How to convert a string containing two hex digits to a character?

How to convert a string containing two hex digits to a character?

[Leslie]

Hi, it's me again. :-)

        I need to be able to convert a string variable containing two
hexadecimal digits (e.g. "7C") to its equivalent character
value.  I see from ARM 3.5.5 "Operations of Discrete Types" that
I can use 

  function S'Val(Arg : universal_integer) return S'Base

where universal_integer would be the numeric value of the string,
but how do I get from "7C" to 16#7C# when 7C is a string value,
not a constant?  I.e.,

  Hex_Pair : unbounded_string;

  begin
    Hex_Pair := slice(some_string,1,2);
    Char_Code : character;
    -- now what?
  end;

Leslie

Re: How to convert a string containing two hex digits to a character?

[Leslie]

Leslie wrote:

> Hi, it's me again. :-)
> 
>         I need to be able to convert a string variable
>         containing two
> hexadecimal digits (e.g. "7C") to its equivalent character
> value.  I see from ARM 3.5.5 "Operations of Discrete Types"
> that I can use
> 
>   function S'Val(Arg : universal_integer) return S'Base
> 
> where universal_integer would be the numeric value of the
> string, but how do I get from "7C" to 16#7C# when 7C is a
> string value,
> not a constant?  I.e.,
> 
OOPS!  Should be
>   Hex_Pair : unbounded_string;
>   Char_Code : character;    -- moved this out of the block. :-)
> 
>   begin
>     Hex_Pair := slice(some_string,1,2);
>     -- now what?
>   end;
> 
> Leslie

Re: How to convert a string containing two hex digits to a character?

[Hibou57 (Yannick Duchêne)]

You may first convert from two 4 bits digits, that is, convert "7" and
"C".

16#7#; -- 7
16#C#; -- 12

To get 16#7C# from 16#7# and 16#C#, just do ((16#7# * 16) + 16#C#)

To get 16#7# and 16#C#, first get '7' and 'C' from the string, you may
do something like the following (providing your string is named S, and
it is an ASCII string) :

   High_Order_Digit_Position : constant Positive := S'First;
   Low_Order_Digit_Position : constant Positive := Positive'Succ
(High_Order_Digit_Position);
   High_Order_Digit_Representation : constant Character := S
(High_Order_Digit_Position);
   Low_Order_Digit_Representation : constant Character := S
(Low_Order_Digit_Position);

To get a digit Natural value from its Character representation, you
may do something like the following (providing your character is
stored in a Character named C) :

   case C is
      when '0' .. '9' =>
         Digit_Value := 0 + (Character'Pos (C) - Character'Pos ('0'));
      when 'A' .. 'F' =>
         Digit_Value := 10 + (Character'Pos (C) - Character'Pos
('A'));
      when 'a' .. 'f' =>
         Digit_Value := 10 + (Character'Pos (C) - Character'Pos
('a'));
      when others =>
         raise Program_Error;
   end case;

Wrap the latter in a function like “ function Hexadecimal_Digit_Value
(C : Character) return Natural; ”

Then do :

   Hexadecimal_Base : constant Natural := 16;
   High_Order_Digit : constant Natural := Hexadecimal_Digit_Value
(High_Order_Digit_Representation);
   Low_Order_Digit : constant Natural := Hexadecimal_Digit_Value
(Low_Order_Digit_Representation);
   Number : constant Natural := (High_Order_Digit * Hexadecimal_Base)
+ Low_Order_Digit;

Re: How to convert a string containing two hex digits to a character?

[Hibou57 (Yannick Duchêne)]

Oops, I forget the last step.

Once you have your Number, which is the character code (it will always
be a valid character code, as it will not go beyond 255), do
Character'Val (Character_Code)

Re: How to convert a string containing two hex digits to a character?

[Leslie]

Hibou57 (Yannick Duchêne) wrote:

> Oops, I forget the last step.
> 
> Once you have your Number, which is the character code (it will
> always be a valid character code, as it will not go beyond
> 255), do Character'Val (Character_Code)

        Okay; this will take a little while to digest. :-)  No wonder I
was struggling! :-)

        Thanks,

Leslie

Re: How to convert a string containing two hex digits to a character?

