[Tutor] Question regarding the len function of a list while using a loop

Brian van den Broek bvande at po-box.mcgill.ca
Thu Apr 14 19:15:44 CEST 2005


Ben Markwell said unto the world upon 2005-04-14 08:14:
> Could somebody explain to me why the code I used to complete this exercise 
> doesn't work. 
> And how do you send an integer to len?
> 
> Thanks
> 
> Ben
> 
> ==================================
> 
> 
> *As an exercise, write a loop that traverses a list and prints the length of 
> each element. What happens if you send an integer to len?
> 
> *>>> foo = ['spam!', 1, ['brie', 'cheddar', 'swiss'], [1, 2, 3]]
> 
>>>>i = 0
>>>>while i < len(foo):
> 
> print len(foo[i])
> i = i+1
> 
> 
> 5
> 
> Traceback (most recent call last):
> File "<pyshell#28>", line 2, in -toplevel-
> print len(foo[i])
> TypeError: len() of unsized object


Hi Ben,

Max and Alberto have already explained why you got the error message.

Depending on the context in which you are doing this, the having the 
error crash the program might be what is desired. (I get that the 
context is simply an exercise, but let's pretend it is an actual 
program, written in anger :-).) Or, you might want your program to 
handle the problem more elegantly.

Using the general framework Alberto suggested, here's a way to do that 
(I've also thrown some string formatting in to make the output more 
friendly):

 >>> foo = ['spam!', 1, ['brie', 'cheddar', 'swiss'], [1, 2, 3]]
 >>> for item in foo:
... 	try:
... 		print "Item: %s has length %s" %(item, len(item))
... 	except TypeError:
... 		print "Item: %s has no length" %item
... 		
Item: spam! has length 5
Item: 1 has no length
Item: ['brie', 'cheddar', 'swiss'] has length 3
Item: [1, 2, 3] has length 3
 >>>

This works by attempting to do the thing in the try part, and catching 
any case than raises a TypeError (as your original code did), doing 
the routine of the except block instead.

Does that make sense?

If not, post back and I can try to explain in more detail.

Best,

Brian vdB



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