Problem to read from array

[Laura Creighton]

In Python we don't write while loops that often.
instead we do:

for i in range(NumberOfColumns):
    for j in range(NumberOfRows):
    	do_something()

But for the particular case that you are after, constructing lists,
we have something even neater -- list comprehensions.

So you might write:

 >>> newarray=[]
 >>> for i in range(3):
 ...     newarray[i] = i

And this would not work.

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
IndexError: list assignment index out of range

At this point you might get a bit frustrated.  Python is happily telling
you that you don't have a newarray[0][0] which is hardly news to you who
was trying to initialise the thing.

The trick is to initialise it with a list comprehension.

You write one like this:
[ calculate_something() for i in range(LIMIT)]

This works:
 >>> newarray = [i for i in range(3)]
 >>> newarray 
[0, 1, 2]
 >>> 

As does
 >>> newarray = ["hello" for i in range(3)]
 >>> newarray
['hello', 'hello', 'hello']

You can even build your lists with a condition:
 >>> even_numbers=[x for x in range(11) if x % 2 == 0]
 >>> even_numbers
[0, 2, 4, 6, 8, 10]

You are not limited to one dimentional arrays (lists) here.  You
can have much more complicated expressions as the calculate_something.

 >>> newarray =[[ [x+2000, y*y] for x in range(3)] for y in range(5)]
 >>> newarray
[[[2000, 0], [2001, 0], [2002, 0]], [[2000, 1], [2001, 1], [2002, 1]], 
[[2000, 4], [2001, 4], [2002, 4]], [[2000, 9], [2001, 9], [2002, 9]], 
[[2000, 16], [2001, 16], [2002, 16]]]

Which leads us to a particularly useful construct:
 >>> emptyarray =[[ [] for x in range(3)] for y in range(5)]
 >>> emptyarray
[[[], [], []], [[], [], []], [[], [], []], [[], [], []], [[], [], []]]

Bingo.  Now you won't have any index errors when you try to append
to your x,y values.

Hope this is useful.
Also, there is the tutor mailing list
https://mail.python.org/mailman/listinfo/tutor
which you might be interested in, where we discuss things like this.
Peter Otten is there, too.

Laura