Returning the positions of a list that are non-zero

[Benjamin Goudey]

I have a very large list of integers representing data needed for a
histogram that I'm going to plot using pylab. However, most of these
values (85%-95%) are zero and I would like to remove them to reduce
the amount of memory I'm using and save time when it comes to plotting
the data. To do this, I'm trying to find the best way to remove all of
the zero values and produce a list of indices of where the non-zero
values used to be.

For example, if my original list is [0,0,1,2,1,0,0] I would like to
produce the lists [1,2,1] (the non zero values) and [2,3,4] (indices
of where the non-zero values used to be). Removing non-zero values is
very easy but determining the indicies is where I'm having difficulty.

Thanks in advance for any help

[Rajanikanth Jammalamadaka]

Try this:

>>li=[0,0,1,2,1,0,0]
li

[0, 0, 1, 2, 1, 0, 0]

>>[i for i in range(len(li)) if li[i] != 0]

[2, 3, 4]

Cheers,

Raj

On Tue, Jul 8, 2008 at 10:26 PM, Benjamin Goudey <bw******@gmail.comwrote:

I have a very large list of integers representing data needed for a
histogram that I'm going to plot using pylab. However, most of these
values (85%-95%) are zero and I would like to remove them to reduce
the amount of memory I'm using and save time when it comes to plotting
the data. To do this, I'm trying to find the best way to remove all of
the zero values and produce a list of indices of where the non-zero
values used to be.

For example, if my original list is [0,0,1,2,1,0,0] I would like to
produce the lists [1,2,1] (the non zero values) and [2,3,4] (indices
of where the non-zero values used to be). Removing non-zero values is
very easy but determining the indicies is where I'm having difficulty.

Thanks in advance for any help
--
http://mail.python.org/mailman/listinfo/python-list

--
"For him who has conquered the mind, the mind is the best of friends;
but for one who has failed to do so, his very mind will be the
greatest enemy."

Rajanikanth

[Ben Finney]

Benjamin Goudey <bw******@gmail.comwrites:

For example, if my original list is [0,0,1,2,1,0,0] I would like to
produce the lists [1,2,1] (the non zero values) and [2,3,4] (indices
of where the non-zero values used to be). Removing non-zero values
is very easy but determining the indicies is where I'm having
difficulty.

The built-in 'enumerate' function is made for this. Creating the new
list can be done efficiently and clearly with a list comprehension.
Further, presumably you want to do further operations on these new
series of data; wouldn't it be better to have the values and original
indices actually correlated together?

This code produces a two-element tuple for each original non-zero
value:

>>sparse_data = [0, 0, 1, 2, 1, 0, 0]
nonzero_data = [(index, value)

... for (index, value) in enumerate(sparse_data)
... if value != 0]

>>nonzero_data

[(2, 1), (3, 2), (4, 1)]

>>for (index, value) in nonzero_data:

... print "Value %(value)r was originally at index %(index)r" % vars()
...
Value 1 was originally at index 2
Value 2 was originally at index 3
Value 1 was originally at index 4

--
\ “Experience is that marvelous thing that enables you to |
`\ recognize a mistake when you make it again.� —Franklin P. Jones |
_o__) |
Ben Finney

[Luis Zarrabeitia]

This could work:

l = [0,0,1,2,1,0,0]
indexes, values = zip(*((index,value) for index,value in enumerate(l) if value
!= 0))

But I guess it would be a little less cryptic (and maybe a lot more efficient)
if there were an unzip function instead of using the zip(*sequence) trick..

I think a more readable way would be:

indexes = [index for index,value in enumerate(l) if value != 0]
values = [value for value in l if value != 0]

Cheers.

--
Luis Zarrabeitia
Facultad de Matemática y Computación, UH
http://profesores.matcom.uh.cu/~kyrie
Quoting Benjamin Goudey <bw******@gmail.com>:

I have a very large list of integers representing data needed for a
histogram that I'm going to plot using pylab. However, most of these
values (85%-95%) are zero and I would like to remove them to reduce
the amount of memory I'm using and save time when it comes to plotting
the data. To do this, I'm trying to find the best way to remove all of
the zero values and produce a list of indices of where the non-zero
values used to be.

For example, if my original list is [0,0,1,2,1,0,0] I would like to
produce the lists [1,2,1] (the non zero values) and [2,3,4] (indices
of where the non-zero values used to be). Removing non-zero values is
very easy but determining the indicies is where I'm having difficulty.

Thanks in advance for any help
--
http://mail.python.org/mailman/listinfo/python-list

[Andrii V. Mishkovskyi]

2008/7/9 Benjamin Goudey <bw******@gmail.com>:

I have a very large list of integers representing data needed for a
histogram that I'm going to plot using pylab. However, most of these
values (85%-95%) are zero and I would like to remove them to reduce
the amount of memory I'm using and save time when it comes to plotting
the data. To do this, I'm trying to find the best way to remove all of
the zero values and produce a list of indices of where the non-zero
values used to be.

