On Thu, 13 May 2010 03:58:45 am Su Chu wrote:
> My problem is as follows:
> I have three lists, one with unique values (list 1), one a sequence
> of values that are not necessarily unique (list2), and a shorter list
> with the unique values of list 2 (list 3). List 1 and List 2 are of
> equal lengths.
>
>
> An example:
> list1 = [ 1, 2, 3, 4, 5, 6 ]
> list2 = [ 2, 2, 2, 5, 6, 6 ]
> list3 = [2, 5, 6]
>
> What I would like to do is find and sum the elements of list 1 given
> its corresponding element in list 2 is equal to some element in list
> 3.
Rather than trying to find a single magic command that happens to do
exactly what you want, let's break that up into individual steps and
walk through it by hand.
Start at the end -- you want to match elements from list2 which equals a
particular value. Where that value comes from doesn't matter.
results = []
for el in list2:
if el = value:
results.append(el)
Unfortunately, doing this loses the information we really need: the
*position* of the element. We don't actually care about the element
itself. How can we get the position? That's what the enumerate()
function is for, it takes a list and lazily returns pairs of (position,
element). So let's adapt the for-loop to do the job:
positions = []
for (pos, el) in enumerate(list2):
if el == value:
positions.append(pos)
This can be written as a "list comprehension", which is syntactic sugar
for a loop:
positions = [pos for (pos, el) in enumerate(list2) if el == value]
Now we have a list of positions. We need to extract the elements in
list1 at those positions. Here's a for-loop to do it:
elements = []
for pos in positions: # positions defined above
elements.append(list1[pos])
And as list comps:
positions = [pos for (pos, el) in enumerate(list2) if el == value]
elements = [list1[pos] for pos in positions]
But we don't need two separate loops, we can combine them into one loop:
elements = []
for (pos, el) in enumerate(list2):
if el == value:
elements.append(list1[pos])
And as a single list comp:
elements = [list1[pos] for (pos,el) in enumerate(list2) if el in list3]
And now sum them:
sum(elements)
Now let's put this into a function, so you can call it with different
values:
def extract_and_sum(list1, list2, value):
elements = [list1[i] for (i,x) in enumerate(list2) if x in list3]
return sum(elements)
And now call it in a loop:
sums = []
for value in list3:
sums.append(extract_and_sum(list1, list2, value))
And you are done.
--
Steven D'Aprano
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