python list index - an easy question

BartC bc at freeuk.com
Sat Dec 17 17:53:43 EST 2016


On 17/12/2016 19:10, John wrote:
> Hi,
>
>    I am new to Python, and I believe it's an easy question. I know R and Matlab.
>
> ************
>>>> x=[1,2,3,4,5,6,7]
>>>> x[0]
> 1
>>>> x[1:5]
> [2, 3, 4, 5]
> *************
>
>     My question is: what does x[1:5] mean?

x[A:B] means the slice consisting of x[A], x[A+1],... x[B-1]. (Although 
slices can shorter including those with be 0 or 1 elements.)

> By Python's convention, the first element of a list is indexed as "0".

Or the slice from the (A+1)th element to the B'th element inclusive, if 
you are informally using ordinal indexing (first, second, third etc).

> Doesn't x[1:5] mean a sub-list of x, indexed 1,2,3,4,5?

Sublists and slices, once extracted, are indexed from 0 too.

Play around with some test code, but avoid test data containing numbers 
that are not too different from possible indices as that will be confusing!

Strings might be better:

   x = "ABCDEFGHIJKLM"

   print (x[1:5])

displays: BCDE

   print (x[1:5][0:2])        # slice of a slice

displays: BC

-- 
Bartc


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