A.4.3 Fixed-Length String Handling
The language-defined package Strings.Fixed provides
string-handling subprograms for fixed-length strings; that is, for values
of type Standard.String. Several of these subprograms are procedures
that modify the contents of a String that is passed as an out
or an in out parameter; each has additional parameters
to control the effect when the logical length of the result differs from
the parameter's length.
For each function that returns a String, the lower
bound of the returned value is 1.
The basic model embodied in the package is that a
fixed-length string comprises significant characters and possibly padding
(with space characters) on either or both ends. When a shorter string
is copied to a longer string, padding is inserted, and when a longer
string is copied to a shorter one, padding is stripped. The Move procedure
in Strings.Fixed, which takes a String as an out parameter, allows
the programmer to control these effects. Similar control is provided
by the string transformation procedures.
Static Semantics
The library package
Strings.Fixed has the following declaration:
with Ada.Strings.Maps;
package Ada.Strings.Fixed
is
pragma Preelaborate(Fixed);
-- "Copy" procedure for strings of possibly different lengths
procedure Move (Source :
in String;
Target :
out String;
Drop :
in Truncation := Error;
Justify :
in Alignment := Left;
Pad :
in Character := Space);
-- Search subprograms
function Index (Source :
in String;
Pattern :
in String;
From :
in Positive;
Going :
in Direction := Forward;
Mapping :
in Maps.Character_Mapping := Maps.Identity)
return Natural;
function Index (Source :
in String;
Pattern :
in String;
From :
in Positive;
Going :
in Direction := Forward;
Mapping :
in Maps.Character_Mapping_Function)
return Natural;
function Index (Source :
in String;
Pattern :
in String;
Going :
in Direction := Forward;
Mapping :
in Maps.Character_Mapping
:= Maps.Identity)
return Natural;
function Index (Source :
in String;
Pattern :
in String;
Going :
in Direction := Forward;
Mapping :
in Maps.Character_Mapping_Function)
return Natural;
function Index (Source :
in String;
Set :
in Maps.Character_Set;
From :
in Positive;
Test :
in Membership := Inside;
Going :
in Direction := Forward)
return Natural;
function Index (Source :
in String;
Set :
in Maps.Character_Set;
Test :
in Membership := Inside;
Going :
in Direction := Forward)
return Natural;
function Index_Non_Blank (Source :
in String;
From :
in Positive;
Going :
in Direction := Forward)
return Natural;
function Index_Non_Blank (Source :
in String;
Going :
in Direction := Forward)
return Natural;
function Count (Source :
in String;
Pattern :
in String;
Mapping :
in Maps.Character_Mapping
:= Maps.Identity)
return Natural;
function Count (Source :
in String;
Pattern :
in String;
Mapping :
in Maps.Character_Mapping_Function)
return Natural;
function Count (Source :
in String;
Set :
in Maps.Character_Set)
return Natural;
procedure Find_Token (Source :
in String;
Set :
in Maps.Character_Set;
Test :
in Membership;
First :
out Positive;
Last :
out Natural);
-- String translation subprograms
function Translate (Source :
in String;
Mapping :
in Maps.Character_Mapping)
return String;
procedure Translate (Source :
in out String;
Mapping :
in Maps.Character_Mapping);
function Translate (Source :
in String;
Mapping :
in Maps.Character_Mapping_Function)
return String;
procedure Translate (Source :
in out String;
Mapping :
in Maps.Character_Mapping_Function);
-- String transformation subprograms
function Replace_Slice (Source :
in String;
Low :
in Positive;
High :
in Natural;
By :
in String)
return String;
procedure Replace_Slice (Source :
in out String;
Low :
in Positive;
High :
in Natural;
By :
in String;
Drop :
in Truncation := Error;
Justify :
in Alignment := Left;
Pad :
in Character := Space);
function Insert (Source :
in String;
Before :
in Positive;
New_Item :
in String)
return String;
procedure Insert (Source :
in out String;
Before :
in Positive;
New_Item :
in String;
Drop :
in Truncation := Error);
function Overwrite (Source :
in String;
Position :
in Positive;
New_Item :
in String)
return String;
procedure Overwrite (Source :
in out String;
Position :
in Positive;
New_Item :
in String;
Drop :
in Truncation := Right);
function Delete (Source :
in String;
From :
in Positive;
Through :
in Natural)
return String;
procedure Delete (Source :
in out String;
From :
in Positive;
Through :
in Natural;
Justify :
in Alignment := Left;
Pad :
in Character := Space);
--
String selector subprograms
function Trim (Source :
in String;
Side :
