more newbie help needed

[Steve Holden]

john boy wrote:
[top-posting corrected]
> 
> Steve Holden <steve at holdenweb.com> wrote:john boy wrote:
> 
>>using the following program:
>>
>>fruit = "banana"
>>index = 0
>>while index < len (fruit):
>>letter = fruit[index-1]
>>print letter
>>index= index -1
>>
>>this program is supposed to spell "banana" backwards and in a vertical patern...it does this....but after spelling "banana" it gives an error message:
>>refering to line 4: letter = fruit[index-1]
>>states that it is an IndexError and the string index is out of range
>>Anybody know how to fix this?
>>
> 
> Change the termination condition on your loop. If you print out the 
> values of index as the loop goes round you'll see you are printing 
> characters whose index values are -1, -2, ..., -6
> 
> Unfortunately -7 is less than -6, so your loop keeps on going, with the 
> results you observe.
> 
> Note, however, that there are more pythonic ways to perform this task. 
> You might try:
> 
> for index in range(len(fruit)):
> letter = fruit[-index-1]
> print letter
> 
> as one example. I'm sure other readers will have their own ways to do 
> this, many of them more elegant.
> 
thanks for your example....but for information purposes so I can grasp 
the basics....could you give me an example of how I could terminate the 
"while" loop

Well in this case, when fruit == "banana" the last value you want to use 
is -6 - you've already seen that using -7 causes the interpreter to 
raise an exception.

So you could use, for example,

     while -index <= len(fruit)

though I haven't tested that. Basically you need to work out a condition 
that's true for all useful values of index and false for the first 
non-useful value.

By the way, I've copied this reply to the list. It's customary to keep 
follow-ups on the list, that way if I'd been too busy to answer somebody 
else might have been able to help.

regards
  Steve
-- 
Steve Holden       +44 150 684 7255  +1 800 494 3119
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