26 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> x:= 'a';  IsChar( x );
'a'
true
gap> '*';
'*'

A string is a dense list (see IsList, IsDenseList) of characters (see IsChar); thus strings are always homogeneous (see IsHomogeneousList).

A string literal can either be entered as the list of characters or by writing the characters between doublequotes ". GAP will always output strings in the latter format. However, the input via the double quote syntax enables GAP to store the string in an efficient compact internal representation. See IsStringRep below for more details.

Each character, in particular those which cannot be typed directly from the keyboard, can also be typed in three digit octal notation. And for some special characters (like the newline character) there is a further possibility to type them, see section Special Characters.

gap> s1 := ['H','e','l','l','o',' ','w','o','r','l','d','.'];
"Hello world."
gap> IsString( s1 );
true
gap> s2 := "Hello world.";
"Hello world."
gap> s1 = s2;
true
gap> s3 := "";
""           # the empty string
gap> s3 = [];
true
gap> IsString( [] );
true
gap> IsString( "123" );  IsString( 123 );
true
false
gap> IsString( [ '1', '2', '3' ] );
true
gap> IsString( [ '1', '2', , '4' ] );  IsString( [ '1', '2', 3 ] );
false        # strings must be dense
false        # strings must only contain characters

gap> s := "\007";
"\007"
gap> Print(s); # rings bell in many terminals
gap> 

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 Membership Test for Collections), ask for the length, concatenate strings (see Concatenation), form substrings etc. You can even assign to a mutable 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];
'e'
gap> 'a' in s2;
false
gap> s2[2] := 'a';;  s2;
"Hallo world."
gap> s1{ [1..4] };
"Hell"
gap> Concatenation( s1{ [ 1 .. 6 ] }, s1{ [ 1 .. 4 ] } );
"Hello Hell"

If a string is displayed by View, for example as result of an evaluation (see Main Loop), or by ViewObj and PrintObj, it is displayed with enclosing doublequotes. (But note that there is an ambiguity for the empty string which is also an empty list of arbitrary GAP objects; it is only printed like a string if it was input as empty string or converted to a string with ConvertToStringRep.) The difference between ViewObj and PrintObj is that the latter prints all non-printable and non-ASCII characters in three digit octal notation, while ViewObj sends all printable characters to the output stream. The output of PrintObj can be read back into GAP.

Strings behave differently from other GAP objects with respect to Print, PrintTo, or AppendTo. These commands interpret a string in the sense that they essentially send the characters of the string directly to the output stream/file. (But depending on the type of the stream and the presence of some special characters used as hints for line breaks there may be sent some additional newline (or backslash and newline) characters.

gap> s4:= "abc\"def\nghi";;
gap> View( s4 );  Print( "\n" );
"abc\"def\nghi"
gap> ViewObj( s4 );  Print( "\n" );
"abc\"def\nghi"
gap> PrintObj( s4 );  Print( "\n" );
"abc\"def\nghi"
gap> Print( s4 );  Print( "\n" );
abc"def
ghi
gap> s := "German uses strange characters: \344\366\374\337\n";
"German uses strange characters: äöüß\n"
gap> PrintObj(s);
"German uses strange characters: \344\366\374\337\n"gap> 
gap> Print(s);
German uses strange characters: äöüß

Note that only those line breaks are printed by Print that are contained in the string (\n characters, see Special Characters), as is shown in the example below.

gap> s1;
"Hello world."
gap> Print( s1 );
Hello world.gap> Print( s1, "\nnext line\n" );
Hello world.
next line
gap> 

26.1 Special Characters

There are a number of special character sequences that can be used between the singlequotes of a character literal or between the doublequotes of a string literal to specify characters. They consist of two characters. The first is a backslash \. The second may be any character. If it is an octal digit (from 0 to 7) there must be two more such digits. 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.

\XYZ

with X, Y, Z three octal digits. This is translated to the character correponding to the number X64+Y8+Z modulo 256. This can be used to specify and store arbitrary binary data as a string in GAP.

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.

Only Print, PrintTo, or AppendTo send the characters directly to the output stream.

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.