[Leslie]

Hibou57 (Yannick Duchêne) wrote:

> You may first convert from two 4 bits digits, that is, convert
> "7" and "C".
> 
> 16#7#; -- 7
> 16#C#; -- 12
> 
> To get 16#7C# from 16#7# and 16#C#, just do ((16#7# * 16) +
> 16#C#)
> 
> To get 16#7# and 16#C#, first get '7' and 'C' from the string,
> you may do something like the following (providing your string
> is named S, and it is an ASCII string) :
> 
>    High_Order_Digit_Position : constant Positive := S'First;
>    Low_Order_Digit_Position : constant Positive :=
>    Positive'Succ
> (High_Order_Digit_Position);
>    High_Order_Digit_Representation : constant Character := S
> (High_Order_Digit_Position);
>    Low_Order_Digit_Representation : constant Character := S
> (Low_Order_Digit_Position);
> 
> To get a digit Natural value from its Character representation,
> you may do something like the following (providing your
> character is stored in a Character named C) :
> 
>    case C is
>       when '0' .. '9' =>
>          Digit_Value := 0 + (Character'Pos (C) - Character'Pos
>          ('0'));
>       when 'A' .. 'F' =>
>          Digit_Value := 10 + (Character'Pos (C) - Character'Pos
> ('A'));
>       when 'a' .. 'f' =>
>          Digit_Value := 10 + (Character'Pos (C) - Character'Pos
> ('a'));
>       when others =>
>          raise Program_Error;
>    end case;
> 
> Wrap the latter in a function like “ function
> Hexadecimal_Digit_Value (C : Character) return Natural; ”
> 
> Then do :
> 
>    Hexadecimal_Base : constant Natural := 16;
>    High_Order_Digit : constant Natural :=
>    Hexadecimal_Digit_Value
> (High_Order_Digit_Representation);
>    Low_Order_Digit : constant Natural :=
>    Hexadecimal_Digit_Value
> (Low_Order_Digit_Representation);
>    Number : constant Natural := (High_Order_Digit *
>    Hexadecimal_Base)
> + Low_Order_Digit;

        Okay, wait now; where did Hexadecimal_Digit_Value come
from? It should be declared outside of the function, and is
different than Decimal_Value, is that right?

Leslie

Re: How to convert a string containing two hex digits to a character?

[Leslie]

Leslie wrote:

> Hibou57 (Yannick Duchêne) wrote:
> 
>> You may first convert from two 4 bits digits, that is, convert
>> "7" and "C".
>> 
>> 16#7#; -- 7
>> 16#C#; -- 12
>> 
>> To get 16#7C# from 16#7# and 16#C#, just do ((16#7# * 16) +
>> 16#C#)
>> 
>> To get 16#7# and 16#C#, first get '7' and 'C' from the string,
>> you may do something like the following (providing your string
>> is named S, and it is an ASCII string) :
>> 
>>    High_Order_Digit_Position : constant Positive := S'First;
>>    Low_Order_Digit_Position : constant Positive :=
>>    Positive'Succ
>> (High_Order_Digit_Position);
>>    High_Order_Digit_Representation : constant Character := S
>> (High_Order_Digit_Position);
>>    Low_Order_Digit_Representation : constant Character := S
>> (Low_Order_Digit_Position);
>> 
>> To get a digit Natural value from its Character
>> representation, you may do something like the following
>> (providing your character is stored in a Character named C) :
>> 
>>    case C is
>>       when '0' .. '9' =>
>>          Digit_Value := 0 + (Character'Pos (C) - Character'Pos
>>          ('0'));
>>       when 'A' .. 'F' =>
>>          Digit_Value := 10 + (Character'Pos (C) -
>>          Character'Pos
>> ('A'));
>>       when 'a' .. 'f' =>
>>          Digit_Value := 10 + (Character'Pos (C) -
>>          Character'Pos
>> ('a'));
>>       when others =>
>>          raise Program_Error;
>>    end case;
>> 
>> Wrap the latter in a function like “ function
>> Hexadecimal_Digit_Value (C : Character) return Natural; ”
>> 
>> Then do :
>> 
>>    Hexadecimal_Base : constant Natural := 16;
>>    High_Order_Digit : constant Natural :=
>>    Hexadecimal_Digit_Value
>> (High_Order_Digit_Representation);
>>    Low_Order_Digit : constant Natural :=
>>    Hexadecimal_Digit_Value
>> (Low_Order_Digit_Representation);
>>    Number : constant Natural := (High_Order_Digit *
>>    Hexadecimal_Base)
>> + Low_Order_Digit;
> 
>         Okay, wait now; where did Hexadecimal_Digit_Value come
> from? It should be declared outside of the function, and is
> different than Decimal_Value, is that right?
> 
        Gak! Never mind, that's the function call. :-p
> Leslie

