[Tutor] for vs while

[taserian]

On 9/28/07, James <jtp at nc.rr.com> wrote:
>
> All,
>
> I have a dumb question...hopefully someone can shed some light on the
> difference between for and while in the situation below.
>
> I'm trying to iterate through a list I've created.  The list consists
> of a command, followed by a 'logging' message (a message printed to a
> console or log file after the command is run).
>
> Here's a small snippet of code:
>
>        # a list which includes (1) a command, and (2) something to be
> dumped into a log file after the command runs
>        stuff = [ ["cat /etc/password"] , ["viewed /etc/password"] ]
>
>        #works
>        i = 0 ; j = 1
>        while i < len( stuff ):
>                os.system( str( stuff[ i ] ) )
>                print stuff[ j ]
>                i += 1 ; j += 1


Here you're basing yourself off of the index of the item in the list, so
stuff[0], stuff[1], etc.


The while loop does precisely what it should do: it runs the first
> command using os.system(), and then prints out the string in the
> second position of the list.
>
> Then I tried to do the same thing with a for loop that looks
> logically equivalent.  I replaced the while loop with this for loop:
>
>        # doesn't work
>        for i in len( stuff ):
>                os.system( stuff[ i ] )
>                j = i + 1
>                print stuff[ j ]


Here, you're using *for* in a non-Pythonic way. You mean to use i as an
iterator, but len( stuff ) is a simple integer.

You could do it this way:

       for i in range( len(stuff)):
               os.system( stuff[i] )
               j = i + 1
               print stuff[ j ]

turning the single integer into a range of integers that you can iterate
over.

Tony R.
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