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A.4.3 Fixed-Length String Handling
1
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.
2
For each function that returns a String, the lower
bound of the returned value is 1.
2.a
Discussion: Most operations
that yields a String are provided both as a function and as a procedure.
The functional form is possibly a more aesthetic style but may introduce
overhead due to extra copying or dynamic memory usage in some implementations.
Thus a procedural form, with an in out parameter so that
all copying is done `in place', is also supplied.
3
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
4
The library package
Strings.Fixed has the following declaration:
5
with Ada.Strings.Maps;
package Ada.Strings.Fixed is
pragma Preelaborate(Fixed);
6
-- "Copy" procedure for strings of possibly different lengths
7
procedure Move (Source : in String;
Target : out String;
Drop : in Truncation := Error;
Justify : in Alignment := Left;
Pad : in Character := Space);
8
-- Search subprograms
9
function Index (Source : in String;
Pattern : in String;
Going : in Direction := Forward;
Mapping : in Maps.Character_Mapping
:= Maps.Identity)
return Natural;
10
function Index (Source : in String;
Pattern : in String;
Going : in Direction := Forward;
Mapping : in Maps.Character_Mapping_Function)
return Natural;
11
function Index (Source : in String;
Set : in Maps.Character_Set;
Test : in Membership := Inside;
Going : in Direction := Forward)
return Natural;
12
function Index_Non_Blank (Source : in String;
Going : in Direction := Forward)
return Natural;
13
function Count (Source : in String;
Pattern : in String;
Mapping : in Maps.Character_Mapping
:= Maps.Identity)
return Natural;
14
function Count (Source : in String;
Pattern : in String;
Mapping : in Maps.Character_Mapping_Function)
return Natural;
15
function Count (Source : in String;
Set : in Maps.Character_Set)
return Natural;
16
procedure Find_Token (Source : in String;
Set : in Maps.Character_Set;
Test : in Membership;
First : out Positive;
Last : out Natural);
17
-- String translation subprograms
18
function Translate (Source : in String;
Mapping : in Maps.Character_Mapping)
return String;
19
procedure Translate (Source : in out String;
Mapping : in Maps.Character_Mapping);
20
function Translate (Source : in String;
Mapping : in Maps.Character_Mapping_Function)
return String;
21
procedure Translate (Source : in out String;
Mapping : in Maps.Character_Mapping_Function);
22
-- String transformation subprograms
23
function Replace_Slice (Source : in String;
Low : in Positive;
High : in Natural;
By : in String)
return String;
24
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);
25
function Insert (Source : in String;
Before : in Positive;
New_Item : in String)
return String;
26
procedure Insert (Source : in out String;
Before : in Positive;
New_Item : in String;
Drop : in Truncation := Error);
27
function Overwrite (Source : in String;
Position : in Positive;
New_Item : in String)
return String;
28
procedure Overwrite (Source : in out String;
Position : in Positive;
New_Item : in String;
Drop : in Truncation := Right);
29
function Delete (Source : in String;
From : in Positive;
Through : in Natural)
return String;
30
procedure Delete (Source : in out String;
From : in Positive;
Through : in Natural;
Justify : in Alignment := Left;
Pad : in Character := Space);
31
--String selector subprograms
function Trim (Source : in String;
Side : in Trim_End)
return String;
32
procedure Trim (Source : in out String;
Side : in Trim_End;
Justify : in Alignment := Left;
Pad : in Character := Space);
33
function Trim (Source : in String;
Left : in Maps.Character_Set;
Right : in Maps.Character_Set)
return String;
34
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);
35
function Head (Source : in String;
Count : in Natural;
Pad : in Character := Space)
return String;
36
procedure Head (Source : in out String;
Count : in Natural;
Justify : in Alignment := Left;
Pad : in Character := Space);
37
function Tail (Source : in String;
Count : in Natural;
Pad : in Character := Space)
return String;
38
procedure Tail (Source : in out String;
Count : in Natural;
Justify : in Alignment := Left;
Pad : in Character := Space);
39
--String constructor functions
40
function "*" (Left : in Natural;
Right : in Character) return String;
41
function "*" (Left : in Natural;
Right : in String) return String;
42
end Ada.Strings.Fixed;
43
The effects of the above subprograms are as follows.
44
procedure Move (Source : in String;
Target : out String;
Drop : in Truncation := Error;
Justify : in Alignment := Left;
Pad : in Character := Space);
45
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:
46
- If Justify=Left, then Source is copied into the first Source'Length
characters of Target.
47
- If Justify=Right, then Source is copied into the last Source'Length
characters of Target.
48
- 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.
49
- Pad is copied to each Target character not otherwise assigned.
50
If Source is longer than Target, then the effect
is based on Drop.
51
- If Drop=Left, then the rightmost Target'Length characters
of Source are copied into Target.
52
- If Drop=Right, then the leftmost Target'Length characters
of Source are copied into Target.
53
- 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:
54
- 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.
55
- 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.
56
- Otherwise, Length_Error is propagated.
56.a
Ramification: The Move
procedure will work even if Source and Target overlap.
56.b
Reason: The order of parameters
(Source before Target) corresponds to the order in COBOL's MOVE verb.
57
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;
58
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.
