How to tackle this Inverse Relation homework?

Question

Here is my assignment:

Write a function inverse(rel) that takes a relation rel and returns the inverse relation of the relation rel. The inverse relation InvsetR of the relation R is dened as InvsetR = {(x, y) ∈ S × S |(y, x) ∈ R)}. Example:

inRelation({(1,1), (1,2), (2,3), (4,2)}) should return

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

Here is my code:

def inverse(rel):
   m=set()
   for (x,y) in rel: 
      m.add(y,x)
   return m

It says that I can add only one element. What can I do?

You should add a couple `(y, x)` like this: `m.add((y, x))`. — Laurent LAPORTE, Aug 16 '16 at 20:28
`m.add(x,y)` invokes the `add` function with *two* parameters `x` and `y`. You need to pass *one* parameter: the tuple `(x,y)`, so you need to write `m.add((x,y))`. — Heinrich supports Monica, Aug 16 '16 at 20:30
Thank you very much! it works now.. but instead of {(1,1), (2,1), (3,2), (2,4)} it returns {(3, 2), (1, 1), (2, 4), (2, 1)}.. why? what should i do? — Margarita, Aug 16 '16 at 20:36
@Margarita: in Python, [set](https://docs.python.org/2/library/stdtypes.html#set) is an unordered collection of unique elements. To have an ordered list, use: `sorted(m)`. — Laurent LAPORTE, Aug 16 '16 at 22:11

score 0 · Answer 1 · answered Aug 16 '16 at 20:24

For this particular example, you don't need any custom function of yours, just use the builtins python already provides, a couple of examples:

foo = set([(1, 1), (1, 2), (2, 3), (4, 2)])
inv_foo1 = map(lambda (a, b): (b, a), foo)
inv_foo2 = {(b, a) for (a, b) in foo}
print(foo)
print(inv_foo1)
print(inv_foo2)

score 0 · Answer 2 · answered Aug 16 '16 at 20:24

If a "relation" is a set of (x, y) couples:

>>> relation = {(1,1), (1,2), (2,3), (4,2)}

The inverted relation is:

>>> inverted = {(y, x) for x, y in relation}
>>> inverted
{(3, 2), (1, 1), (2, 4), (2, 1)}

The inRelation can be:

def inRelation(relation):
    return {(y, x) for x, y in relation}

note: I prefer snake case: inv_relation

How to tackle this Inverse Relation homework?

2 Answers2