Re: How to convert a string containing two hex digits to a character?

[Maciej Sobczak]

On 6 Sty, 02:22, Hibou57 (Yannick Duchêne) <yannick_duch...@yahoo.fr>
wrote:

>    High_Order_Digit_Position : constant Positive := S'First;
>    Low_Order_Digit_Position : constant Positive := Positive'Succ
> (High_Order_Digit_Position);

Sorry, but I find it to be overly paranoid.
What has happened to the old good "Something + 1"?

--
Maciej Sobczak * www.msobczak.com * www.inspirel.com

Database Access Library for Ada: www.inspirel.com/soci-ada

Re: How to convert a string containing two hex digits to a character?

[Hibou57 (Yannick Duchêne)]

On 6 jan, 22:54, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
>
> Sorry, but I find it to be overly paranoid.
> What has happened to the old good "Something + 1"?
No such thing as paranoia here, as any way, for numbers, X + 1 and
Number_Type'Succ (X) are strictly equivalent (thus, the Succ function
attribute is not more paranoid than the “ + ” operator).

As the question was from a student, I was just trying to give him/her
some food for dinner. It's always good to know about Succ : it is
required with enumerated types, and it is the only way to do something
like “ + 1 ” when a numeric type is passed as a formal discrete type
parameter of a generic.

Re: How to convert a string containing two hex digits to a character?

[Hibou57 (Yannick Duchêne)]

On 6 jan, 23:19, Hibou57 (Yannick Duchêne) <yannick_duch...@yahoo.fr>
wrote:
> On 6 jan, 22:54, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
>
> > Sorry, but I find it to be overly paranoid.
> > What has happened to the old good "Something + 1"?
>
> No such thing as paranoia here, as any way, for numbers, X + 1 and
> Number_Type'Succ (X) are strictly equivalent (thus, the Succ function
> attribute is not more paranoid than the “ + ” operator).
>
> As the question was from a student, I was just trying to give him/her
> some food for dinner. It's always good to know about Succ : it is
> required with enumerated types, and it is the only way to do something
> like “ + 1 ” when a numeric type is passed as a formal discrete type
> parameter of a generic.

Number_Type'Pred and Number_Type'Succ are also useful when just
increment are decrement are to be done on type whose standard
operators are not visible. It allows to do this computation without
even requiring a "use type".

Re: How to convert a string containing two hex digits to a character?

[Hibou57 (Yannick Duchêne)]

On 9 jan, 11:50, Hibou57 (Yannick Duchêne) <yannick_duch...@yahoo.fr>
wrote:
> Number_Type'Pred and Number_Type'Succ are also useful when just
> increment are decrement are to be done on type whose standard
> operators are not visible. It allows to do this computation without
> even requiring a "use type".

Sorry, I'm back again on the topic (I'm joking with me, alone) : if
I'm welcome to close my comments with more wordings, the addition and
substraction have a formal logic definition which is based on the
notion of successor and predecessor, so the most pure and native way
of doing "+1" and "-1" is to use the successor and predecessor
function. Ada knows that, yeah ;p (as its 'Pred and 'Succ are more
universal than its standard "+ and "-" operators)

Re: How to convert a string containing two hex digits to a character?

