[Tutor] Array indexing

[Joe Abbey]

On 1/16/07, Dave Kuhlman <dkuhlman at rexx.com> wrote:
>
> On Tue, Jan 16, 2007 at 11:28:49AM -0500, Joe Abbey wrote:
> > Hello,
> >
> > I'm using Active Python v2.4.3.11 on a Windows XP machine.
> >
> > Probably more relevant is that I'm just learning Python, as in I've been
> > writing Python for less than 24 hours.
> >
> > While trying to implement a PE parser, I ran into the following problem:
> >
> > #************** START CODE*******************
> > data = file.read(128);
> > directoryTable = struct.unpack('LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL',
> data);
> > i=0;
> > print "Export table           0x%08X + 0x%08x" % (directoryTable[i+=1],
> > directoryTable[i+=1]);
> > print "Import table           0x%08X + 0x%08x" % (directoryTable[i+=1],
> > directoryTable[i+=1]);
> > #************** END CODE*******************
> >
> > This code throws a syntax error at the first i+=1 on "line 4".
> >
> > Why is this the case?
> >
>
> In Python, "i += 1" is a statement.  You have used in as an
> expression.  In Python, an expression returns a value; a statement
> does not.
>
> > It seems like it would be very useful to be able to increment an index
> after
> > referencing into an array.
> >
>
> What you are asking for is viewed by some as useful.  But, I think
> it is too confusing.  Should the variable be incremented before or
> after it is used to index into the array?  C/C++ gives you a
> choice: you can use either "i++" or "++i", which makes code harder
> to read, I think.  And, what about:
>
>     x = y[i+=1] + z[i]
>
> Has the second use of "i" been incremented or not.
>
> > Is my approach busted?  Is there a better way to reference elements?
> >
>
> Instead of:
>
>     x = directoryTable[i] + directoryTable[i+=1]);
>
> use something like:
>
>     x = directoryTable[i] + directoryTable[i+1]
>
> And, by the way, you do not need all those semicolons at the end of
> each line.  In Python, the semicolon is a statement separator, not
> a statement terminator.  It is more Pythonic to use a semicolon
> between statements only when there are more than one statement on a
> line.  And writing more than one statement on a line is usually
> discouraged anyway.
>
> > The "fix" I'm currently using is to write the index I want:
> >
> > (directoryTable[0], directoryTable[1])
>
> Or, if you need an index variable:
>
>     directoryTable[i], directoryTable[i+1])
>
> Dave
>
>
> --
> Dave Kuhlman
> http://www.rexx.com/~dkuhlman
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> http://mail.python.org/mailman/listinfo/tutor
>


Thank you Danny for the iterator tutorial. I'll check the link.

Thanks Dave for the language lesson.  As you could tell from my code
snippet, C\C++ is what I have programmed the most in.

For this case I believe the iterator example would be most preferred.

#************** START CODE*******************
dTable = iterator(directoryTable)
print "Export table           0x%08X + 0x%08x" % (dTable.next(), dTable.next
())
#************** END CODE*******************

But for now the explicit indexing will work fine :)

Thanks!

Joe
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