58.a
Discussion: There is no
default value for the Mapping parameter that is a Character_Mapping_Function;
if there were, a call would be ambiguous since there is also a default
for the Mapping parameter that is a Character_Mapping.
59
function Index (Source : in String;
Set : in Maps.Character_Set;
Test : in Membership := Inside;
Going : in Direction := Forward)
return Natural;
60
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.
61
function Index_Non_Blank (Source : in String;
Going : in Direction := Forward)
return Natural;
62
Returns Index(Source,
Maps.To_Set(Space), Outside, Going)
63
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;
64
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.
64.a
Reason: We say `maximum
number' because it is possible to slice a source string in different
ways yielding different numbers of matches. For example if Source is
"ABABABA" and Pattern is "ABA", then Count yields
2, although there is a partitioning of Source that yields just 1 match,
for the middle slice. Saying `maximum number' is equivalent to saying
that the pattern match starts either at the low index or the high index
position.
65
function Count (Source : in String;
Set : in Maps.Character_Set)
return Natural;
66
Returns the number
of occurrences in Source of characters that are in Set.
67
procedure Find_Token (Source : in String;
Set : in Maps.Character_Set;
Test : in Membership;
First : out Positive;
Last : out Natural);
68/1
{
8652/0049}
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 {Constraint_Error
(raised by failure of run-time check)} is raised.
69
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;
70
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.
71
procedure Translate (Source : in out String;
Mapping : in Maps.Character_Mapping);
procedure Translate (Source : in out String;
Mapping : in Maps.Character_Mapping_Function);
72
Equivalent to
Source := Translate(Source, Mapping).
73
function Replace_Slice (Source : in String;
Low : in Positive;
High : in Natural;
By : in String)
return String;
74/1
{
8652/0049}
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),
and if High < Low then the returned string is Insert(Source, Before=>Low,
New_Item=>By).74.1/1
- {8652/0049} If
High >= Low, then the returned string comprises Source(Source'First..Low-1)
& By & Source(High+1..Source'Last), but with lower bound 1.
74.2/1
- {8652/0049} If
High < Low, then the returned string is Insert(Source, Before=>Low,
New_Item=>By).
75
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);
76
Equivalent to
Move(Replace_Slice(Source, Low, High, By), Source, Drop, Justify, Pad).
77
function Insert (Source : in String;
Before : in Positive;
New_Item : in String)
return String;
78
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.
79
procedure Insert (Source : in out String;
Before : in Positive;
New_Item : in String;
Drop : in Truncation := Error);
80
Equivalent to
Move(Insert(Source, Before, New_Item), Source, Drop).
81
function Overwrite (Source : in String;
Position : in Positive;
New_Item : in String)
return String;
82
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.
83
procedure Overwrite (Source : in out String;
Position : in Positive;
New_Item : in String;
Drop : in Truncation := Right);
84
Equivalent to
Move(Overwrite(Source, Position, New_Item), Source, Drop).
85
function Delete (Source : in String;
From : in Positive;
Through : in Natural)
return String;
86/1
{
8652/0049}
If From <= Through, the returned string is Replace_Slice(Source, From, Through,
""), otherwise it is Source
with lower bound 1.
87
procedure Delete (Source : in out String;
From : in Positive;
Through : in Natural;
Justify : in Alignment := Left;
Pad : in Character := Space);
88
Equivalent to
Move(Delete(Source, From, Through), Source, Justify => Justify, Pad
=> Pad).
89
function Trim (Source : in String;
Side : in Trim_End)
return String;
90
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).
91
procedure Trim (Source : in out String;
Side : in Trim_End;
Justify : in Alignment := Left;
Pad : in Character := Space);
92
Equivalent to
Move(Trim(Source, Side), Source, Justify=>Justify, Pad=>Pad).
93
function Trim (Source : in String;
Left : in Maps.Character_Set;
Right : in Maps.Character_Set)
return String;
94
Returns the string
obtained by removing from Source all leading characters in Left and all
trailing characters in Right.
95
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);
96
Equivalent to
Move(Trim(Source, Left, Right), Source, Justify => Justify, Pad=>Pad).
97
function Head (Source : in String;
Count : in Natural;
Pad : in Character := Space)
return String;
98
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.
99
procedure Head (Source : in out String;
Count : in Natural;
Justify : in Alignment := Left;
Pad : in Character := Space);
100
Equivalent to
Move(Head(Source, Count, Pad), Source, Drop=>Error, Justify=>Justify,
Pad=>Pad).
101
function Tail (Source : in String;
Count : in Natural;
Pad : in Character := Space)
return String;
102
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.
103
procedure Tail (Source : in out String;
Count : in Natural;
Justify : in Alignment := Left;
Pad : in Character := Space);
104
Equivalent to
Move(Tail(Source, Count, Pad), Source, Drop=>Error, Justify=>Justify,
Pad=>Pad).
105
function "*" (Left : in Natural;
Right : in Character) return String;
function "*" (Left : in Natural;
Right : in String) return String;
106/1
{
8652/0049}
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 1otherwise.
107
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.
108
10 In the Insert subprograms,
inserting at the end of a string is obtained by passing Source'Last+1
as the Before parameter.
109
11 {Constraint_Error
(raised by failure of run-time check)} If
a null Character_Mapping_Function is passed to any of the string handling
subprograms, Constraint_Error is propagated.
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