in Trim_End)
return String;
procedure Trim (Source :
in out String;
Side :
in Trim_End;
Justify :
in Alignment := Left;
Pad :
in Character := Space);
function Trim (Source :
in String;
Left :
in Maps.Character_Set;
Right :
in Maps.Character_Set)
return String;
procedure Trim (Source :
in out String;
Left :
in Maps.Character_Set;
Right :
in Maps.Character_Set;
Justify :
in Alignment := Strings.Left;
Pad :
in Character := Space);
function Head (Source :
in String;
Count :
in Natural;
Pad :
in Character := Space)
return String;
procedure Head (Source :
in out String;
Count :
in Natural;
Justify :
in Alignment := Left;
Pad :
in Character := Space);
function Tail (Source :
in String;
Count :
in Natural;
Pad :
in Character := Space)
return String;
procedure Tail (Source :
in out String;
Count :
in Natural;
Justify :
in Alignment := Left;
Pad :
in Character := Space);
--String constructor functions
function "*" (Left : in Natural;
Right : in Character) return String;
function "*" (Left : in Natural;
Right : in String) return String;
end Ada.Strings.Fixed;
The effects of the above subprograms are as follows.
procedure Move (Source : in String;
Target : out String;
Drop : in Truncation := Error;
Justify : in Alignment := Left;
Pad : in Character := Space);
The Move procedure
copies characters from Source to Target. If Source has the same length
as Target, then the effect is to assign Source to Target. If Source is
shorter than Target then:
If Justify=Left, then Source is copied
into the first Source'Length characters of Target.
If Justify=Right, then Source is copied
into the last Source'Length characters of Target.
If Justify=Center, then Source is copied
into the middle Source'Length characters of Target. In this case, if
the difference in length between Target and Source is odd, then the extra
Pad character is on the right.
Pad is copied to each Target character
not otherwise assigned.
If Source is longer than Target, then the effect
is based on Drop.
If Drop=Left, then the rightmost Target'Length
characters of Source are copied into Target.
If Drop=Right, then the leftmost Target'Length
characters of Source are copied into Target.
If Drop=Error,
then the effect depends on the value of the Justify parameter and also
on whether any characters in Source other than Pad would fail to be copied:
If Justify=Left, and if each of
the rightmost Source'Length-Target'Length characters in Source is Pad,
then the leftmost Target'Length characters of Source are copied to Target.
If Justify=Right, and if each of
the leftmost Source'Length-Target'Length characters in Source is Pad,
then the rightmost Target'Length characters of Source are copied to Target.
Otherwise,
Length_Error is propagated.
function Index (Source : in String;
Pattern : in String;
From : in Positive;
Going : in Direction := Forward;
Mapping : in Maps.Character_Mapping := Maps.Identity)
return Natural;
function Index (Source : in String;
Pattern : in String;
From : in Positive;
Going : in Direction := Forward;
Mapping : in Maps.Character_Mapping_Function)
return Natural;
Each Index function
searches, starting from From, for a slice of Source, with length Pattern'Length,
that matches Pattern with respect to Mapping; the parameter Going indicates
the direction of the lookup. If From is not in Source'Range, then Index_Error
is propagated. If Going = Forward, then Index returns the smallest index
I which is greater than or equal to From such that the slice of Source
starting at I matches Pattern. If Going = Backward, then Index returns
the largest index I such that the slice of Source starting at I matches
Pattern and has an upper bound less than or equal to From. If there is
no such slice, then 0 is returned. If Pattern is the null string, then
Pattern_Error is propagated.
function Index (Source : in String;
Pattern : in String;
Going : in Direction := Forward;
Mapping : in Maps.Character_Mapping
:= Maps.Identity)
return Natural;
function Index (Source : in String;
Pattern : in String;
Going : in Direction := Forward;
Mapping : in Maps.Character_Mapping_Function)
return Natural;
If Going = Forward,
returns
Index (Source, Pattern, Source'First, Forward, Mapping);
otherwise returns
Index (Source, Pattern, Source'Last, Backward, Mapping);
function Index (Source : in String;
Set : in Maps.Character_Set;
From : in Positive;
Test : in Membership := Inside;
Going : in Direction := Forward)
return Natural;
Index searches for
the first or last occurrence of any of a set of characters (when Test=Inside),
or any of the complement of a set of characters (when Test=Outside).
