what does := means simply?

[Chris Angelico]

On Sat, May 19, 2018 at 8:33 PM, bartc <bc at freeuk.com> wrote:
> On 19/05/2018 02:26, Chris Angelico wrote:
>>
>> On Sat, May 19, 2018 at 11:10 AM, bartc <bc at freeuk.com> wrote:
>
>
>>> The .ppm (really .pbm) file which was the subject of this sub-thread has
>>> its
>>> header defined using ASCII. I don't think an EBCDIC 'P4' etc will work.
>>>
>>
>> "Defined using ASCII" is a tricky concept. There are a number of file
>> formats that have certain parts defined because of ASCII mnemonics,
>> but are actually defined numerically. The PNG format begins with the
>> four bytes 89 50 4E 47, chosen because three of those bytes represent
>> the letters "PNG" in ASCII. But it's defined as those byte values. The
>> first three represent "i&+" in EBCDIC, and that would be just as
>> valid, because you get the correct bytes.
>>
>> Your file contains bytes. Not text.
>
>
> Not you understand why some of us don't bother with 'text mode' files.
>
> However if you have an actual EBCDIC system and would to read .ppm files,
> then you will have trouble reading the numeric parameters as they are
> expressed using sequences of ASCII digits.
>
> The simplest way would be to pass each byte through an ASCII to EBCDIC
> lookup table (so that code 0x37 for ASCII '7', which is EOT in EBCDIC, is
> turned into 0xF8 which is EBCDIC '7').
>
> But then you are acknowledging the file is, in fact, ASCII.

Cool! So what happens if you acknowledge that a file is ASCII, and
then it starts with a byte value of E3 ?

ChrisA