Re: [Tutor] Summing arrays
Peter Otten wrote: > (Specifying dtype=float forces floating point arithmetic which is inexact) I found a way to avoid the error: >>> import numpy as np >>> def sum2(N): ... a = np.arange(1, N+1, dtype=object) ... return (2*(1+3**(a-1))).sum() ... >>> sum2(50) 717897987691852588770348 As this operates on int objects rather than C/machine language numbers this will probably be significantly slower than dtype=int or dtype=float. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Summing arrays
On 03/16/2017 10:38 AM, Wolfgang Maier wrote: In addition, I'd agree with Alan that the advantage of using numpy functionality in this function seems quite questionable. You'd probably get pretty much the same performance with just: def sum2(N): return sum(2*(1+3**(i-1)) for i in range(1,N+1,1)) Or, if you really want to leverage the power of numpy (and are willing to pay the price of possibly inexact calculations) rewrite your function as Peter shows in his reply. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Summing arrays
On 03/16/2017 10:12 AM, Alan Gauld via Tutor wrote: On 16/03/17 05:11, Aaliyah Ebrahim wrote: def sum2(N): b = np.arange(1,N+1,1) mylist = [ ] for i in b: terms = 2*(1+3**(i-1)) a = mylist.append[terms] return np.sum(mylist) terms = 2*(1+3**(i-1))> 9 mylist.append[terms] 10 return np.sum(mylist) 11 TypeError: 'builtin_function_or_method' object is not subscriptable You are using [] to call append instead of (). It should be a = mylist.append(terms) However, most of your function could be replaced by a list comprehension: def sum2(N): mylist = [2*(1+3**(i-1)) for i in np.arange(1,N+1,1) ] return np.sum(mylist) And I'm not sure the np versions of the functions will give much advantage over the standard range() and sum() Any time you see a structure like aList = [] for aList.append() You should consider whether a comprehension would be more suitable Alternatively, you could even use a generator expression (essentially, just omitting the brackets from Alan's suggestion) and avoid keeping mylist in memory like this: def sum2(N): return np.sum(2*(1+3**(i-1)) for i in np.arange(1,N+1,1)) In addition, I'd agree with Alan that the advantage of using numpy functionality in this function seems quite questionable. You'd probably get pretty much the same performance with just: def sum2(N): return sum(2*(1+3**(i-1)) for i in range(1,N+1,1)) Best, Wolfgang ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Summing arrays
Aaliyah Ebrahim wrote:
> Hi guys! I hope you can assist me with understanding and fixing my error
> below. Thanks for this amazing forum :)
>
>
> def sum2(N):
>
> b = np.arange(1,N+1,1)
> mylist = [ ]
> for i in b:
>
>
> terms = 2*(1+3**(i-1))
> a = mylist.append[terms]
The immediate problem is that append is a method, the line above should be
mylist.append(terms)
As the append() method always returns None the assignment a = ...
does not do anything useful and should be avoided.
