# Clarification Python Analysis basics step 3

Hi, I struggle to understand the slicing code here “decade = decade[:-1]” :

``````for age in final_ages:
if age == "Unknown":

else :
``````

I see the difference when looking at the final “decades” list, but I am missing why.

Pretty sure it has been seen in a previous lesson, so if you happen to know which one tell me I will retake it as I dont understand my mistake.

Hi Nicolas, welcome to the community!

I think this little summary of this type of list slicing will help:

• `a_list[:x]` when we want to select the first `x` elements.
• `a_list[-x:]` when we want to select the last `x` elements.
• `a_list[x:]` when we want to select all the elements after the first `x` elements
• `a_list[:-x]` when we want to select all the elements up to the last `x` elements

In the case of decades, we want to keep all but the last digit and change it so that 34 becomes 30s, 103 becomes 100s, and so on.

With `decade[1:]`, this means we’re keeping all but the first element. So then `34` would become `40s` instead of `30s`.

With `decade[:-1]`, we’re keeping all but the last element, so `34` becomes `30s`

I hope that helps clear things up.

1 Like

Hello april,

Thank you very much for the summary and explanation !
It does indeed clarify everything.