[Dmitry A. Kazakov]

On Sat, 9 Jan 2010 03:13:38 -0800 (PST), Hibou57 (Yannick Duch�ne) wrote:

> I'm welcome to close my comments with more wordings, the addition and
> substraction have a formal logic definition which is based on the
> notion of successor and predecessor, so the most pure and native way
> of doing "+1" and "-1" is to use the successor and predecessor
> function. Ada knows that, yeah ;p (as its 'Pred and 'Succ are more
> universal than its standard "+ and "-" operators)

No. Addition is an operation (in an Abelian group) that may exist
independently on the order relation. As an example consider the complex
numbers, which is a field. As such it has addition and subtraction.
Nevertheless it lacks order, and thus there is no Complex'Pred or
Complex'Succ. And conversely, there can be an order, but no addition.
Example: String.

The guide line should be the semantics. If it is an iteration intended,
then T'Succ is a clear favorite, just because x + 1 has nothing to do with
enumeration. Unfortunately T'Succ is much too verbose, because it
unnecessarily refers to the type.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: How to convert a string containing two hex digits to a character?

[Hibou57 (Yannick Duchêne)]

On 9 jan, 12:33, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> No. Addition is an operation (in an Abelian group) that may exist
> independently on the order relation. As an example consider the complex
> numbers, which is a field. As such it has addition and subtraction.
> Nevertheless it lacks order, and thus there is no Complex'Pred or
> Complex'Succ. And conversely, there can be an order, but no addition.
> Example: String.
>
> The guide line should be the semantics. If it is an iteration intended,
> then T'Succ is a clear favorite, just because x + 1 has nothing to do with
> enumeration. Unfortunately T'Succ is much too verbose, because it
> unnecessarily refers to the type.
Nice to read you

Not “ No ”, while True as well. Right that it is not the only way to
define it.

You may define the attraction as the opposite of the repulsion, or you
may define the repulsion as the opposite of the attraction. In some
area, you may define Pred and Succ based on “ + ” and “ - ”, or in an
as good way, define + and - based on Pred and Succ.

There are no Pred and Succ on reals, complexes and the likes, but
their “ + ” and “ - ” are not either the same too (homonym, but
different things), and that's because their “ + ” and “ - ” are not
the same that they do not have a Pred and Succ.

This topic is a lot attractive

Re: How to convert a string containing two hex digits to a character?

[Dmitry A. Kazakov]

On Sat, 9 Jan 2010 06:50:17 -0800 (PST), Hibou57 (Yannick Duchêne) wrote:

> On 9 jan, 12:33, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>> No. Addition is an operation (in an Abelian group) that may exist
>> independently on the order relation. As an example consider the complex
>> numbers, which is a field. As such it has addition and subtraction.
>> Nevertheless it lacks order, and thus there is no Complex'Pred or
>> Complex'Succ. And conversely, there can be an order, but no addition.
>> Example: String.
>>
>> The guide line should be the semantics. If it is an iteration intended,
>> then T'Succ is a clear favorite, just because x + 1 has nothing to do with
>> enumeration. Unfortunately T'Succ is much too verbose, because it
>> unnecessarily refers to the type.
> Nice to read you
> 
> Not “ No ”, while True as well. Right that it is not the only way to
> define it.
> 
> You may define the attraction as the opposite of the repulsion, or you
> may define the repulsion as the opposite of the attraction.

Huh, not in intuitionism. The law of excluded middle is not an axiom, even
in physics. Take diffuse reflection instead of repulsion as *an* opposite
to attraction.

> In some
> area, you may define Pred and Succ based on “ + ” and “ - ”, or in an
> as good way, define + and - based on Pred and Succ.

I would rather say Succ *consistent* with +, IFF both exist.

> There are no Pred and Succ on reals, complexes and the likes,

BTW, there is Float'Succ! I tempted to say due to a language design flaw,
because it exposes the nature of Float as a model.