Note in particular that 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, an input 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 input line, except the last line must be a backslash. Both backslash and newline are thrown away by GAP while reading the string. 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 therefore conti\
nued on the next line."

Note that the output is also continued, but at a different place that is determined by the value of SizeScreen (see SizeScreen).

26.2 Internally Represented Strings

IsStringRep is a special (internal) representation of dense lists of characters. Dense lists of characters can be converted into this representation using ConvertToStringRep. Note that calling IsString does not change the representation.

If obj is a dense internally represented list of characters then ConvertToStringRep changes the representation to IsStringRep. This is useful in particular for converting the empty list [], which usually is in IsPlistRep, to IsStringRep. If obj is not a string then ConvertToStringRep signals an error.

IsEmptyString returns true if str is the empty string in the representation IsStringRep, and false otherwise. Note that the empty list [] and the empty string "" have the same type, the recommended way to distinguish them is via IsEmptyString. For formatted printing, this distinction is sometimes necessary.

gap> l:= [];;  IsString( l );  IsEmptyString( l );  IsEmpty( l );
true
false
true
gap> l;  ConvertToStringRep( l );  l;
[  ]
""
gap> IsEmptyString( l );  IsEmptyString( "" );  IsEmptyString( "abc" );
true
true
false
gap> ll:= [ 'a', 'b' ];  IsStringRep( ll );  ConvertToStringRep( ll );
"ab"
false
gap> ll;  IsStringRep( ll );
"ab"
true

Each character lies in the family CharFamily, each nonempty string lies in the collections family of this family. Note the subtle differences between the empty list [] and the empty string "" when both are printed.

26.3 Recognizing Characters

checks whether the character c is a digit, i.e., occurs in the string "0123456789".

checks whether the character c is a lowercase alphabet letter, i.e., occurs in the string "abcdefghijklmnopqrstuvwxyz".

checks whether the character c is an uppercase alphabet letter, i.e., occurs in the string "ABCDEFGHIJKLMNOPQRSTUVWXYZ".

checks whether the character c is either a lowercase or an uppercase alphabet letter.

26.4 Comparisons of Strings

The equality operator = returns to true if the two strings string1 and string2 are equal and false otherwise. The inequality operator <> returns true if the two strings string1 and string2 are not equal and false otherwise.

gap> "Hello world.\n" = "Hello world.\n";
true
gap> "Hello World.\n" = "Hello world.\n";
false # string comparison is case sensitive
gap> "Hello world." = "Hello world.\n";
false # the first string has no <newline>
gap> "Goodbye world.\n" = "Hello world.\n";
false
gap> [ 'a', 'b' ] = "ab";
true

The ordering of strings is lexicographically according to the order implied by the underlying, system dependent, character set.

gap> "Hello world.\n" < "Hello world.\n";
false # the strings are equal
gap> "Hello World.\n" < "Hello world.\n";
true # in ASCII uppercase letters come before lowercase letters
gap> "Hello world." < "Hello world.\n";
true # prefixes are always smaller
gap> "Goodbye world.\n" < "Hello world.\n";
true # `G' comes before `H', in ASCII at least

Strings can be compared via < with certain GAP objects that are not strings, see Comparisons for the details.

26.5 Operations to Produce or Manipulate Strings

String returns a representation of obj, which may be an object of arbitrary type, as a string. This string should approximate as closely as possible the character sequence you see if you print obj.

If length is given it must be an integer. The absolute value gives the minimal length of the result. If the string representation of obj takes less than that many characters it is filled with blanks. If length is positive it is filled on the left, if length is negative it is filled on the right.

