Velocity Reviews > first and last index as in matlab

# first and last index as in matlab

Evan
Guest
Posts: n/a

 12-17-2006
In matlab I can do the following:

>> ind = [3,5,7,2,4,7,8,24]

ind = 3 5 7 2 4 7 8 24
>> ind(1) ans = 3
>> ind(end) ans = 24
>> ind([1 end]) ans = 3 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

Rob Williscroft
Guest
Posts: n/a

 12-17-2006
Evan wrote in news:(E-Mail Removed) ups.com in
comp.lang.python:

> In matlab I can do the following:
>
>>> ind = [3,5,7,2,4,7,8,24]

> ind = 3 5 7 2 4 7 8 24
>>> ind(1) ans = 3
>>> ind(end) ans = 24
>>> ind([1 end]) ans = 3 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/

Paul McGuire
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Posts: n/a

 12-17-2006
"Evan" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ups.com...
> In matlab I can do the following:
>
>>> ind = [3,5,7,2,4,7,8,24]

> ind = 3 5 7 2 4 7 8 24
>>> ind(1) ans = 3
>>> ind(end) ans = 24
>>> ind([1 end]) ans = 3 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:

>>> lst = list("ABCDEFG")
>>> lst

['A', 'B', 'C', 'D', 'E', 'F', 'G']
>>> lst[0::len(lst)-1]

['A', 'G']

-- Paul

Beliavsky
Guest
Posts: n/a

 12-17-2006

Evan wrote:
> In matlab I can do the following:
>
> >> ind = [3,5,7,2,4,7,8,24]

> ind = 3 5 7 2 4 7 8 24
> >> ind(1) ans = 3
> >> ind(end) ans = 24
> >> ind([1 end]) ans = 3 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.

Robert Kern
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Posts: n/a

 12-17-2006
Beliavsky wrote:
> Evan wrote:
>> In matlab I can do the following:
>>
>>>> ind = [3,5,7,2,4,7,8,24]

>> ind = 3 5 7 2 4 7 8 24
>>>> ind(1) ans = 3
>>>> ind(end) ans = 24
>>>> ind([1 end]) ans = 3 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.

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
an underlying truth."
-- Umberto Eco

sturlamolden
Guest
Posts: n/a

 12-17-2006

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.

Evan
Guest
Posts: n/a

 12-18-2006
Thanks for all the replies, it's much clearer now.

-Evan