[Tutor] Q?

[Daniel Yoo]

On Fri, 1 Sep 2000, deng wei wrote:

>   The Python said this is wrong.But I think it's right.Why?
> 
> 	# The while practice:
> 	a=['hello','world','i','am','a','worker']
> 	i=0
> 	while i<len(a):
> 		b[i]=a[len(a)-i]
> 		i=i+1
> 	print b

I'm guessing that this piece of code reverses the order of list 'a', and
places it into list 'b'.  This would work, except that we haven't sized
'b' yet as a new list.  Try it on the interpreter:

###
>>> a = ['hello', 'world', 'i', 'am', 'a', 'worker']
>>> i = 0
>>> while i < len(a):
...     b[i] = a[len(a) - i]
...     i = i + 1
... 
Traceback (innermost last):
  File "<stdin>", line 2, in ?
IndexError: list index out of range
###

To fix this, we need to properly set b to a list of equal size to
a.  Here's one quick way of doing it:

###
>>> b = list(range(len(a)))
>>> b
[0, 1, 2, 3, 4, 5]
###


I create a tuple 'b' of size 'len(a)', and convert it to a list, so that
we can change elements of b.  Afterwards, we should be ok:

###
>>> i = 0
>>> while i < len(a):
...     b[i] = a[len(a) - i]
...     i = i + 1
... 
Traceback (innermost last):
  File "<stdin>", line 2, in ?
IndexError: list index out of range
###


Hmm... I spoke too soon!  Let's think about this.  When i=0, then we try
to execute:

    b[i] = a[5 - 0]


Ah!  That's off the list!  We can read from a[0], a[1], a[2], ..., a[4],
but not a[5].  This is why we're getting the IndexError.

The solution is to subtract 1 from the offset.  If we do this, then the
loop will go through through b[4], b[3], ..., b[0].

###
    b[i] = a[len(a) - i - 1]
###

should be the corrected line.