[Tutor] Array indexing

[Danny Yoo]

> 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".


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
<tupleiterator object at 0x6dd50>
##################################

'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#SECTION0011900000000000000000

for a quick-and-dirty introduction to iterators.


Best of wishes!