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Groups > comp.lang.python > #46746

Re: How to get an integer from a sequence of bytes

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On Sun, 02 Jun 2013 21:25:45 +0200, Mok-Kong Shen
<mok-kong.shen@t-online.de> declaimed the following in
gmane.comp.python.general:


> b'7' is the byte with the character 7 in a certain code, so that's
> ok. In other PLs one assigns an int to a byte, with that int in either

	In other languages "byte" is an 8-bit signed/unsigned numeric.

	But what you have is a Python 3.x "bytes" structure -- similar to a
character string in Python 2.x...

> decimal notation or hexadecimal notation, or else one assigns a
> character to it, in which case it gets the value of the character
> in a certain code. What I don't yet understand is why Python is
> apprently different from other PLs in that point in not allowing direct
> coersion of a byte to an int.
>

	As you've been shown, the first step is that you may have to
subscript it; even with just one byte, the structure is still a
"string/array". NOTE: that example doesn't work in 2.7, since
subscripting what is a "string" still returns a substring (of one
character).

	Python doesn't have a "numeric" byte type -- the b"..." is an
"array" of 8-bit values in Python 3.x, and is just a character string in
2.x

	A language like C didn't have a "string" type... "char" was a
pseudonym for "numeric byte" (and some even support "unsigned char" vs
"signed char").

	Maybe you'd like to program in Ada... Where "7" is a "string of
length 1" and '7' is a character -- and you have to do type conversions
to assign the latter to the former.

	Heck:

with Text_IO; use Text_IO;

procedure Bytes is
    
begin
    if "7" = '7' then
	Put_Line ("string 7 is equal to character 7");
    else
	Put_Line ("string 7 is NOT equal to character 7");
    end if;
end Bytes;

WON'T compile... string can not be compared to character!

with Text_IO; use Text_IO;

procedure Bytes is
    
    A_String : String (1 .. 1);
    A_Char : Character := '7';
    
    
begin
    A_String := A_Char;
    
end Bytes;

	The above fails to compile, whereas the following is valid Ada

with Text_IO; use Text_IO;

procedure Bytes is
    
    A_String : String (1 .. 1);
    A_Char : Character := '7';
    
    
begin
    A_String(1) := A_Char;
    
end Bytes;

	Don't even ask about /numeric/ bytes and strings (or characters). Or
lets...

with Text_IO; use Text_IO;

procedure Bytes is
    
    type Byte is mod 256;
    
    A_String : String (1 .. 1);
    Char : Byte := 7;

    
begin
    A_String (1) := Char;
    
end Bytes;

Fails... But...

with Text_IO; use Text_IO;

procedure Bytes is
    
    type Byte is mod 256;
    
    A_String : String (1 .. 1);
    Char : Byte := 7;
    
begin
    A_String (1) := Character'Val (Char);
    
end Bytes;

	That takes a byte data type (unsigned 8-bit value)... Asks for the
CHARACTER data type having the value equivalent to the "position" of the
byte... And then stuff that into the only element of a STRING data type.

-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/

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Thread

How to get an integer from a sequence of bytes Mok-Kong Shen <mok-kong.shen@t-online.de> - 2013-05-27 16:45 +0200
  Re: How to get an integer from a sequence of bytes Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-27 15:00 +0000
    RE: How to get an integer from a sequence of bytes Carlos Nepomuceno <carlosnepomuceno@outlook.com> - 2013-05-28 03:37 +0300
  Re: How to get an integer from a sequence of bytes Ned Batchelder <ned@nedbatchelder.com> - 2013-05-27 11:30 -0400
    Re: How to get an integer from a sequence of bytes Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-28 00:31 +0000
      Re: How to get an integer from a sequence of bytes Dave Angel <davea@davea.name> - 2013-05-27 20:41 -0400
    Re: How to get an integer from a sequence of bytes Mok-Kong Shen <mok-kong.shen@t-online.de> - 2013-05-30 20:26 +0200
      Re: How to get an integer from a sequence of bytes Ian Kelly <ian.g.kelly@gmail.com> - 2013-05-30 12:42 -0600
        Re: How to get an integer from a sequence of bytes jmfauth <wxjmfauth@gmail.com> - 2013-05-30 11:53 -0700
      Re: How to get an integer from a sequence of bytes Ned Batchelder <ned@nedbatchelder.com> - 2013-05-30 15:22 -0400
        Re: How to get an integer from a sequence of bytes Mok-Kong Shen <mok-kong.shen@t-online.de> - 2013-06-02 21:25 +0200
          Re: How to get an integer from a sequence of bytes Chris Angelico <rosuav@gmail.com> - 2013-06-03 05:54 +1000
          Re: How to get an integer from a sequence of bytes Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-06-02 21:48 -0400
            Re: How to get an integer from a sequence of bytes Grant Edwards <invalid@invalid.invalid> - 2013-06-03 14:31 +0000
              Re: How to get an integer from a sequence of bytes Dave Angel <d@davea.name> - 2013-06-03 18:07 -0400
                Re: How to get an integer from a sequence of bytes Grant Edwards <invalid@invalid.invalid> - 2013-06-03 22:34 +0000
              Re: How to get an integer from a sequence of bytes Dan Stromberg <drsalists@gmail.com> - 2013-06-03 15:41 -0700
                Re: How to get an integer from a sequence of bytes Grant Edwards <invalid@invalid.invalid> - 2013-06-04 13:39 +0000
                Re: How to get an integer from a sequence of bytes Joshua Landau <joshua.landau.ws@gmail.com> - 2013-06-04 20:51 +0100
                Re: How to get an integer from a sequence of bytes Chris Angelico <rosuav@gmail.com> - 2013-06-05 07:49 +1000
                Re: How to get an integer from a sequence of bytes Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-06-04 20:34 -0400
                Re: How to get an integer from a sequence of bytes Tim Roberts <timr@probo.com> - 2013-06-04 22:11 -0700
                Re: How to get an integer from a sequence of bytes Fábio Santos <fabiosantosart@gmail.com> - 2013-06-12 15:00 +0100
                Re: How to get an integer from a sequence of bytes Grant Edwards <invalid@invalid.invalid> - 2013-06-12 14:42 +0000
              RE: How to get an integer from a sequence of bytes Carlos Nepomuceno <carlosnepomuceno@outlook.com> - 2013-06-04 02:18 +0300
                Re: How to get an integer from a sequence of bytes Grant Edwards <invalid@invalid.invalid> - 2013-06-04 13:42 +0000
                RE: How to get an integer from a sequence of bytes Carlos Nepomuceno <carlosnepomuceno@outlook.com> - 2013-06-04 16:58 +0300
              Re: How to get an integer from a sequence of bytes Dan Stromberg <drsalists@gmail.com> - 2013-06-03 16:47 -0700
  Re: How to get an integer from a sequence of bytes Grant Edwards <invalid@invalid.invalid> - 2013-05-28 16:04 +0000

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