Hello, I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] And would like to compute the cumulative sum of all the integers from index zero into another array. So for the array above, I should get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ] What is the best way (or pythonic way) to get this.
dhruvbird wrote: > I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] > And would like to compute the cumulative sum of all the integers > from index zero into another array. So for the array above, I should > get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ] > What is the best way (or pythonic way) to get this.
Homework?
>>> def cumulative_sum(values, start=0):
... for v in values: ... start += v ... yield start ...
> Hello, > I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] > And would like to compute the cumulative sum of all the integers > from index zero into another array. So for the array above, I should > get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ] > What is the best way (or pythonic way) to get this.
> Regards, > -Dhruv. > --
Hi, just a straightworward, naive approach...:
lst_int = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] acc_int = 0 output_lst = [] for i in lst_int: acc_int += i output_lst.append(acc_int) print output_lst
> Hello, > I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] > And would like to compute the cumulative sum of all the integers > from index zero into another array. So for the array above, I should > get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ] > What is the best way (or pythonic way) to get this.
<python>
import copy import itertools
def acc(items, copy=copy.deepcopy): items = iter(items) result = next(items) yield copy(result) for item in items: result += item yield copy(result)
> Hello, > I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] > And would like to compute the cumulative sum of all the integers > from index zero into another array. So for the array above, I should > get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ] > What is the best way (or pythonic way) to get this.
> Regards, > -Dhruv.
Maybe not pythonic, but straight-forward:
>>> import numpy >>> numpy.cumsum(x)
array([ 0, 1, 3, 4, 5, 5, 5, 7, 10])
An example with a class
class CumulativeSum(object): def __init__(self, start=0): self._current = start def __call__(self, value): self._current += value return self._current
On Mon, 19 Jul 2010 04:18:48 -0700, dhruvbird wrote: > Hello, > I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] And would > like to compute the cumulative sum of all the integers > from index zero into another array. So for the array above, I should > get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ]
[pedant] The above are *lists*, not arrays. Python has arrays, but you have to call "import array" to get them. [/pedant]
> What is the best way (or pythonic way) to get this.
Others have given you a plethora of advanced, complicated and obscure ways to solve this question, but I haven't seen anyone give the simplest method (simple as in no tricks or advanced features):
data = [0, 1, 2, 1, 1, 0, 0, 2, 3] csums = [] for x in data: if csums: y = x + csums[-1] else: y = x csums.append(y)
We can save some code with the ternary operator:
data = [0, 1, 2, 1, 1, 0, 0, 2, 3] csums = [] for x in data: csums.append((x + csums[-1]) if csums else x)
Here's a version that writes the cumulative sum in place:
data = [0, 1, 2, 1, 1, 0, 0, 2, 3] for i in range(1, len(data)): data[i] += data[i-1]
dhruvbird wrote: > Hello, > I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] > And would like to compute the cumulative sum of all the integers > from index zero into another array. So for the array above, I should > get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ] > What is the best way (or pythonic way) to get this.
Now that Steven's given you the simple, pythonic way, I'll just mention the advanced, complicated and obscure way that might be vaguely familiar if you're coming from a functional programming background:
x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] def running_sum(result, current_value): return result + [result[-1]+current_value if result else current_value]
reduce(running_sum, x, [])
Having offered this, I don't recall ever seeing reduce used in real python code, and explicit iteration is almost always preferred.
> dhruvbird wrote: > > Hello, > > I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] > > And would like to compute the cumulative sum of all the integers > > from index zero into another array. So for the array above, I should > > get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ] > > What is the best way (or pythonic way) to get this.
> Now that Steven's given you the simple, pythonic way, I'll just mention > the advanced, complicated and obscure way that might be vaguely familiar > if you're coming from a functional programming background:
> x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] > def running_sum(result, current_value): > return result + [result[-1]+current_value if result else current_value]
> reduce(running_sum, x, [])
> Having offered this, I don't recall ever seeing reduce used in real > python code, and explicit iteration is almost always preferred.
Yes, even I have noticed that reduce is a tad under-used function.
So, I guess no function like "accumulate" below exists in the standard lib.
def accumulate(proc, seed, seq): ret = [] for i in seq: ret.append(proc(seed, i)) return ret
On Jul 19, 4:28 pm, Peter Otten <__pete...@web.de> wrote:
> dhruvbird wrote: > > I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] > > And would like to compute the cumulative sum of all the integers > > from index zero into another array. So for the array above, I should > > get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ] > > What is the best way (or pythonic way) to get this.
> Homework?
not really :)
It's just that I was wondering if a built-in function for doing such things (which I find myself doing increasingly with an explicit loop) exists.
dhruvbird <dhruvb...@gmail.com> wrote: > On Jul 19, 4:28 pm, Peter Otten <__pete...@web.de> wrote: >> dhruvbird wrote: >> > I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] >> > And would like to compute the cumulative sum of all the integers >> > from index zero into another array. So for the array above, I should >> > get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ] >> > What is the best way (or pythonic way) to get this.
>> Homework?
> not really :)
> It's just that I was wondering if a built-in function for doing such > things (which I find myself doing increasingly with an explicit loop) > exists.
Why would you find yourself doing it more than once? Write it once in a function and then just re-use the code.
