[Tutor] New to programming question

[Brian van den Broek]

Ben Markwell said unto the world upon 2005-04-12 12:56:
> This is an exercise from "How to think like a Computer Scientist."
> 
> 
> The following example shows how to use concatenation and a for loop to 
> generate an abecedarian series. "Abecedarian" refers to a series or list in 
> which the elements appear in alphabetical order. For example, in Robert 
> McCloskey's book *Make Way for Ducklings*, the names of the ducklings are 
> Jack, Kack, Lack, Mack, Nack, Ouack, Pack, and Quack. This loop outputs 
> these names in order: 
> 
> prefixes = "JKLMNOPQ" 
> suffix = "ack" 
> 
> for letter in prefixes: 
> print letter + suffix 
> 
> The output of this program is: 
> 
> Jack 
> Kack 
> Lack 
> Mack 
> Nack 
> Oack 
> Pack 
> Qack 
> 
> Of course, that's not quite right because "Ouack" and "Quack" are 
> misspelled.*
> *
> 
> *As an exercise, modify the program to fix this error.
> *
> 
> ==================================================
>  
> In trying to solve the problem I have come up with the following:
>  
> prefixes = 'JKLMNOPQ'
> suffix = 'ack'
> xsuffix = 'uack'
> 
> 
> for letter in prefixes:
> n = 0
> if prefixes[n] == 'O' or 'Q':
> print prefixes[n] + xsuffix
> else:
> print letter + suffix
>  
> --- I know it doesn't work, but want to know if I am on the right track. And 
> what is the solution?
>  
> Thanks
>  
> Ben
> **
>

Hi Ben,

in generally, it is a good idea to say *why* it doesn't work.
Sometimes it won't be clear what you expected as output, so it also
won't be clear why you are disappointed.

That said, see if this helps:

>>> if 'Q' == 'O' or 'Q': print "Yep (or is it?)"
...
Yep (or is it?)
>>> if 'B' == 'O' or 'Q': print "Yep (or is it?)"
...
Yep (or is it?)


Probably not what is wanted. What happens here is Python first evaluates
'Q' == 'O'
and, if it evaluates to true, returns it. But, in neither case does it 
evaluate to true. So Python then turns to evaluating 'Q'. But that 
*always* evaluates to true. So Python returns 'Q', and the if test 
above is always met. See:

>>> if 'Q': print "That evaluated to True"
...
That evaluated to True
>>> False or 'Q'
'Q'
>>> 42==42 or 'Q'
True
>>> 42==17 or 'Q'
'Q'
>>>

There are a couple of different ways to get the test I think you want.
Here's what I'd do:

>>> if 'Q' in ('O', 'Q'): print "Thank goodness!"
...
Thank goodness!
>>> if 'B' in ('O', 'Q'): print "Thank goodness!"
...
>>>

Now, fix that up so it is not testing a hardcoded value (i.e. make it
other than "if 'Q' ... ") and see if that helps.

Post again if not.

Best,

Brian vdB