Re: [Tutor] Array indexing
On 1/16/07, Dave Kuhlman <[EMAIL PROTECTED]> 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('',
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
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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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Re: [Tutor] Array indexing
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('', 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@python.org
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Re: [Tutor] Array indexing
> While trying to implement a PE parser, I ran into the following problem:
>
> #** START CODE***
> data = file.read(128);
> directoryTable = struct.unpack('', 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".
Hi Joe,
Yes. Python's assignments aren't expressions --- in Python, assignments
are meant to visually stand out. Unfortunately, this means you can't put
the assignment within the array indexing expression.
There are a few workarounds. One is to treat the directoryTable as a
stream of values that we can iterate across. For example:
##
>>> values = (3, 1, 4, 1, 5)
>>> i = iter(values)
>>> i
##
'i' here is an "iterator" that we can repeatedly use to get sequential
elements:
#
>>> i.next()
3
>>> i.next()
1
>>> i.next()
4
#
In some sense, this should allow you to do what you had in your original
code, since i.next() will both give you the value and, internally, move
the iterator forward.
See:
http://www.python.org/doc/tut/node11.html#SECTION001190
for a quick-and-dirty introduction to iterators.
Best of wishes!
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