Testing for list membership

schmu
3 ●1 ●1 ●2

I'd like to adopt Sage in my teaching this fall, so I've started to play around with some basic manipulations that I would expect my students to be able to do. I am an absolute beginner in Sage and Python.

Suppose that I want all the partitions of 6 that contain the number "2". My first attempt failed:

L=Partitions(6).list(); L

[[6], [5, 1], [4, 2], [4, 1, 1], [3, 3], [3, 2, 1], [3, 1, 1, 1], [2, 2, 2], [2, 2, 1, 1], [2, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1]]

[j for j in L if j.count(2)>0]

"Error in lines 1-1\nTraceback (most recent call last):\n
File \"/projects/6313dd6a-b1df-46ed-885a-a1bdf71f3fc3/.sagemathcloud/sage_server.py\", line 879, in execute\n exec compile(block+'\n', '', 'single') in namespace, locals\n File \"\", line 1, in <module>\n File \"sage/structure/element.pyx\", line 431, in sage.structure.element.Element.__getattr__ (build/cythonized/sage/structure/element.c:4644)\n
return getattr_from_other_class(self, P._abstract_element_class, name)\n File \"sage/structure/misc.pyx\", line 253, in sage.structure.misc.getattr_from_other_class (build/cythonized/sage/structure/misc.c:1582)\n
raise dummy_attribute_error\nAttributeError: 'Partitions_n_with_category.element_class' object has no attribute 'count'\n"

However, if I create a new list E by copying and pasting the above, it works fine.

E=[[6], [5, 1], [4, 2], [4, 1, 1], [3, 3], [3, 2, 1], [3, 1, 1, 1], [2, 2, 2], [2, 2, 1, 1], [2, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1]]

[j for j in E if j.count(2)>0]

[[4, 2], [3, 2, 1], [2, 2, 2], [2, 2, 1, 1], [2, 1, 1, 1, 1]]

Is there a better way to do this, without copying and pasting??

answered 10 years ago

tmonteil
27493 ●32 ●205 ●519 http://wiki.sagemath.o...

updated 10 years ago

You should notice that the elements of L are not lists, but partitions (that do not provide a count() method as you saw with the AttributeError):

sage: L[0]
[6]
sage: L[0].parent()
Partitions of the integer 6
sage: type(L[0])
<class 'sage.combinat.partition.Partitions_n_with_category.element_class'>

Of course, you can transform them as lists with the list function:

sage: list(L[0])
[6]
sage: type(list(L[0]))
<type 'list'>

Hence, you can do;

sage: [j for j in L if list(j).count(2)>0]
[[4, 2], [3, 2, 1], [2, 2, 2], [2, 2, 1, 1], [2, 1, 1, 1, 1]]

But more elegantly, instead of counting, you can check that 2 belongs to the partition with the in operator:

sage: [j for j in L if 2 in j]
[[4, 2], [3, 2, 1], [2, 2, 2], [2, 2, 1, 1], [2, 1, 1, 1, 1]]

link

Comments

Thanks very much, this is exactly what I needed!

schmu ( 10 years ago )

Thierry, isn't there also an option somewhere in http://doc.sagemath.org/html/en/refer... to get this directly?

kcrisman ( 10 years ago )

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