For example, if my original list is [0,0,1,2,1,0,0] I would like to
produce the lists [1,2,1] (the non zero values) and [2,3,4] (indices
of where the non-zero values used to be). Removing non-zero values is
very easy but determining the indicies is where I'm having difficulty.

Thanks in advance for any help

>>l = [0, 0, 1, 2, 1, 0, 0]
zip(*[(item, index) for (index, item) in enumerate(l) if item != 0])

[(1, 2, 1), (2, 3, 4)]

--
http://mail.python.org/mailman/listinfo/python-list

--
Wbr, Andrii Mishkovskyi.

He's got a heart of a little child, and he keeps it in a jar on his desk.

[Chris]

On Jul 9, 7:48*am, "Rajanikanth Jammalamadaka" <[e-mail]>
wrote:

Try this:

>li=[0,0,1,2,1,0,0]
li

[0, 0, 1, 2, 1, 0, 0]>>[i for i in range(len(li)) if li[i] != 0]

[2, 3, 4]

Cheers,

Raj

On Tue, Jul 8, 2008 at 10:26 PM, Benjamin Goudey <[e-mail]:

I have a very large list of integers representing data needed for a
histogram that I'm going to plot using pylab. However, most of these
values (85%-95%) are zero and I would like to remove them to reduce
the amount of memory I'm using and save time when it comes to plotting
the data. To do this, I'm trying to find the best way to remove all of
the zero values and produce a list of indices of where the non-zero
values used to be.

For example, if my original list is [0,0,1,2,1,0,0] I would like to
produce the lists [1,2,1] (the non zero values) and [2,3,4] (indices
of where the non-zero values used to be). Removing non-zero values is
very easy but determining the indicies is where I'm having difficulty.

Thanks in advance for any help
--
http://mail.python.org/mailman/listinfo/python-list

--
"For him who has conquered the mind, the mind is the best of friends;
but for one who has failed to do so, his very mind will be the
greatest enemy."

Rajanikanth

That's a waste

>>li=[0,0,1,2,1,0,0]
[i for i in li if i]

That's all you need. :)

[Chris]

On Jul 9, 7:48*am, "Rajanikanth Jammalamadaka" <[e-mail]>
wrote:

Try this:

>li=[0,0,1,2,1,0,0]
li

[0, 0, 1, 2, 1, 0, 0]>>[i for i in range(len(li)) if li[i] != 0]

[2, 3, 4]

Cheers,

Raj

On Tue, Jul 8, 2008 at 10:26 PM, Benjamin Goudey <[e-mail]:

I have a very large list of integers representing data needed for a
histogram that I'm going to plot using pylab. However, most of these
values (85%-95%) are zero and I would like to remove them to reduce
the amount of memory I'm using and save time when it comes to plotting
the data. To do this, I'm trying to find the best way to remove all of
the zero values and produce a list of indices of where the non-zero
values used to be.

For example, if my original list is [0,0,1,2,1,0,0] I would like to
produce the lists [1,2,1] (the non zero values) and [2,3,4] (indices
of where the non-zero values used to be). Removing non-zero values is
very easy but determining the indicies is where I'm having difficulty.

Thanks in advance for any help
--
http://mail.python.org/mailman/listinfo/python-list

--
"For him who has conquered the mind, the mind is the best of friends;
but for one who has failed to do so, his very mind will be the
greatest enemy."

Rajanikanth

Whoops, misread the question

li =[0,0,1,2,1,0,0]
[(index,data) for index,data in enumerate(li) if data]

[Paul McGuire]

On Jul 9, 12:26*am, Benjamin Goudey <[e-mail]:

I have a very large list of integers representing data needed for a
histogram that I'm going to plot using pylab. However, most of these
values (85%-95%) are zero and I would like to remove them to reduce
the amount of memory I'm using and save time when it comes to plotting
the data. To do this, I'm trying to find the best way to remove all of
the zero values and produce a list of indices of where the non-zero
values used to be.

For example, if my original list is [0,0,1,2,1,0,0] I would like to
produce the lists [1,2,1] (the non zero values) and [2,3,4] (indices
of where the non-zero values used to be). Removing non-zero values is
very easy but determining the indicies is where I'm having difficulty.

>>sparse_data = [0, 0, 1, 2, 1, 0, 0]
values,locns = zip(*[ (x,i) for i,x in enumerate(sparse_data) if x ])
print values

(1, 2, 1)

>>print locns

(2, 3, 4)

>>>

-- Paul