Lessons · Python · build the list outside the loop
Build the list outside the loop
Create an empty list before the loop, append inside it, and use it after. One list, filled one item at a time.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
Filtering matching records, gathering the lines that failed, collecting every price over a limit: half of everyday code is walk a sequence, keep some of it.
How to think about it
Three places: the empty list above the loop, the append inside, the use below. If the list is created inside the loop it starts over every time and you keep one item.
Worked example
out = []Empty, created once, before the loop.
for x in [1, 2, 3, 4]:Walk every item.
if x % 2 == 0:Keep the even ones.
out.append(x * 10)Add inside the loop.
print(out)[20, 40]: both kept, in order.
Your turn
Keep every word longer than three letters.
long = []
for w in words:
if len(w) > 3:
long.(w)Solve one with the tests running
The trap
out = out.append(x) sets out to None, because append returns nothing. Call it on its own line.
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