Problems with Python sets

[Jonah Thomas]

I am getting some failures with Python sets.

a={1,2,3} gets an error, but

a=set((1,1,1,2))  gets no error.

print(a) then gives a lot of code, but

print([a]) gives  [{1, 2}]

a=set(1,2)
print([a]) gives [{}] I can live with that.

 

if 1 in a: print(1) gives 1. Good.

But I don't find any way to put tuples into a set.

a=set([1,2], [1,3], [1,1])
print([a]) gives [{1, 2}] as if it processes the first item.

a=set(([1,2], [1,3], [1,1]))
print([a]) gives [{[1, 2], [1, 3], [1, 1]}] as if it keeps the whole thing as one set element. But I can't find a way to get it to say that item is in the set. I haven't found any combination of writing them as tuples (instead of the lists as above) that works.

a=set((1,2,3))
b=set((4,5,6))
c=a.union(b)
print([c]) gives [{1, 2, 3, 4, 5, 6}] Good.

Do tuples just not work? Or is there a workable syntax I didn't find?

--

Jonah Thomas

We are limited, but the limits aren't where we think they are. We find them only by exceeding them.

[Bruce Sherwood]

Thanks for the report that lists and tuples don't work in sets. I'll comment that JavaScript, which Web VPython compiles to in order to be able to run in the browser, does not have something equivalent to the Python tuple.

I don't see the problems you see with elements that are numbers. Is it possible that you are using an old version of Web VPython?

Web VPython 3.2
# OK:
s = {1,2,3}
print(s, 2 in s, 7 in s) # {1, 2, 3} True False

# OK:
s = set((1,2,3))
print(s) # {1, 2, 3}

# Does not handle lists
s = {[1,2], [3,4], [5,6]}
print(s) # {[1, 2], [3, 4], [5, 6]}
s = set(([1,2], [3,4], [5,6]))
print(s, [3,4] in s) # {[1, 2], [3, 4], [5, 6]} False
s = set(((1,2), (3,4), (5,6)))
print(s) # {[1, 2], [3, 4], [5, 6]}
print([3,4] in s) # False; should be True

[Harlan Gilbert]

All the statements below work properly for me:

Web VPython 3.2

a = {1,2,3}
b=set((1,2,3))
print(a,b)
c=set(([1,2],[3,4]))
d={[5,6],[7,8]}

print(c,d)

But funny things happen in the following, including a different treatment of tuples and lists as parts of sets. Tuples are dissolved and merged, lists are retained as separate structures:

s = set(((1,2), (3,4), (5,6)))

print(s) #  {1, 2, 3, 4, 5, 6}  merges tuples

s = set(([1,2], [3,4], [5,6]))

print(s) # {[1, 2], [3, 4], [5, 6]}  does not merge lists
print([3,4] in s) # False; but still does not recognize them as members of the set

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--

Harlan Gilbert, Ph.D.

High School Chair and Math, Physics, Computer Science, and Philosophy Teacher

Green Meadow Waldorf High School

Chestnut Ridge, NY 10977

(845) 356-2514 x. 320

[Bruce Sherwood]

The major problem is that the "in" operation fails with lists. I thought of a workaround.

Make a list of the lists to go into the set.

Place in the set the indices of those lists in the list.

Then when locating an item in the set, use it to index into the list of lists.

Bruce

[Harlan Gilbert]

The other problem is that tuples are not properly handled:

s = set(((1,2), (3,4), (5,6)))

print(s)  

The above should give 

{(1,2), (3,4), (5,6)}

But instead gives

 {1, 2, 3, 4, 5, 6} 

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