There is a big difference between real and complex numbers. The former have
the relation >. Real'Succ does not exist because the set {t|t>x} is open,
therefore there is no Min{t|t>x}, only Inf{t|t>x} = x. Complex numbers do
not even have >.

> but
> their “ + ” and “ - ” are not either the same too (homonym, but
> different things), and that's because their “ + ” and “ - ” are not
> the same that they do not have a Pred and Succ.

There exists a subset of Complex isomorphic to Real. In mathematics that
subset is indistinguishable from Real. In a typed system of Ada they are
different not because of their properties, but nominally due to their
attribution to different types. Moreover even type X is new Float is not
Float in Ada.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: How to convert a string containing two hex digits to a character?

[Adam Beneschan]

On Jan 9, 8:04 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:

> > There are no Pred and Succ on reals, complexes and the likes,
>
> BTW, there is Float'Succ! I tempted to say due to a language design flaw,
> because it exposes the nature of Float as a model.

I don't see this is a flaw.  It's a reminder that a Float, on any
computer, in any language, is actually limited to a subset of rational
numbers, which means you need to exercise some caution when using
them.  Programmers who think "Float" can represent any real number,
and that arithmetic on them is just like doing arithmetic on real
numbers, are going to get into trouble, so it doesn't seem like a good
thing to hide "the nature of Float as a model".

Anyway, Float'Succ was not present in Ada 83 but was added in Ada 95,
so someone must have thought it was useful for some practical purpose.

                               -- Adam

Re: How to convert a string containing two hex digits to a character?

[Dmitry A. Kazakov]

On Mon, 11 Jan 2010 07:53:14 -0800 (PST), Adam Beneschan wrote:

> On Jan 9, 8:04�am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
> 
>>> There are no Pred and Succ on reals, complexes and the likes,
>>
>> BTW, there is Float'Succ! I tempted to say due to a language design flaw,
>> because it exposes the nature of Float as a model.
> 
> I don't see this is a flaw.  It's a reminder that a Float, on any
> computer, in any language, is actually limited to a subset of rational
> numbers,

Technically no:

1. the radix can be irrational = you can have a finite set or irrational
numbers

2. infinite sets of rational numbers still have no Succ

But I know what you meant. Yes any set of model numbers is finite. BUT that
alone is not enough to have Succ well-defined. Consider the model x=P/Q,
where P and Q are bignums. In this model of real numbers there is no
meaningful Succ.

> which means you need to exercise some caution when using
> them.  Programmers who think "Float" can represent any real number,
> and that arithmetic on them is just like doing arithmetic on real
> numbers, are going to get into trouble, so it doesn't seem like a good
> thing to hide "the nature of Float as a model".

But also using operations specific to the given implementation (model) make
the program fragile. I think that Succ is OK for fixed-point numbers, but
less sure in the case of floating-point ones.

> Anyway, Float'Succ was not present in Ada 83 but was added in Ada 95,
> so someone must have thought it was useful for some practical purpose.

They also added 'Adjacent, which could serve same purpose. However upper
and lower pairs +,-,*,/ would be more useful. (Upper + is greater or equal
to the exact +).

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: How to convert a string containing two hex digits to a character?

[Hibou57 (Yannick Duchêne)]

On 11 jan, 16:53, Adam Beneschan <a...@irvine.com> wrote:
> I don't see this is a flaw.  It's a reminder that a Float, on any
> computer, in any language, is actually limited to a subset of rational
> numbers, which means you need to exercise some caution when using
> them.  Programmers who think "Float" can represent any real number,
> and that arithmetic on them is just like doing arithmetic on real
> numbers, are going to get into trouble, so it doesn't seem like a good
> thing to hide "the nature of Float as a model".
>
> Anyway, Float'Succ was not present in Ada 83 but was added in Ada 95,
> so someone must have thought it was useful for some practical purpose.
>
>                                -- Adam

While I agree with Dimitry Pred and Succ on Float are mostly a strange
idea, I agree Float (computer floats) and reals are not the same.

