Hi,
I am wondering how this is evaluated.
a=(x for x in [1,2,3,4])
p=[4,5]
c=[x for x in p if x in list(a)]
c is []
but if I expand a first, like a = list(a)
c is [4]
So it seems that the "if" part don't get expanded ?
3  1419  bo****@gmail.com wrote: Hi,
I am wondering how this is evaluated.
a=(x for x in [1,2,3,4])
p=[4,5]
c=[x for x in p if x in list(a)]
c is []
but if I expand a first, like a = list(a)
c is [4]
So it seems that the "if" part don't get expanded ?
Well, for every element in p it recalculates list(a). Since the
generator is exhausted after making a list from it, it gives you
nothing afterwards. So after it checks the first element, it's
equivalent to [x for x in p if x in []]. bo****@gmail.com wrote: Hi,
I am wondering how this is evaluated.
a=(x for x in [1,2,3,4])
p=[4,5]
c=[x for x in p if x in list(a)]
c is []
No it isn't.
In [1]: a=(x for x in [1,2,3,4])
In [2]: p=[4,5]
In [3]: c=[x for x in p if x in list(a)]
In [4]: c
Out[4]: [4]
I'm willing to bet that you used up the 'a' iterator before you ran that
list comprehension, though.
In [5]: c=[x for x in p if x in list(a)]
In [6]: c
Out[6]: []
Note that "x in list(a)" gets executed on each iteration, but the
iterator is used up on the first time.
In [7]: a=(x for x in [1,2,3,4])
In [8]: p = [4, 5, 2, 3]
In [9]: c=[x for x in p if x in list(a)]
In [10]: c
Out[10]: [4]
--
Robert Kern rk***@ucsd.edu
"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter
Ah, no wonder. I test with p=[5,4].
thanks. so basically, I still need to expand it first given this
behaviour.
Robert Kern wrote: bo****@gmail.com wrote: Hi,
I am wondering how this is evaluated.
a=(x for x in [1,2,3,4])
p=[4,5]
c=[x for x in p if x in list(a)]
c is []
No it isn't.
In [1]: a=(x for x in [1,2,3,4])
In [2]: p=[4,5]
In [3]: c=[x for x in p if x in list(a)]
In [4]: c
Out[4]: [4]
I'm willing to bet that you used up the 'a' iterator before you ran that
list comprehension, though.
In [5]: c=[x for x in p if x in list(a)]
In [6]: c
Out[6]: []
Note that "x in list(a)" gets executed on each iteration, but the
iterator is used up on the first time.
In [7]: a=(x for x in [1,2,3,4])
In [8]: p = [4, 5, 2, 3]
In [9]: c=[x for x in p if x in list(a)]
In [10]: c
Out[10]: [4]
--
Robert Kern
rk***@ucsd.edu
"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
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