Function to remove elements from a list not working (corrected)

[Girish Sahani]

Hi,
I am trying to modify a list of pairs (l4) by removing those
pairs which are not present in a third list called pairList.
The following is a simplified part of the routine i have written. However
it does not give the correct output. Please help!
Its possible i have made a trivial mistke since i am a newbie.

def getl5():
l5 = []
pairList = [[1,2],[3,4],[3,5],[3,6],[9,7],[8,9],[8,7],[7,9],[11,10]]
l4 = [[4,2],[4,7],[4,10],[4,12],[9,2],[9,7],[9,10],[9,12],[11,2],[11,7]]
for pair in l4:
if pair not in pairList:
l4.remove(pair)
print "l4 is",l4

The output given is:
l4 is [[4, 7], [4, 12], [9, 7], [9, 12], [11, 7]]

 

[wittempj]

Hi,
I am trying to modify a list of pairs (l4) by removing those
pairs which are not present in a third list called pairList.
The following is a simplified part of the routine i have written. However
it does not give the correct output. Please help!
Its possible i have made a trivial mistke since i am a newbie.

def getl5():
l5 = []
pairList = [[1,2],[3,4],[3,5],[3,6],[9,7],[8,9],[8,7],[7,9],[11,10]]
l4 = [[4,2],[4,7],[4,10],[4,12],[9,2],[9,7],[9,10],[9,12],[11,2],[11,7]]
for pair in l4:
if pair not in pairList:
l4.remove(pair)
print "l4 is",l4

The output given is:
l4 is [[4, 7], [4, 12], [9, 7], [9, 12], [11, 7]]

It is better to iterate over a copy, e.g. like this:

pairList = [[1,2],[3,4],[3,5],[3,6],[9,7],[8,9],[8,7],[7,9],[11,10]]
l4 =
[[4,2],[4,7],[4,10],[4,12],[9,2],[9,7],[9,10],[9,12],[11,2],[11,7]]
for pair in l4[:]:
if pair not in pairList:
l4.remove(pair)
print "l4 is",l4

 

[Girish Sahani]

Thank you Mark....this works too...
Btw going slightly off-topic, when i try to run a code like below with
around 50 elements (pairs) in l4,python just hangs. Any ideas why this is
happening...the data is not that large (

Hi,
I am trying to modify a list of pairs (l4) by removing those
pairs which are not present in a third list called pairList.
The following is a simplified part of the routine i have written.
However
it does not give the correct output. Please help!
Its possible i have made a trivial mistke since i am a newbie.

def getl5():
l5 = []
pairList = [[1,2],[3,4],[3,5],[3,6],[9,7],[8,9],[8,7],[7,9],[11,10]]
l4 =
[[4,2],[4,7],[4,10],[4,12],[9,2],[9,7],[9,10],[9,12],[11,2],[11,7]]
for pair in l4:
if pair not in pairList:
l4.remove(pair)
print "l4 is",l4

The output given is:
l4 is [[4, 7], [4, 12], [9, 7], [9, 12], [11, 7]]

It is better to iterate over a copy, e.g. like this:

pairList = [[1,2],[3,4],[3,5],[3,6],[9,7],[8,9],[8,7],[7,9],[11,10]]
l4 =
[[4,2],[4,7],[4,10],[4,12],[9,2],[9,7],[9,10],[9,12],[11,2],[11,7]]
for pair in l4[:]:
if pair not in pairList:
l4.remove(pair)
print "l4 is",l4

 

[Boris Borcic]

Hi,
I am trying to modify a list of pairs (l4) by removing those
pairs which are not present in a third list called pairList.
The following is a simplified part of the routine i have written. However
it does not give the correct output. Please help!
Its possible i have made a trivial mistke since i am a newbie.

def getl5():
l5 = []
pairList = [[1,2],[3,4],[3,5],[3,6],[9,7],[8,9],[8,7],[7,9],[11,10]]
l4 = [[4,2],[4,7],[4,10],[4,12],[9,2],[9,7],[9,10],[9,12],[11,2],[11,7]]
for pair in l4:
if pair not in pairList:
l4.remove(pair)
print "l4 is",l4

The output given is:
l4 is [[4, 7], [4, 12], [9, 7], [9, 12], [11, 7]]

use sets

def gets5() :
pairSet = set([(1,2),(3,4),(3,5),(3,6),(9,7),(8,9),(8,7),(7,9),(11,10)])
s4= set([(4,2),(4,7),(4,10),(4,12),(9,2),(9,7),(9,10),(9,12),(11,2),(11,7)])
s4 &= pairSet
print "s4 is",s4

the output is

s4 is set([(9, 7)])

 

[Paul McGuire]

Hi,
I am trying to modify a list of pairs (l4) by removing those
pairs which are not present in a third list called pairList.
The following is a simplified part of the routine i have written. However
it does not give the correct output. Please help!
Its possible i have made a trivial mistke since i am a newbie.

You've fallen victim to one of the Classic Blunders! The First is "Never
start a land war in Asia!", but the second, only slightly lesser known is
"Never modify a list that you are iterating over!"

-- Paul

 

[Fredrik Lundh]

Btw going slightly off-topic, when i try to run a code like below with
around 50 elements (pairs) in l4,python just hangs. Any ideas why this is
happening...the data is not that large (

building a filtered new list by repeatedly removing stuff from a copy
of the original list isn't exactly the fastest way to do things, but
there's no way the following code will "hang" with 50 items instead of
10, unless your computer is ludicrously slow (it takes about 0.000113
seconds on my machine).

maybe you meant to write 50k items ? (11 seconds on my machine)

pairList = [[1,2],[3,4],[3,5],[3,6],[9,7],[8,9],[8,7],[7,9],[11,10]]
l4 =
[[4,2],[4,7],[4,10],[4,12],[9,2],[9,7],[9,10],[9,12],[11,2],[11,7]]
for pair in l4[:]:
if pair not in pairList:
l4.remove(pair)
print "l4 is",l4

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