In matlab I can do the following:

ind = 3 5 7 2 4 7 8 24

but I can't get the last line in python:

In [690]: ind = [3,5,7,2,4,7,8,24]
In [691]: ind[0] Out[691]: 3
In [692]: ind[-1:] Out[692]: [24]
In [693]: ??

How do I pull out multiple indices as in matlab?


Thanks, Evan

Evan wrote in news:1166379931.054933.77450@j72g2000cwa.googlegro ups.com in
comp.lang.python:

In matlab I can do the following:
>

ind = 3 5 7 2 4 7 8 24

>
but I can't get the last line in python:
>
In [690]: ind = [3,5,7,2,4,7,8,24]
In [691]: ind[0] Out[691]: 3
In [692]: ind[-1:] Out[692]: [24]
In [693]: ??
>
How do I pull out multiple indices as in matlab?

[ind[0], ind[-1]]

or if you need something that can be generalised:

[ind[i] for i in [0, -1]]

so if you have:

indexes_of_ind = [0, 2, -1, -2]

you can write:

[ind[i] for i in indexes_of_ind]


Rob.
--
http://www.victim-prime.dsl.pipex.com/

"Evan" <evanmason@gmail.comwrote in message
news:1166379931.054933.77450@j72g2000cwa.googlegro ups.com...

In matlab I can do the following:
>

ind = 3 5 7 2 4 7 8 24

>
but I can't get the last line in python:
>
In [690]: ind = [3,5,7,2,4,7,8,24]
In [691]: ind[0] Out[691]: 3
In [692]: ind[-1:] Out[692]: [24]
In [693]: ??
>
How do I pull out multiple indices as in matlab?
>
>
Thanks, Evan
>

Or use the third element of a slice, which defines a stepsize, and pick a
step that will go from the first to the last element:

['A', 'B', 'C', 'D', 'E', 'F', 'G']

['A', 'G']

-- Paul

Evan wrote:

In matlab I can do the following:
>

ind = 3 5 7 2 4 7 8 24

>
but I can't get the last line in python:
>
In [690]: ind = [3,5,7,2,4,7,8,24]
In [691]: ind[0] Out[691]: 3
In [692]: ind[-1:] Out[692]: [24]
In [693]: ??
>
How do I pull out multiple indices as in matlab?

If you want functionality similar to Matlab in Python, you should use
Numpy, which has the "take" function to do what you want.

Beliavsky wrote:

Evan wrote:

>
If you want functionality similar to Matlab in Python, you should use
Numpy, which has the "take" function to do what you want.

Actually, in numpy, we also have "fancy indexing" similar to Matlab's:


In [1]: from numpy import *

In [2]: ind = array([3,5,7,2,4,7,8,24])

In [3]: ind[[0, -1]]
Out[3]: array([ 3, 24])


--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

It's quite straight forward, actually. What you need to know is that -1
is the index of the last element in a sequence, and that slicing
excludes the 'end' value in 'start:end'. So if you type arr[0:N], you
get the subsequence

[arr[0], arr[1], arr[2], ..., arr[N-1]]

When comparing with Matlab, Python slicing works like this:

arr(1:end) - arr[:] or arr[0:]
arr(1:end-1) -arr[:-1] or arr[0:-1]
arr(1:end-N) -arr[:-N] or arr[0:-N]
arr(end) -arr[-1]
arr(1) - arr[0]
arr(1:2:end) - arr[::2] or arr[0::2]
arr(1:2:end-1) -arr[:-1:2] or arr[0:-1:2]

Python slicing is not completely like Matlab, because it was adoped
from Haskell. It can do the same as Matlab's indexing, but the syntax
is different. If you think Matlab's indexing is more intuitive it is
just because you are more used to it.

Thanks for all the replies, it's much clearer now.

-Evan