If From is not in Source'Range, then Index_Error is propagated. Otherwise,
it returns the smallest index I >= From (if Going=Forward) or the
largest index I <= From (if Going=Backward) such that Source(I) satisfies
the Test condition with respect to Set; it returns 0 if there is no such
Character in Source.
function Index (Source : in String;
Set : in Maps.Character_Set;
Test : in Membership := Inside;
Going : in Direction := Forward)
return Natural;
If Going = Forward,
returns
Index (Source, Set, Source'First, Test, Forward);
otherwise returns
Index (Source, Set, Source'Last, Test, Backward);
function Index_Non_Blank (Source : in String;
From : in Positive;
Going : in Direction := Forward)
return Natural;
Returns Index (Source,
Maps.To_Set(Space), From, Outside, Going);
function Index_Non_Blank (Source : in String;
Going : in Direction := Forward)
return Natural;
Returns Index(Source,
Maps.To_Set(Space), Outside, Going)
function Count (Source : in String;
Pattern : in String;
Mapping : in Maps.Character_Mapping
:= Maps.Identity)
return Natural;
function Count (Source : in String;
Pattern : in String;
Mapping : in Maps.Character_Mapping_Function)
return Natural;
Returns the maximum
number of nonoverlapping slices of Source that match Pattern with respect
to Mapping. If Pattern is the null string then Pattern_Error is propagated.
function Count (Source : in String;
Set : in Maps.Character_Set)
return Natural;
Returns the number
of occurrences in Source of characters that are in Set.
procedure Find_Token (Source : in String;
Set : in Maps.Character_Set;
Test : in Membership;
First : out Positive;
Last : out Natural);
Find_Token returns
in First and Last the indices of the beginning and end of the first slice
of Source all of whose elements satisfy the Test condition, and such
that the elements (if any) immediately before and after the slice do
not satisfy the Test condition. If no such slice exists, then the value
returned for Last is zero, and the value returned for First is Source'First;
however, if Source'First is not in Positive then Constraint_Error
is
raised.
function Translate (Source : in String;
Mapping : in Maps.Character_Mapping)
return String;
function Translate (Source : in String;
Mapping : in Maps.Character_Mapping_Function)
return String;
Returns the string
S whose length is Source'Length and such that S(I) is the character to
which Mapping maps the corresponding element of Source, for I in 1..Source'Length.
procedure Translate (Source : in out String;
Mapping : in Maps.Character_Mapping);
procedure Translate (Source : in out String;
Mapping : in Maps.Character_Mapping_Function);
Equivalent to Source
:= Translate(Source, Mapping).
function Replace_Slice (Source : in String;
Low : in Positive;
High : in Natural;
By : in String)
return String;
If Low > Source'Last+1,
or High < Source'First–1, then Index_Error is propagated. Otherwise:
If High >= Low, then the returned string
comprises Source(Source'First..Low–1) & By & Source(High+1..Source'Last),
but with lower bound 1.
If High < Low, then the returned string
is Insert(Source, Before=>Low, New_Item=>By).
procedure Replace_Slice (Source : in out String;
Low : in Positive;
High : in Natural;
By : in String;
Drop : in Truncation := Error;
Justify : in Alignment := Left;
Pad : in Character := Space);
Equivalent to Move(Replace_Slice(Source,
Low, High, By), Source, Drop, Justify, Pad).
function Insert (Source : in String;
Before : in Positive;
New_Item : in String)
return String;
Propagates Index_Error
if Before is not in Source'First .. Source'Last+1; otherwise returns
Source(Source'First..Before–1) & New_Item & Source(Before..Source'Last),
but with lower bound 1.
