4.	STRINGS

 
A string is an object which represents a sequence of characters.  Interlisp provides functions for creating strings, concatenating strings, and creating sub-strings of a string.
The input syntax for a string is a double quote ("), followed by a sequence of any characters except double quote and %, terminated by a double quote.  The % and double quote characters may be included in a string by preceding them with the character %.
Strings are printed by PRINT and PRIN2 with initial and final double quotes, and %s inserted where necessary for it to read back in properly.  Strings are printed by PRIN1 without the delimiting double quotes and extra %s.
A "null string" containing no characters is input as "".  The null string is printed by PRINT and PRIN2 as "".  (PRIN1 "") doesn't print anything.
Internally a string is stored in two parts; a "string pointer" and the sequence of characters.  Several string pointers may reference the same character sequence, so a substring can be made by creating a new string pointer, without copying any characters.  Functions that refer to "strings" actually manipulate string pointers.  Some functions take an "old string" argument, and re-use the string pointer.
(STRINGP X)  	[Function]
Returns X if X is a string, NIL otherwise.

(STREQUAL X Y)  	[Function]
Returns T if X and Y are both strings and they contain the same sequence of characters, otherwise NIL.  EQUAL uses STREQUAL.  Note that strings may be STREQUAL without being EQ.  For instance,
(STREQUAL "ABC" "ABC")  =>  T
(EQ "ABC" "ABC")  =>  NIL
STREQUAL returns T if X and Y are the same string pointer, or two different string pointers which point to the same character sequence, or two string pointers which point to different character sequences which contain the same characters.  Only in the first case would X and Y be EQ.

(STRING-EQUAL X Y)  	[Function]
Returns T if X and Y are either strings or litatoms, and they contain the same sequence of characters, ignoring case.  For instance,
(STRING-EQUAL "FOO" "Foo")  =>  T
(STRING-EQUAL "FOO" 'Foo)  =>  T
This is useful for comparing things that might want to be considered "equal" even though they're not both litatoms in a consistent case, such as file names and user names.

(ALLOCSTRING N INITCHAR OLD FATFLG)  	[Function]
Creates a string of length N characters of INITCHAR (which can be either a character code or something coercible to a character).  If INITCHAR is NIL, it defaults to character code 0.  if OLD is supplied, it must be a string pointer, which is modified and returned.
If FATFLG is non-NIL, the string is allocated using full 16-bit NS characters (see page X.XX) instead of 8-bit characters.  This can speed up some string operations if NS characters are later inserted into the string.  This has no other effect on the operation of the string functions.

(MKSTRING X FLG RDTBL)  	[Function]
If X is a string, returns X.  Otherwise, creates and returns a string containing the print name of X.  Examples:
(MKSTRING "ABC")  =>  "ABC"
(MKSTRING '(A B C))  =>  "(A B C)"
(MKSTRING NIL)  =>  "NIL"
Note that the last example returns the string "NIL", not the atom NIL.
If FLG is T, then the PRIN2-name of X is used, computed with respect to the readtable RDTBL.  For example,
(MKSTRING "ABC" T)  =>  "%"ABC%""

(NCHARS X FLG RDTBL)  	[Function]
Returns the number of characters in the print name of X.  If FLG=T, the PRIN2-name is used.  For example,
(NCHARS 'ABC)  =>  3
(NCHARS "ABC" T)  =>  5
Note:  NCHARS works most efficiently on litatoms and strings, but can be given any object.

(SUBSTRING X N M OLDPTR)  	[Function]
Returns the substring of X consisting of the Nth through Mth characters of X.  If M is NIL, the substring contains the Nth character thru the end of X.  N and M can be negative numbers, which are interpreted as counts back from the end of the string, as with NTHCHAR (page X.XX).  SUBSTRING returns NIL if the substring is not well defined, e.g., N or M specify character positions outside of X, or N corresponds to a character in X to the right of the character indicated by M).  Examples:
(SUBSTRING "ABCDEFG" 4 6)  =>  "DEF"
(SUBSTRING "ABCDEFG" 3 3)  =>  "C"
(SUBSTRING "ABCDEFG" 3 NIL)  =>  "CDEFG"
(SUBSTRING "ABCDEFG" 4 -2)  =>  "DEF"
(SUBSTRING "ABCDEFG" 6 4)  =>  NIL
(SUBSTRING "ABCDEFG" 4 9)  =>  NIL
If X is not a string, it is converted to one.  For example,
(SUBSTRING '(A B C) 4 6)  =>  "B C"
SUBSTRING does not actually copy any characters, but simply creates a new string pointer to the characters in X.   If OLDPTR is a string pointer, it is modified and returned.

(GNC X)  	[Function]
"Get Next Character."  Returns the next character of the string X (as an atom); also removes the character from the string, by changing the string pointer.  Returns NIL if X is the null string.  If X isn't a string, a string is made.  Used for sequential access to characters of a string.  Example:
←(SETQ FOO "ABCDEFG")
"ABCDEFG"
←(GNC FOO)
A
←(GNC FOO)
B
←FOO
"CDEFG"
Note that if A is a substring of B, (GNC A) does not remove the character from B.

(GLC X)  	[Function]
"Get Last Character."  Returns the last character of the string X (as an atom); also removes the character from the string.  Similar to GNC.  Example:
←(SETQ FOO "ABCDEFG")
"ABCDEFG"
←(GLC FOO)
G
←(GLC FOO)
F
←FOO
"ABCDE"

(CONCAT X1 X2 ... XN)  	[NoSpread Function]
Returns a new string which is the concatenation of (copies of) its arguments.  Any arguments which are not strings are transformed to strings.  Examples:
(CONCAT "ABC" 'DEF "GHI")  =>  "ABCDEFGHI"
(CONCAT '(A B C) "ABC")  =>  "(A B C)ABC"
(CONCAT) returns the null string, "".

