Hey i just need help with this bit of code.
def purify(lista):
for i in range(len(lista)):
if lista[i] % 2 != 0:
del lista[i]
return lista
Im just wondering why it doesn’t work. It’s purpose is to take a list and return the list without the odd elements.
Could be an indentation issue. But we can’t tell b/c the code isn’t formatted. Plus, there’s no code that informs me that you’ve called the function w/an argument.
Please use the “</>” button to format your code.
Okay, i think I’ve found what doesn’t make sense
def purify1(lista):
even = lista
for i in range(len(lista)):
if lista[i] % 2 != 0:
del(even[i])
return even
def purify2(lista):
odd = []
even = []
for i in range(len(lista)):
if lista[i] % 2 != 0:
odd.append(lista[i])
else:
even.append(lista[i])
return even
L = [1,2,3,4,5,6]
print purify2(L)
print purify1(L)
It’s the del operator. It somehoes messes the code up. It’s in the first code. In the second one it works just fine. 
The reason it doesn’t work correctly (or as expected) is because it is mutating the list (as in shortening) it is iterating. even is not a true copy, only a duplicate reference to the same list.
Bottom line, deleting members of a list causes everything to the right to move one index to the left to take up the void and maintain a contiguous index. When i is incremented it subsequently points to the next position, in effect skipping the one it should be pointing to.
Aside
To make an independent shallow copy of a list, there are two ways (three actually) to do this:
Changes made to listb will not be mirrored in lista.
Now let’s consider one approach to mutating a list directly while iterating.
la = [1, 2, 3, 4, 5]
lb = la[:]
for x in lb:
if x % 2 == 0:
la.remove(x)
print la
[1, 3, 5]
you could also use a list comprehension,
lst1 = [1, 7, 12, 3, 44, 52]
blah = [v for i, v in enumerate(lst1) if i % 2 != 0]
print(blah)
[7, 3, 52]
#not sure why lists are now greyed out like comments when formatting the code.
Ok, thank you I get it now. I had no idea that when I changed even I also changed lista. It’s this about shallow copy and deep copy, I have to read more.
After the opening triple tick type python
```python
I am using the button, “</>” to insert formatted code and never had an issue with it. I will use your suggestion next time.