can't see the error with a list method in python

Question

I'am working on a exercise and I can't see where I get it wrong. I'm trying to add the indexes of the sum of two numbers to a list, but instead of getting the indexes, I'm actually getting the numbers. Here is my code, and many thanks in advance for your precious help and time.

a_list = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

target_list = []

counter = 0

for i in range(len(a_list)):
  for j in a_list:
      if i + j == 10:
          target_list.append((a_list.index(i), a_list.index(j)))
          counter += 1

print(target_list)

how can I do it with list comprehension. Thank you again and again.

What purpose does the counter variable serve? It is also not entirely clear what output you expect since the indeces are equal to the values in your example. Pls chose more general sample data and show the expected output. — user2390182
– user2390182, Commented Dec 20, 2020 at 11:08
you have no problem in your specific case cuz item IS index, what are you expecting? — adir abargil
– adir abargil, Commented Dec 20, 2020 at 11:10

U13-Forward · Accepted Answer · 2020-12-20 11:08:36Z

1

You are getting the indexes, here is an example, if you change your list to:

a_list = [0, 10, 20, 30, 40, 50, 60, 70, 80, 90]

The output would be different:

[(0, 1)]

answered Dec 20, 2020 at 11:08

U13-Forward

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Comments

adir abargil · Accepted Answer · 2020-12-20 11:17:25Z

what you did doesnt make any sense.. :

a_list = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

target_list = []
for i in range(len(a_list)):
  for j in a_list:
      if i + j == 10:
          target_list.append((a_list.index(i), a_list.index(j)))
>>> target_list 
[(1, 9), (2, 8), (3, 7), (4, 6), (5, 5), (6, 4), (7, 3), (8, 2), (9, 1)]

which is equal to :

a_list = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

target_list = []
for i in a_list:
  for j in a_list:
      if i + j == 10:
          target_list.append((a_list.index(i), a_list.index(j)))
>>> target_list 
[(1, 9), (2, 8), (3, 7), (4, 6), (5, 5), (6, 4), (7, 3), (8, 2), (9, 1)]

the point in python loops is that when you call for <item> in <Iterable>: it inserts the items to item and iterating each one by order, when you call range(len(list_a)) it generates a list from 0 to the length of a_list , that is eaqual to [0,1,2,3,4,5,6,7,8,9] and this is the same thing as you list...

you could also do the following:

target_list = [(i,j) for j in range(10) for i in range(10) if j+i ==10]
Target_list 
>>>
[(9, 1), (8, 2), (7, 3), (6, 4), (5, 5), (4, 6), (3, 7), (2, 8), (1, 9)]

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