A.4.3 Fixed-Length String Handling

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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; 
                    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; 
                    Test   : in Membership := Inside; 
                    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;
                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;

Each Index function searches 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 Going = Forward, then Index returns the smallest index I 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. 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;
                Set    : in Maps.Character_Set;
                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). It returns the smallest index I (if Going=Forward) or the largest index I (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_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.

Notes

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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