In the two argument case, the string returned is a new mutable string (in particular not a part of any other object); it can be modified safely, and MakeImmutable may be safely applied to it.

gap> String(123);String([1,2,3]);
"123"
"[ 1, 2, 3 ]"

returns a string which represents the integer int with hexa-decimal digits (using A-F as digits 10-15). The inverse translation can be achieved with IntHexString.

returns a string representing the prime factor decomposition of the integer int.

gap> StringPP(40320);
"2^7*3^2*5*7"

returns a string that is the nr-th word over the alphabet list alpha, w.r.t. word length and lexicographical order. The empty word is WordAlp( alpha, 0 ).

gap> List([0..5],i->WordAlp("abc",i));
[ "", "a", "b", "c", "aa", "ab" ]

returns a lowercase version of the string string, that is, a string in which each uppercase alphabet character is replaced by the corresponding lowercase character.

gap> LowercaseString("This Is UpperCase");
"this is uppercase"

This function accepts a string string and lists seps and, optionally, wspace of characters. Now string is split into substrings at each occurrence of a character in seps or wspace. The characters in wspace are interpreted as white space characters. Substrings of characters in wspace are treated as one white space character and they are ignored at the beginning and end of a string.

Both arguments seps and wspace can be single characters.

Each string in the resulting list of substring does not contain any characters in seps or wspace.

A character that occurs both in seps and wspace is treated as a white space character.

A separator at the end of a string is interpreted as a terminator; in this case, the separator does not produce a trailing empty string. Also see Chomp.

gap> SplitString( "substr1:substr2::substr4", ":" );
[ "substr1", "substr2", "", "substr4" ]
gap> SplitString( "a;b;c;d;", ";" );
[ "a", "b", "c", "d" ]
gap> SplitString( "/home//user//dir/", "", "/" );
[ "home", "user", "dir" ]

replaces occurrences of the string old in string by new, starting from the left and always replacing the first occurrence. To avoid infinite recursion, characters which have been replaced already, are not subject to renewed replacement.

gap> ReplacedString("abacab","a","zl");
"zlbzlczlb"
gap> ReplacedString("ababa", "aba","c");
"cba"
gap> ReplacedString("abacab","a","ba");
"babbacbab"

This function changes the string string in place. The characters (space), \n, \r and \t are considered as white space. Leading and trailing white space characters in string are removed. Sequences of white space characters between other characters are replaced by a single space character.

See NormalizedWhitespace for a non-destructive version.

gap> s := "   x y \n\n\t\r  z\n   \n";
"   x y \n\n\t\r  z\n   \n"
gap> NormalizeWhitespace(s);
gap> s;
"x y z"

This function returns a copy of string str to which NormalizeWhitespace was applied.

For the possibility to print GAP objects to strings, see String Streams.

joins list (a list of strings) after interpolating sep (or "," if the second argument is omitted) between each adjacent pair of strings; sep should be a string.

Examples

gap> list := List([1..10], String);                                  
[ "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" ]
gap> JoinStringsWithSeparator(list);
"1,2,3,4,5,6,7,8,9,10"
gap> JoinStringsWithSeparator(["The", "quick", "brown", "fox"], " ");
"The quick brown fox"
gap> JoinStringsWithSeparator(["a", "b", "c", "d"], ",\n    ");    
"a,\n    b,\n    c,\n    d"
gap> Print("    ", last, "\n");
    a,
    b,
    c,
    d

Recall, last is the last expression output by GAP.

Like the similarly named Perl function, Chomp removes a trailing newline character (or carriage-return line-feed couplet) from a string argument str if present and returns the result. If str is not a string or does not have such trailing character(s) it is returned unchanged. This latter property means that Chomp is safe to use in cases where one is manipulating the result of another function which might sometimes return fail, for example.

gap> Chomp("The quick brown fox jumps over the lazy dog.\n");
"The quick brown fox jumps over the lazy dog."
gap> Chomp("The quick brown fox jumps over the lazy dog.\r\n");
"The quick brown fox jumps over the lazy dog."
gap> Chomp("The quick brown fox jumps over the lazy dog.");
"The quick brown fox jumps over the lazy dog."
gap> Chomp(fail);
fail
gap> Chomp(32);
32

Note: Chomp only removes a trailing newline character from str. If your string contains several newline characters and you really want to split str into lines at the newline characters (and remove those newline characters) then you should use SplitString (see SplitString), e.g.

gap> str := "The quick brown fox\njumps over the lazy dog.\n";
"The quick brown fox\njumps over the lazy dog.\n"
gap> SplitString(str, "", "\n");
[ "The quick brown fox", "jumps over the lazy dog." ]
gap> Chomp(str);
"The quick brown fox\njumps over the lazy dog."