(CONCATLIST L)  	[Function]
L is a list of strings and/or other objects.  The objects are transformed to strings if they aren't strings.  Returns a new string which is the concatenation of the strings.  Example:
(CONCATLIST '(A B (C D) "EF"))  =>  "AB(C D)EF"

(RPLSTRING X N Y)  	[Function]
Replaces the characters of string X beginning at character position N with string Y.  X and Y are converted to strings if they aren't already.  N may be positive or negative, as with SUBSTRING.  Characters are smashed into (converted) X.  Returns the string X.  Examples:
(RPLSTRING "ABCDEF" -3 "END")  =>  "ABCEND"
(RPLSTRING "ABCDEFGHIJK" 4 '(A B C))  =>  "ABC(A B C)K"
Generates an error if there is not enough room in X for Y, i.e., the new string would be longer than the original.  If Y was not a string, X will already have been modified since RPLSTRING does not know whether Y will "fit" without actually attempting the transfer.
Warning:  In some implementations of Interlisp, if X is a substring of Z, Z will also be modified by the action of RPLSTRING or RPLCHARCODE.  However, this is not guaranteed to be true in all cases, so programmers should not rely on RPLSTRING or RPLCHARCODE altering the characters of any string other than the one directly passed as argument to those functions.

(RPLCHARCODE X N CHAR)  	[Function]
Replaces the Nth character of the string X with the character code CHAR.  N may be positive or negative.  Returns the new X.  Similar to RPLSTRING.  Example:
(RPLCHARCODE "ABCDE" 3 (CHARCODE F))  =>  "ABFDE"

(STRPOS PAT STRING START SKIP ANCHOR TAIL CASEARRAY BACKWARDSFLG)  	[Function]
STRPOS is a function for searching one string looking for another.  PAT and STRING are both strings (or else they are converted automatically).  STRPOS searches STRING beginning at character number START, (or 1 if START is NIL) and looks for a sequence of characters equal to PAT.  If a match is found, the character position of the first matching character in STRING is returned, otherwise NIL.  Examples:
(STRPOS "ABC" "XYZABCDEF")  =>  4
(STRPOS "ABC" "XYZABCDEF" 5)  =>  NIL
(STRPOS "ABC" "XYZABCDEFABC" 5)  =>  10
SKIP can be used to specify a character in PAT that matches any character in STRING.  Examples:
(STRPOS "A&C&" "XYZABCDEF" NIL '&)  =>  4
(STRPOS "DEF&" "XYZABCDEF" NIL '&)  =>  NIL
If ANCHOR is T, STRPOS compares PAT with the characters beginning at position START (or 1 if START is NIL).  If that comparison fails, STRPOS returns NIL without searching any further down STRING.  Thus it can be used to compare one string with some portion of another string.  Examples:
(STRPOS "ABC" "XYZABCDEF" NIL NIL T)  =>  NIL
(STRPOS "ABC" "XYZABCDEF" 4 NIL T)  =>  4
If TAIL is T, the value returned by STRPOS if successful is not the starting position of the sequence of characters corresponding to PAT, but the position of the first character after that, i.e., the starting position plus (NCHARS PAT).  Examples:
(STRPOS "ABC" "XYZABCDEFABC" NIL NIL NIL T)  =>  7
(STRPOS "A" "A" NIL NIL NIL T)  =>  2
If TAIL=NIL, STRPOS returns NIL, or a character position within STRING which can be passed to SUBSTRING.  In particular, (STRPOS "" "")  =>  NIL.  However, if TAIL=T, STRPOS may return a character position outside of STRING.  For instance, note that the second example above returns 2, even though "A" has only one character.
If CASEARRAY is non-NIL, this should be a casearray like that given to FILEPOS (page X.XX).  The casearray is used to map the string characters before comparing them to the search string.
If BACKWARDSFLG is non-NIL, the search is done backwards from the end of the string.

(STRPOSL A STRING START NEG BACKWARDSFLG)  	[Function]
STRING is a string (or else it is converted automatically to a string), A is a list of characters or character codes.  STRPOSL searches STRING beginning at character number START (or else 1 if START=NIL) for one of the characters in A.  If one is found, STRPOSL returns as its value the corresponding character position, otherwise NIL.  Example:
(STRPOSL '(A B C) "XYZBCD")  =>  4
If NEG=T, STRPOSL searches for a character not on A.  Example:
(STRPOSL '(A B C) "ABCDEF" NIL T)  =>  4
If any element of A is a number, it is assumed to be a character code.  Otherwise, it is converted to a character code via CHCON1.  Therefore, it is more efficient to call STRPOSL with A a list of character codes.
If A is a bit table, it is used to specify the characters (see MAKEBITTABLE below)
If BACKWARDSFLG is non-NIL, the search is done backwards from the end of the string.

STRPOSL uses a "bit table" data structure to search efficiently.  If A is not a bit table, it is converted to a bit table using MAKEBITTABLE.  If STRPOSL is to be called frequently with the same list of characters, a considerable savings can be achieved by converting the list to a bit table once, and then passing the bit table to STRPOSL as its first argument.
(MAKEBITTABLE L NEG A)  	[Function]
Returns a bit table suitable for use by STRPOSL.  L is a list of characters or character codes, NEG is the same as described for STRPOSL.  If A is a bit table, MAKEBITTABLE modifies and returns it.  Otherwise, it will create a new bit table.
Note:  if NEG=T, STRPOSL must call MAKEBITTABLE whether A is a list or a bit table.  To obtain bit table efficiency with NEG=T, MAKEBITTABLE should be called with NEG=T, and the resulting "inverted" bit table should be given to STRPOSL with NEG=NIL.


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