A.10.9 Input-Output for Real Types
Static Semantics
1
The following procedures are defined in the generic
packages Float_IO, Fixed_IO, and Decimal_IO, which have to be instantiated
for the appropriate floating point, ordinary fixed point, or decimal
fixed point type respectively (indicated by Num in the specifications).
2
Values are output as
decimal literals without low line characters. The format of each value
output consists of a Fore field, a decimal point, an Aft field, and (if
a nonzero Exp parameter is supplied) the letter E and an Exp field. The
two possible formats thus correspond to:
3
Fore . Aft
4
and to:
5
Fore . Aft E Exp
6
without any spaces between these fields. The Fore
field may include leading spaces, and a minus sign for negative values.
The Aft field includes only decimal digits (possibly with trailing zeros).
The Exp field includes the sign (plus or minus) and the exponent (possibly
with leading zeros).
7
For floating point
types, the default lengths of these fields are defined by the following
variables that are declared in the generic package Float_IO:
8
Default_Fore : Field := 2;
Default_Aft : Field := Num'Digits-1;
Default_Exp : Field := 3;
9
For ordinary or decimal
fixed point types, the default lengths of these fields are defined by
the following variables that are declared in the generic packages Fixed_IO
and Decimal_IO, respectively:
10
Default_Fore : Field := Num'Fore;
Default_Aft : Field := Num'Aft;
Default_Exp : Field := 0;
11
The following procedures
are provided:
12
procedure Get(File : in File_Type; Item : out Num; Width : in Field := 0);
procedure Get(Item : out Num; Width : in Field := 0);
13
If the value
of the parameter Width is zero, skips any leading blanks, line terminators,
or page terminators, then reads the longest possible sequence of characters
matching the syntax of any of the following (see
2.4):
14
15
16
17
- [+|–]base#based_numeral.#[exponent]
18
- [+|–]base#.based_numeral#[exponent]
19
If a nonzero value of Width is supplied, then
exactly Width characters are input, or the characters (possibly none)
up to a line terminator, whichever comes first; any skipped leading blanks
are included in the count.
20
Returns in the parameter Item the value of
type Num that corresponds to the sequence input, preserving the sign
(positive if none has been specified) of a zero value if Num is a floating
point type and Num'Signed_Zeros is True.
21
The exception
Data_Error is propagated if the sequence input does not have the required
syntax or if the value obtained is not of the subtype Num.
22
procedure Put(File : in File_Type;
Item : in Num;
Fore : in Field := Default_Fore;
Aft : in Field := Default_Aft;
Exp : in Field := Default_Exp);
procedure Put(Item : in Num;
Fore : in Field := Default_Fore;
Aft : in Field := Default_Aft;
Exp : in Field := Default_Exp);
23
Outputs the value of the parameter Item as
a decimal literal with the format defined by Fore, Aft and Exp. If the
value is negative, or if Num is a floating point type where Num'Signed_Zeros
is True and the value is a negatively signed zero, then a minus sign
is included in the integer part. If Exp has the value zero, then the
integer part to be output has as many digits as are needed to represent
the integer part of the value of Item, overriding Fore if necessary,
or consists of the digit zero if the value of Item has no integer part.
24
If Exp has a value greater than zero, then
the integer part to be output has a single digit, which is nonzero except
for the value 0.0 of Item.
25
In both cases, however, if the integer part
to be output has fewer than Fore characters, including any minus sign,
then leading spaces are first output to make up the difference. The number
of digits of the fractional part is given by Aft, or is one if Aft equals
zero. The value is rounded; a value of exactly one half in the last place
is rounded away from zero.
26
If Exp has the
value zero, there is no exponent part. If Exp has a value greater than
zero, then the exponent part to be output has as many digits as are needed
to represent the exponent part of the value of Item (for which a single
digit integer part is used), and includes an initial sign (plus or minus).
If the exponent part to be output has fewer than Exp characters, including
the sign, then leading zeros precede the digits, to make up the difference.
For the value 0.0 of Item, the exponent has the value zero.
27
procedure Get(From : in String; Item : out Num; Last : out Positive);
28
Reads a real value from the beginning of the
given string, following the same rule as the Get procedure that reads
a real value from a file, but treating the end of the string as a file
terminator. Returns, in the parameter Item, the value of type Num that
corresponds to the sequence input. Returns in Last the index value such
that From(Last) is the last character read.
29
The exception
Data_Error is propagated if the sequence input does not have the required
syntax, or if the value obtained is not of the subtype Num.
30
procedure Put(To : out String;
Item : in Num;
Aft : in Field := Default_Aft;
Exp : in Field := Default_Exp);
31
Outputs the value
of the parameter Item to the given string, following the same rule as
for output to a file, using a value for Fore such that the sequence of
characters output exactly fills the string, including any leading spaces.
32
Float_Text_IO is a
library package that is a nongeneric equivalent to Text_IO.Float_IO for
the predefined type Float:
33
with Ada.Text_IO;
package Ada.Float_Text_IO
is new Ada.Text_IO.Float_IO(Float);
34
For each predefined floating point type, a nongeneric
equivalent to Text_IO.Float_IO is provided, with names such as Ada.Long_Float_Text_IO.
Implementation Permissions
35
An implementation may extend Get [and Put] for floating
point types to support special values such as infinities and NaNs.
35.a
Discussion: See also the similar permission
for the Wide_Value attribute in
3.5.
36
The implementation of Put need not produce an output
value with greater accuracy than is supported for the base subtype. The
additional accuracy, if any, of the value produced by Put when the number
of requested digits in the integer and fractional parts exceeds the required
accuracy is implementation defined.
36.a
Discussion: The required accuracy is
thus Num'Base'Digits digits if Num is a floating point subtype. For a
fixed point subtype the required accuracy is a function of the subtype's
Fore, Aft, and Delta attributes.
36.b
Implementation defined: The accuracy
of the value produced by Put.
37
The nongeneric equivalent packages may, but need
not, be actual instantiations of the generic package for the appropriate
predefined type.
38
31 For an item with a positive value, if
output to a string exactly fills the string without leading spaces, then
output of the corresponding negative value will propagate Layout_Error.
39
32 The rules for the Value attribute (see
3.5) and the rules for Get are based on the
same set of formats.
Examples
40/1
This paragraph
was deleted.
41
package Real_IO is new Float_IO(Real); use Real_IO;
-- default format used at instantiation, Default_Exp = 3
42
X : Real := -123.4567; --
digits 8 (see 3.5.7)43
Put(X); -- default format "–1.2345670E+02"
Put(X, Fore => 5, Aft => 3, Exp => 2); -- "bbb–1.235E+2"
Put(X, 5, 3, 0); -- "b–123.457"
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