> return np.sum(mylist)
>
> print(sum2(N=50))
However, with the fix above you will still run into overflow errors with
your code. I suggest that you use Python's integer arithmetic and forget
about numpy for now:
$ cat tmp1.py
import numpy as np
def sum2(N):
b = np.arange(1,N+1,1)
mylist = []
for i in b:
terms = 2*(1+3**(i-1))
mylist.append(terms)
return np.sum(mylist)
def sum2a(N):
mylist = []
for i in range(1, N+1):
mylist.append(2*(1+3**(i-1)))
return sum(mylist)
print("N=5:", sum2(5), sum2a(5))
print("N=50:", sum2(50), sum2a(50))
$ python3 tmp1.py
N=5: 252 252
tmp1.py:7: RuntimeWarning: overflow encountered in long_scalars
terms = 2*(1+3**(i-1))
N=50: 6048575297968530476 717897987691852588770348
Both the numpy and the non-numpy version can be written more concisely (and
for the numpy version the rewritten version is more efficient, too):
$ cat tmp2.py
import numpy as np
def sum2(N):
b = np.arange(1, N+1, 1, dtype=float)
a = 2*(1+3**(b-1))
return a.sum()
def sum2a(N):
return sum(2*(1+3**(i-1)) for i in range(1, N+1))
print("N=5:", sum2(5), sum2a(5))
print("N=50:", sum2(50), sum2a(50))
$ python3 tmp2.py
N=5: 252.0 252
N=50: 7.17897987692e+23 717897987691852588770348
(Specifying dtype=float forces floating point arithmetic which is inexact)
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Re: [Tutor] Summing arrays
On 16/03/17 05:11, Aaliyah Ebrahim wrote: > def sum2(N): > > b = np.arange(1,N+1,1) > mylist = [ ] > for i in b: > terms = 2*(1+3**(i-1)) > a = mylist.append[terms] > return np.sum(mylist) > terms = 2*(1+3**(i-1))> 9 mylist.append[terms] 10 > return np.sum(mylist) 11 > TypeError: 'builtin_function_or_method' object is not subscriptable You are using [] to call append instead of (). It should be a = mylist.append(terms) However, most of your function could be replaced by a list comprehension: def sum2(N): mylist = [2*(1+3**(i-1)) for i in np.arange(1,N+1,1) ] return np.sum(mylist) And I'm not sure the np versions of the functions will give much advantage over the standard range() and sum() Any time you see a structure like aList = [] for aList.append() You should consider whether a comprehension would be more suitable -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ http://www.amazon.com/author/alan_gauld Follow my photo-blog on Flickr at: http://www.flickr.com/photos/alangauldphotos ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
[Tutor] Summing arrays
Hi guys! I hope you can assist me with understanding and fixing my error below. Thanks for this amazing forum :) def sum2(N): b = np.arange(1,N+1,1) mylist = [ ] for i in b: terms = 2*(1+3**(i-1)) a = mylist.append[terms] return np.sum(mylist) print(sum2(N=50)) ---TypeError Traceback (most recent call last) in () 10 return np.sum(mylist) 11 ---> 12 print(sum2(N=50)) in sum2(N) 7 8 terms = 2*(1+3**(i-1))> 9 mylist.append[terms] 10 return np.sum(mylist) 11 TypeError: 'builtin_function_or_method' object is not subscriptable Aaliyah ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
[Tutor] summing arrays, along a dimension
d array([[0, 0, 1], [1, 2, 3], [2, 2, 4], [3, 6, 8]]) e=reshape((d[:,-2]+d[:,-1]),(4,1)) e array([[ 1], [ 5], [ 6], [14]]) is there a better way to accomplish this? ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] summing arrays, along a dimension
John wrote: d array([[0, 0, 1], [1, 2, 3], [2, 2, 4], [3, 6, 8]]) e=reshape((d[:,-2]+d[:,-1]),(4,1)) e array([[ 1], [ 5], [ 6], [14]]) is there a better way to accomplish this? d [[0, 0, 1], [1, 2, 3], [2, 2, 4], [3, 6, 8]] e = [sum(i) for i in d] e [1, 6, 8, 17] f = [sum([L[i] for L in d]) for i in xrange(len(d[0]))] f [6, 10, 16] ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] summing arrays, along a dimension
John [EMAIL PROTECTED] wrote d array([[0, 0, 1], [1, 2, 3], [2, 2, 4], [3, 6, 8]]) e=reshape((d[:,-2]+d[:,-1]),(4,1)) e array([[ 1], [ 5], [ 6], [14]]) is there a better way to accomplish this? Better? Maybe. More readable I think is: e1 = [ [row[-2]+row[-1]] for row in d ] You can convert the 2D list to an array object if you want later Alan G. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] summing arrays, along a dimension
John wrote: d array([[0, 0, 1], [1, 2, 3], [2, 2, 4], [3, 6, 8]]) e=reshape((d[:,-2]+d[:,-1]),(4,1)) e array([[ 1], [ 5], [ 6], [14]]) is there a better way to accomplish this? Which module are you using? In APL we'd write +/d to reduce along the rows using +. Does the array object have a similar reduce method? ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
