newbie question

Fri Apr 1 17:03:03 EDT 2011

Ah, all right. Thank you very much, eryksun!
On 2011年04月01日 22:48:44 +0200, eryksun () said:
> Regarding the format of your post, please use plain text only.
>> On Friday, April 1, 2011 3:52:24 PM UTC-4, Karl wrote:
>>>> aList = [0, 1, 2, 3, 4]
>> bList = [2*i for i in aList]
>> sum = 0
>> for j in bList:
>> sum = sum + bList[j]
>> print j
>>>> 0
>> 2
>> 4
>>>> IndexError: list index out of range
>>>> Why is j in the second run 2 and not 1 in the for-loop?? I think
>> j is a control variable with 0, 1, 2, 3, ...
>> The for loop yields the elements of bList, not their index (it's like a 
> foreach statement in other languages). Here's the full printout:
>> In [1]: aList = [0, 1, 2, 3, 4]
> In [2]: bList = [2*i for i in aList]
> In [3]: for j in bList:
> ...: print j
>> 0
> 2
> 4
> 6
> 8
>> Since bList is 5 elements long, bList[6] is indeed "out of range". 
> Here's a more direct way to accomplish this task:
>> In [4]: b_sum = sum(bList)
> In [5]: b_sum
> Out[5]: 20
>> Note that I'm not overwriting the name of the built-in function 'sum'. 
> Try to avoid doing that. It's pretty simple to type the name in the 
> interpreter and see if it's already bound to something you might want 
> to keep around.
>> Here's one way to get the index in a loop:
>> In [6]: b_sum = 0
> In [7]: for j, b in enumerate(bList):
> ...: b_sum += b
> ...: print j
>> 0
> 1
> 2
> 3
> 4
>> In [9]: b_sum
> Out[9]: 20
>> The enumerate function returns an iterable that's a tuple of the index 
> and item. A for loop iterates over an iterable object such as a 
> sequence.