first and last index as in matlab

[Evan]

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]

Evan wrote in 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 for i in [0, -1]]

so if you have:

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

you can write:

[ind for i in indexes_of_ind]


Rob.

 

[Paul McGuire]

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]

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]

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
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

 

[sturlamolden]

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]

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

-Evan