procedure Insert (Source : in out String;
Before : in Positive;
New_Item : in String;
Drop : in Truncation := Error);
Equivalent to Move(Insert(Source,
Before, New_Item), Source, Drop).
function Overwrite (Source : in String;
Position : in Positive;
New_Item : in String)
return String;
Propagates Index_Error
if Position is not in Source'First .. Source'Last+1; otherwise returns
the string obtained from Source by consecutively replacing characters
starting at Position with corresponding characters from New_Item. If
the end of Source is reached before the characters in New_Item are exhausted,
the remaining characters from New_Item are appended to the string.
procedure Overwrite (Source : in out String;
Position : in Positive;
New_Item : in String;
Drop : in Truncation := Right);
Equivalent to Move(Overwrite(Source,
Position, New_Item), Source, Drop).
function Delete (Source : in String;
From : in Positive;
Through : in Natural)
return String;
If From <= Through,
the returned string is Replace_Slice(Source, From, Through, ""),
otherwise it is Source with lower bound 1.
procedure Delete (Source : in out String;
From : in Positive;
Through : in Natural;
Justify : in Alignment := Left;
Pad : in Character := Space);
Equivalent to Move(Delete(Source,
From, Through), Source, Justify => Justify, Pad => Pad).
function Trim (Source : in String;
Side : in Trim_End)
return String;
Returns the string
obtained by removing from Source all leading Space characters (if Side
= Left), all trailing Space characters (if Side = Right), or all leading
and trailing Space characters (if Side = Both).
procedure Trim (Source : in out String;
Side : in Trim_End;
Justify : in Alignment := Left;
Pad : in Character := Space);
Equivalent to Move(Trim(Source,
Side), Source, Justify=>Justify, Pad=>Pad).
function Trim (Source : in String;
Left : in Maps.Character_Set;
Right : in Maps.Character_Set)
return String;
Returns the string
obtained by removing from Source all leading characters in Left and all
trailing characters in Right.
procedure Trim (Source : in out String;
Left : in Maps.Character_Set;
Right : in Maps.Character_Set;
Justify : in Alignment := Strings.Left;
Pad : in Character := Space);
Equivalent to Move(Trim(Source,
Left, Right), Source, Justify => Justify, Pad=>Pad).
function Head (Source : in String;
Count : in Natural;
Pad : in Character := Space)
return String;
Returns a string
of length Count. If Count <= Source'Length, the string comprises the
first Count characters of Source. Otherwise its contents are Source concatenated
with Count–Source'Length Pad characters.
procedure Head (Source : in out String;
Count : in Natural;
Justify : in Alignment := Left;
Pad : in Character := Space);
Equivalent to Move(Head(Source,
Count, Pad), Source, Drop=>Error, Justify=>Justify, Pad=>Pad).
function Tail (Source : in String;
Count : in Natural;
Pad : in Character := Space)
return String;
Returns a string
of length Count. If Count <= Source'Length, the string comprises the
last Count characters of Source. Otherwise its contents are Count-Source'Length
Pad characters concatenated with Source.
procedure Tail (Source : in out String;
Count : in Natural;
Justify : in Alignment := Left;
Pad : in Character := Space);
Equivalent to Move(Tail(Source,
Count, Pad), Source, Drop=>Error, Justify=>Justify, Pad=>Pad).
function "*" (Left : in Natural;
Right : in Character) return String;
function "*" (Left : in Natural;
Right : in String) return String;
These functions replicate a character or string
a specified number of times. The first function returns a string whose
length is Left and each of whose elements is Right. The second function
returns a string whose length is Left*Right'Length and whose value is
the null string if Left = 0 and otherwise is (Left–1)*Right &
Right with lower bound 1.
9 In the Index and Count functions taking
Pattern and Mapping parameters, the actual String parameter passed to
Pattern should comprise characters occurring as target characters of
the mapping. Otherwise the pattern will not match.
10 In the Insert subprograms, inserting
at the end of a string is obtained by passing Source'Last+1 as the Before
parameter.
11
If a null Character_Mapping_Function
is passed to any of the string handling subprograms, Constraint_Error
is propagated.
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