Brian Victor <homeusen...@brianhv.org> writes: > def running_sum(result, current_value): > return result + [result[-1]+current_value if result else current_value]
> reduce(running_sum, x, [])
That is not really any good because Python lists are actually vectors, so result+[...] actually copies the whole old list, making your function take quadratic time. It would be ok in a FP language where lists were chains of cons nodes and result+[...] just allocated a single cons.
I think Peter Otten's solution involving a generator is the one most in the current Python spirit. It's cleaner (for my tastes) than the ones that use things like list.append.
>> I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] >> And would like to compute the cumulative sum of all the integers >> from index zero into another array. So for the array above, I should >> get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ] >> What is the best way (or pythonic way) to get this.
> On Jul 19, 9:12 pm, Brian Victor<homeusen...@brianhv.org> wrote: >> dhruvbird wrote: >> Having offered this, I don't recall ever seeing reduce used in real >> python code, and explicit iteration is almost always preferred.
> Yes, even I have noticed that reduce is a tad under-used function.
Yes, I had a use case for it once, but it wasn't worth the trouble. "map" is often useful, but "reduce", not so much.
Python isn't really a functional language. There's no bias toward functional solutions, lambdas aren't very general, and the performance isn't any better. Nor is any concurrency provided by "map" or "reduce". So there's no win in trying to develop cute one-liners.
> > On Jul 19, 9:12 pm, Brian Victor<homeusen...@brianhv.org> wrote: > >> dhruvbird wrote: > >> Having offered this, I don't recall ever seeing reduce used in real > >> python code, and explicit iteration is almost always preferred.
> > Yes, even I have noticed that reduce is a tad under-used function.
> Yes, I had a use case for it once, but it wasn't worth the trouble. > "map" is often useful, but "reduce", not so much.
> Python isn't really a functional language. There's no bias toward > functional solutions, lambdas aren't very general, and the performance > isn't any better. Nor is any concurrency provided by "map" or "reduce". > So there's no win in trying to develop cute one-liners.
Yes agreed.
However, there is:
1. now scope for optimization (for example returning generators instead of lists) at every stage if using functions -- these functions can be internally changed as long as the external guarantees they provide remain essentially unchanged.
2. readability wins because you express your intent (operations) rather than anything else. For example, if I want the product of the square roots of all odd integers in an array, I can say: answer = reduce(product, map(math.sqrt, filter(lambda x: x%2 == 0, some_array_with_ints)))
While I agree that python may not have been initially seen as a functional language, it is powerful and flexible enough to be one or at least decently support such paradigms.
On Wed, Jul 21, 2010 at 8:17 AM, John Nagle <na...@animats.com> wrote: > On 7/19/2010 9:56 AM, dhruvbird wrote:
>> On Jul 19, 9:12 pm, Brian Victor<homeusen...@brianhv.org> wrote:
>>> dhruvbird wrote:
>>> Having offered this, I don't recall ever seeing reduce used in real >>> python code, and explicit iteration is almost always preferred.
>> Yes, even I have noticed that reduce is a tad under-used function.
> Yes, I had a use case for it once, but it wasn't worth the trouble. > "map" is often useful, but "reduce", not so much.
> Python isn't really a functional language. There's no bias toward > functional solutions, lambdas aren't very general, and the performance > isn't any better. Nor is any concurrency provided by "map" or "reduce". > So there's no win in trying to develop cute one-liners.
Too bad about the lack of concurrency, would be many places where that would be nice.
> On Wed, Jul 21, 2010 at 8:17 AM, John Nagle wrote: >> On 7/19/2010 9:56 AM, dhruvbird wrote:
>>> On Jul 19, 9:12 pm, Brian Victor wrote:
>>>> dhruvbird wrote:
>>>> Having offered this, I don't recall ever seeing reduce used in real >>>> python code, and explicit iteration is almost always preferred.
>>> Yes, even I have noticed that reduce is a tad under-used function.
>> Yes, I had a use case for it once, but it wasn't worth the trouble. >> "map" is often useful, but "reduce", not so much.
>> Python isn't really a functional language. There's no bias toward >> functional solutions, lambdas aren't very general, and the performance >> isn't any better. Nor is any concurrency provided by "map" or "reduce". >> So there's no win in trying to develop cute one-liners.
> Too bad about the lack of concurrency, would be many places where that > would be nice.
Besides the many places where the current properties match just fine, there are some places where concurrency would be helpful. So I wouldn't call it "lack" of concurrency, as that seems to imply that it's a missing feature in what both builtins are targeted to provide. Just use one of the map-reduce frameworks that are out there if you need concurrency in one way or another. Special needs are not what builtins are there for.
On 19 Jul, 13:18, dhruvbird <dhruvb...@gmail.com> wrote:
> Hello, > I have a list of integers: x = [ 0, 1, 2, 1, 1, 0, 0, 2, 3 ] > And would like to compute the cumulative sum of all the integers > from index zero into another array. So for the array above, I should > get: [ 0, 1, 3, 4, 5, 5, 5, 7, 10 ] > What is the best way (or pythonic way) to get this.
At least for large arrays, this is the kind of task where NumPy will help.
In article <7xpqyjgvjm....@ruckus.brouhaha.com>, Paul Rubin <no.em...@nospam.invalid> wrote:
>I think Peter Otten's solution involving a generator is the one most in >the current Python spirit. It's cleaner (for my tastes) than the ones >that use things like list.append.