Just to give a good example about, a Python methods associated to
Float : float.as_integer_ratio(), which is defined as :
> Return a pair of integers whose ratio is exactly equal to the original float
> and with a positive denominator. Raises OverflowError on infinities and a
> ValueError on NaNs.
Found here : http://docs.python.org/library/stdtypes.html

Funny to apply this on Pi :P

Yes, Floats are a special things, they are not reals, although this
can be formalized

Re: How to convert a string containing two hex digits to a character?

[John B. Matthews]

In article 
<95653745-d21d-4cf9-8604-315ff185021a@j5g2000yqm.googlegroups.com>,
 Hibou57 (Yannick Duchêne) <yannick_duchene@yahoo.fr> wrote:

> Found here : http://docs.python.org/library/stdtypes.html
> 
> Funny to apply this on Pi :P

Well, I had to try it:

>>> float.as_integer_ratio(math.pi);
(884279719003555L, 281474976710656L)

It's not as good as my favorite, due to Arima Yoriyuki:

<http://en.wikipedia.org/wiki/Arima_Yoriyuki>

$ cat pi.bc 
#!/usr/bin/bc -lq
scale = 30
print "python: ",884279719003555/281474976710656,"\n"
print "Arima:  ",428224593349304/136308121570117,"\n"
print "4*a(1): ",4*a(1),"\n"
print "Wiki:   3.141592653589793238462643383279\n"
quit

$ ./pi.bc 
python: 3.141592653589793115997963468544
Arima:  3.141592653589793238462643383275
4*a(1): 3.141592653589793238462643383276
Wiki:   3.141592653589793238462643383279

-- 
John B. Matthews
trashgod at gmail dot com
<http://sites.google.com/site/drjohnbmatthews>

Re: How to convert a string containing two hex digits to a character?

[tmoran]

  Hex_Pair : String(1 .. 2) := "7C";

  Char : Character
    := Character'val(Integer'value("16#" & Hex_Pair & "#"));

Of course if this is homework in an Intro to Ada class, the teacher may not
believe you came up with this by yourself.

Re: How to convert a string containing two hex digits to a character?

[Leslie]

tmoran@acm.org wrote:

>   Hex_Pair : String(1 .. 2) := "7C";
> 
>   Char : Character
>     := Character'val(Integer'value("16#" & Hex_Pair & "#"));
> 
> Of course if this is homework in an Intro to Ada class, the
> teacher may not believe you came up with this by yourself.

        No, this is not for school. :-)  After 30 years of COBOL and
REXX, I'm trying to teach myself Ada, which I'm finding a lot
more understandable than C, especially for text manipulation.

        Thanks very much for this elegant solution; text handling seems
to be a somewhat neglected subject in the Ada books and web
sites I've been reading.

Leslie

Re: How to convert a string containing two hex digits to a character?

[Hibou57 (Yannick Duchêne)]

On 6 jan, 06:17, tmo...@acm.org wrote:
>   Hex_Pair : String(1 .. 2) := "7C";
>
>   Char : Character
>     := Character'val(Integer'value("16#" & Hex_Pair & "#"));
>
> Of course if this is homework in an Intro to Ada class, the teacher may not
> believe you came up with this by yourself.

We do not know about the context, and depending on the context, as en
example, if he/she was supposed to learn more about attributes, then
your way would be the better one (not easy to give a good answer to a
student when we do not know about the context and the real
expectations)

Re: How to convert a string containing two hex digits to a character?

[John B. Matthews]

In article <hi0m2j$df4$1@news.albasani.net>,
 Leslie <jlturriff@centurytel.net> wrote:

> I need to be able to convert a string variable containing two
> hexadecimal digits (e.g. "7C") to its equivalent character
> value.

It's not directly related to your question, but you might enjoy looking 
at this program for converting the other way:

<http://sites.google.com/site/drjohnbmatthews/hexdump> 

-- 
John B. Matthews
trashgod at gmail dot com
<http://sites.google.com/site/drjohnbmatthews>