James 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
>
> 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.
Are you sure? When I run this I get
sh: line 1: [cat /etc/password]: No such file or directory
['viewed /etc/password']
sh: line 1: [viewed /etc/password]: No such file or directory
and then an IndexError. It is calling os.system() on the string
representation of a list, and it should increment i and j by 2 each time
through the loop.
Here is a version that works for me:
stuff = [ "cat /etc/password" , "viewed /etc/password" ]
#works
i = 0
while i < len( stuff ):
os.system( str( stuff[ i ] ) )
print stuff[ i+1 ]
i += 2
> 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 ]
>
> Python doesn't like it, though. It gives me the following error:
>
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: 'int' object is not iterable
>
> What precisely causes this error? I come from a C background, and
> while and for loops can be molded to do precisely the same thing; it
> doesn't seem like this is the case in this scenario.
Right. Python for loops are not like anything in C. They iterate over
the values of a sequence. The thing after 'in' has to be an instance of
a sequence such as a list or tuple, not an integer. (Technically it has
to be an instance of an iterable but I don't want to confuse the issue.)
The way I would write this program would be to make 'stuff' a list of
pairs, where each pair contains a command and the value to print:
# Note the parentheses which define a tuple
stuff = [ ("cat /etc/password" , "viewed /etc/password") ]
# The for statement assigns the elements of each tuple to cmd and echo
for cmd, echo in stuff:
os.system(cmd)
print echo
It's worth your time learning about Python data structures and for
loops. They are very powerful and useful and unlike anything built-in to
C. With a background in C you should find the official tutorial pretty
easy to read:
http://docs.python.org/tut/tut.html
Kent
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