CODEWITHSUNDEEP
python
Oleg Pavliv
Oleg Pavliv

Reputation: 21162

python operations with sets

I use python only occasionally, sorry for a seemingly trivial question

>>> a = set(((1,1),(1,6),(6,1),(6,6)))
>>> a
set([(6, 1), (1, 6), (1, 1), (6, 6)])
>>> a - set(((1,1)))
set([(6, 1), (1, 6), (1, 1), (6, 6)])
>>> a.remove((1,1))
>>> a
set([(6, 1), (1, 6), (6, 6)])

why '-' operator didn't delete the element but the remove did ?

Upvotes: 1

Views: 136

Answers (2)

6502
6502

Reputation: 114481

You missed a comma when trying to specify a tuple of one element. Syntax for tuples is indeed somewhat tricky...

  • A tuple is built using commas, not parenthesis
  • sometimes it is mandatory to add parenthesis around it
  • an empty tuple is however represented by an empty pair of parenthesis
  • not always a pair of parenthesis wrapping zero or more expression separated by commas is a tuple

Some examples

w = 1, 2, 3             # creates a tuple, no parenthesis needed
w2 = (1, 2, 3)          # works too, like x+y is the same as (x+y)
x, y, z = w             # unpacks a tuple
k0 = ()                 # creates an empty tuple
k1 = (1,)               # a tuple with one element (note the comma)
k = (1)                 # just a number, NOT a tuple
foo(1, 2, 3)            # call passing three numbers, not a tuple
bar((1, 2, 3))          # call passing a tuple
if x in 1, 2:           # syntax error, parenthesis are needed
   pass
for x in 1, 2:          # ok here
   pass
gen = (x for x in 1, 2) # error, parenthesis needed here around (1, 2)

Upvotes: 2

JBernardo
JBernardo

Reputation: 33397

Because you missed a comma:

>>> set(((1,1)))
set([1])

should be:

>>> set(((1,1),))
set([(1, 1)])

or, to be more readable:

set([(1,1)])

or even (Py2.7+):

{(1,1)}

Upvotes: 8

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