Question

def compare_two_lists(list1,list2):
   i=0
   j=0
   while i < len(list2) :
      if i%2 == 0:
         j == 0
      else:
         j == 1
      for sublist2 in list2[i:] :
         for sublist in list1[j:]:
            #sublist.intersection(sublist2)
            intersect =  [x for x in sublist if x in sublist2]
            print('from set ',sublist, len(intersect),' matched number(s): ', intersect)
            i=i +1

compare_two_lists([[1,2,3,4,5],[20,30]],[[5,3,7,8,1],[20,10],[4,10,1,7,8],[30,20]])

I am trying to get list 0 and 1 of list 1 to compare appropriately list 0, 1, 2 and 3 of list 2 and return matches. The program almost works in the sense that it does return matches for the lists amongst other iterations. I cannot seem to be able to get the iteration to occur twice and return [1,3,5],[20], [1,4],[20,30]. Please help. I am going quite mad trying to understand how to space functions correctly and use loops logically!!

Answer 1

def compare_two_lists(list1,list2):
    lRet = []                         #a
    for i in range(len(list2)):       #b
        j= i%2                        #c
        sublist1 = list1[j]           
        sublist2 = list2[i]           
        lRet.append([x for x in sublist1 if x in sublist2])   
    return lRet 

compare_two_lists([[1,2,3,4,5],[20,30]],[[5,3,7,8,1],[20,10],[4,10,1,7,8],[30,20]])

This seems to do the trick. Here’s an explanation (see line labels above):

• a: lRet is initialised. We are doing to build up a list of lists in this variable
• b: i runs through every index of list2
• c: if i is even then j is zero, otherwise j is one. you had j==1 and j==0 in your code. These operators don’t change any operand values. you meant j=1 etc. but this way is quicker

For explaining the rest I’ll just talk through the actual iteration with your example input lists:

i = 0 (the first iteration)

j=0 
sublist1 = [1,2,3,4,5]
sublist2 = [5,3,7,8,1]
intersection is [1,3,5]. we APPEND this to lRet. 
thus lRet = [[1,3,5],]

i=1

j=1
sublist1 = [20,30]
sublist2 = [20,10]
the intersection is [20,]
thus lRet = [[1,3,5],[20]]

i=3

j=0
sublist1 = [1,2,3,4,5]
sublist2 = [4,10,1,7,8]
etc