
    "This is one line.\nThis is another line.\n"
    gap> Print( last );
    This is one line.
    This is another line. 

It is not allowed to enclose a newline inside the string. You can use the special character sequence \ n to write strings that include newline characters. If, however, a string is too long to fit on a single line it is possible to continue it over several lines. In this case the last character of each line, except the last must be a backslash. Both backslash and newline are thrown away. Note that the same continuation mechanism is available for identifiers and integers.

    gap> "This is a very long string that does not fit on a line \
    gap> and is therefore continued on the next line.";
    "This is a very long string that does not fit on a line and is therefo\
    re continued on the next line."
    # note that the output is also continued, but at a different place 

This chapter contains sections describing the function that creates the printable representation of a string (see String), the functions that create new strings (see ConcatenationString, SubString), the functions that tests if an object is a string (see IsString), the string comparisons (see Comparisons of Strings), and the function that returns the length of a string (see LengthString).

Subsections

1. String
2. ConcatenationString
3. SubString
4. Comparisons of Strings
5. IsString
6. LengthString