[Tutor] question about list

[Liam Clarke]

Hi Lin, 

I  ran into this problem myself once, and I forgot how I fixed it. I
can, however, tell you what's going wrong -

a=['a','b','c','d','e','f','g']
b=[1,2,4]

for in in b:
del a[i]

Watch this 

for i in b:
   del a[1]

a becomes ['a','c','d','e','f','g']

next one around

del a[2]

a[2] is now 'd' !  so a becomes ['a','c','e','f','g']

del a[4]

a[4] is now 'g' so you are going to get -

a=['a','c','e','f']

So that's what's going wrong for you. 

I worked around this once by the following - but bear in mind that all
my workarounds are messy, and there will be a better way, which
hopefully someone will post shortly, because I'm curious also.

work-around - 

a=['a','b','c','d','e','f','g']
b=[1,2,4]

for i  in b:
   a[i]="XX"    <-- this makes a=['a','XX','XX','d','XX','f','g']

while a.count("XX"):  While the number of occurences of 'XX' in a are
above zero, do this...
      a.remove("XX")   Remove the first occurrence of 'XX' found

which gives you 

a=['a', 'd', 'f', 'g']


But like I said, there'll be an easier way.


Hope it helps, 

Liam Clarke





On Fri, 12 Nov 2004 07:24:50 +0800, Lin Jin <jinlin555 at msn.com> wrote:
> > i am new to python.i have a question about list.if i have two list:
> > a=[a,b,c,d,e,f,g]
> > b=[1,2,4]
> > and i want to remove the element of a using b,that is i want
> a=[a,d,f,g],my
> > code is like this:
> > >>>for i in b:
> > >>>    del a[i]
> >
> > but it is not working, it can't remove the correct items.what should i do
> 
> > to make it correct?thx
> 
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