26.6 Operations to Evaluate Strings

return either an integer (Int and IntHexString), or a rational (Rat) as represented by the string str. Int returns fail if non-digit characters occur in str. For Rat, the argument string may start with the sign character '-', followed by either a sequence of digits or by two sequences of digits that are separated by one of the characters '/' or '.', where the latter stands for a decimal dot. (The methods only evaluate numbers but do not perform arithmetic!)

IntHexString evaluates an integer written with hexa-decimal digits. Here the letters a-f or A-F are used as digits 10-15. An error occurs when a wrong character is found in the string. This function can be used (together with HexStringInt) for efficiently storing and reading large integers from respectively into GAP. Note that the translation between integers and their hexa-decimal representation costs linear computation time in terms of the number of digits, while translation from and into decimal representation needs substantial computations.

gap> Int("12345")+1;
12346
gap> Int("123/45");
fail
gap> Int("1+2");
fail
gap> Int("-12");
-12
gap> Rat("123/45");
41/15
gap> Rat( "123.45" );
2469/20
gap> IntHexString("-abcdef0123456789");
-12379813738877118345
gap> HexStringInt(last);
"-ABCDEF0123456789"

returns the ordinal of the integer n as a string.

gap> Ordinal(2);  Ordinal(21);  Ordinal(33);  Ordinal(-33);
"2nd"
"21st"
"33rd"
"-33rd"

passes expr (a string) through an input text stream so that GAP interprets it, and returns the result. The following trivial example demonstrates its use.

gap> a:=10;
10
gap> EvalString("a^2");
100

EvalString is intended for single expressions. A sequence of commands may be interpreted by using the functions InputTextString (see InputTextString) and ReadAsFunction (see ReadAsFunction!for streams) together; see Operations for Input Streams for an example.

26.7 Calendar Arithmetic

All calendar functions use the Gregorian calendar.

returns the number of days in a year.

returns the number of days in month number month of year (and fail if month is integer not in valid range.

gap> DaysInYear(1998);
365
gap> DaysInMonth(3,1998);
31

converts a number of days, starting 1-Jan-1970 to a list [day,month,year] in Gregorian calendar counting.

returns the number of days from 01-Jan-1970 to the day given by dmy. dmy must be a list of the form [day,month,year] in Gregorian calendar counting. The result is fail on input outside valid ranges.

Note that this makes not much sense for early dates like: before 1582 (no Gregorian calendar at all), or before 1753 in many English countries or before 1917 in Russia.

returns the weekday of a day given by date. date can be a number of days since 1-Jan-1970 or a list [day,month,year].

converts date to a readable string. date can be a number of days since 1-Jan-1970 or a list [day,month,year].

gap> DayDMY([1,1,1970]);DayDMY([2,1,1970]);
0
1
gap> DMYDay(12345);
[ 20, 10, 2003 ]
gap> WeekDay([11,3,1998]);
"Wed"
gap> StringDate([11,3,1998]);
"11-Mar-1998"

converts a number msec of milliseconds into a list [hour,min,sec,milli].

is the reverse of HMSMSec.

converts time (given as a number of milliseconds or a list [hour, min, sec, milli]) to a readable string.

gap> HMSMSec(Factorial(10));
[ 1, 0, 28, 800 ]
gap> SecHMSM([1,10,5,13]);
4205013
gap> StringTime([1,10,5,13]);
" 1:10:05.013"

returns the number of seconds from 01-Jan-1970, 00:00:00, to the time given by DMYhms. DMYhms must be a list of the form [day,month,year,hour,minute,second]. The remarks on the Gregorian calendar in the section on DayDMY apply here as well. The last three arguments must lie in the appropriate ranges.

This is the inverse function to SecondsDMYhms.

gap> SecondsDMYhms([ 9, 9, 2001, 1, 46, 40 ]);
1000000000
gap> DMYhmsSeconds(-1000000000);
[ 24, 4, 1938, 22, 13, 20 ]

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GAP 4 manual
May 2002