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[x for x in <> while <>]?

It seems that I rather frequently need a list or iterator of the form
[x for x in <while <>]
And there is no one like this.
May be there is another short way to write it (not as a loop). Is
there?
Thanks
Jun 27 '08 #1
14 1138

"urikaluzhny" <uk*******@nds.comwrote in message
news:f8**********************************@w7g2000h sa.googlegroups.com...
| It seems that I rather frequently need a list or iterator of the form
| [x for x in <while <>]

I can think of two ways to interpret that.

| And there is no one like this.
| May be there is another short way to write it (not as a loop). Is
| there?

Using loops to write an example of what you mean would make the above
clearer.

tjr

Jun 27 '08 #2
urikaluzhny a écrit :
It seems that I rather frequently need a list or iterator of the form
[x for x in <while <>]
And there is no one like this.
May be there is another short way to write it (not as a loop). Is
there?
The answer is very probably in the itertools module.

Jun 27 '08 #3
urikaluzhny <uk*******@nds.comwrites:
It seems that I rather frequently need a list or iterator of the form
[x for x in <while <>]
And there is no one like this.
May be there is another short way to write it (not as a loop). Is there?
itertools.takewhile(condition, seq)
Jun 27 '08 #4
On May 15, 10:06*am, "Terry Reedy" <tjre...@udel.eduwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message

news:f8**********************************@w7g2000h sa.googlegroups.com...
| It seems that I rather frequently need a list or iterator of the form
| [x for x in <while <>]

I can think of two ways to interpret that.
I mean like [x for x in <Aif <B>], only that it breaks the loop when
the expression <Bis false.
Jun 27 '08 #5
"urikaluzhny" <uk*******@nds.comwrote in message
news:51**********************************@m44g2000 hsc.googlegroups.com...
On May 15, 10:06 am, "Terry Reedy" <tjre...@udel.eduwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message

news:f8**********************************@w7g2000h sa.googlegroups.com...
| It seems that I rather frequently need a list or iterator of the form
| [x for x in <while <>]

I can think of two ways to interpret that.
>I mean like [x for x in <Aif <B>], only that it breaks the loop when
the expression <Bis false.
def gen(a):
for x in a:
if B: break
yield x

a_gen = gen(A)

# now iterate over a_gen

--
Geoff
Jun 27 '08 #6
The following proposed solution is not intended to be a solution, it
goes completely against the zen of python. [Type import this into the
python command interpreter]

I brought it down to two lines:-

l = range(6)
[1 if b!=4 else l.__delslice__(0,len(l)) for b in l][:-1]

itertools would still be a better approach in my opinion.

Just because I'm curious to know, can anyone bring it shorter[even if
its cryptic] than this without invoking any Python Library.

P.S. Once again I would not recommend using this as Explicit is better
than Implicit
P.P.S. It is strongly undesirable for us humans to use anything
starting with __ :)
On May 15, 5:10*pm, "Geoffrey Clements"
<geoffrey.clement...@SPAMbaesystems.comwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message

news:51**********************************@m44g2000 hsc.googlegroups.com...
On May 15, 10:06 am, "Terry Reedy" <tjre...@udel.eduwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message
news:f8**********************************@w7g2000h sa.googlegroups.com...
| It seems that I rather frequently need a list or iterator of the form
| [x for x in <while <>]
I can think of two ways to interpret that.
I mean like [x for x in <Aif <B>], only that it breaks the loop when
the expression <Bis false.

def gen(a):
* * for x in a:
* * * * if B: break
* * * * yield x

a_gen = gen(A)

# now iterate over a_gen

--
Geoff
Jun 27 '08 #7
On May 15, 6:07*pm, afrobeard <afrobe...@gmail.comwrote:
The following proposed solution is not intended to be a solution, it
goes completely against the zen of python. [Type import this into the
python command interpreter]

I brought it down to two lines:-

l = range(6)
[1 if b!=4 else l.__delslice__(0,len(l)) for b in l][:-1]

itertools would still be a better approach in my opinion.

Just because I'm curious to know, can anyone bring it shorter[even if
its cryptic] than this without invoking any Python Library.

P.S. Once again I would not recommend using this as Explicit is better
than Implicit
P.P.S. It is strongly undesirable for us humans to use anything
starting with __ :)

On May 15, 5:10*pm, "Geoffrey Clements"

<geoffrey.clement...@SPAMbaesystems.comwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message
news:51**********************************@m44g2000 hsc.googlegroups.com...
On May 15, 10:06 am, "Terry Reedy" <tjre...@udel.eduwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message
>news:f8**********************************@w7g2000 hsa.googlegroups.com....
| It seems that I rather frequently need a list or iterator of the form
| [x for x in <while <>]
I can think of two ways to interpret that.
>I mean like [x for x in <Aif <B>], only that it breaks the loop when
>the expression <Bis false.
def gen(a):
* * for x in a:
* * * * if B: break
* * * * yield x
a_gen = gen(A)
# now iterate over a_gen
--
Geoff- Hide quoted text -

- Show quoted text -
In your original, you have:
l = range(6)
[1 if b!=4 else l.__delslice__(0,len(l)) for b in l][:-1]
You may be hyperextending the use of '..if..else..', which is one of
my fears regarding 'with x as y'. "l.__delslice__(0,len(l))" is not
an expression.
Jun 27 '08 #8
According to http://en.wikipedia.org/wiki/Expression_(programming)

"An expression in a programming language is a combination of values,
variables, operators, and functions that are interpreted (evaluated)
according to the particular rules of precedence and of association for
a particular programming language, which computes and then produces
(returns, in a stateful environment) another value."

l.__delslice__(0,len(l)) is an expression because it returns None
[which also happens to be a value] in this case.

On May 16, 4:23*am, castironpi <castiro...@gmail.comwrote:
On May 15, 6:07*pm, afrobeard <afrobe...@gmail.comwrote:
The following proposed solution is not intended to be a solution, it
goes completely against the zen of python. [Type import this into the
python command interpreter]
I brought it down to two lines:-
l = range(6)
[1 if b!=4 else l.__delslice__(0,len(l)) for b in l][:-1]
itertools would still be a better approach in my opinion.
Just because I'm curious to know, can anyone bring it shorter[even if
its cryptic] than this without invoking any Python Library.
P.S. Once again I would not recommend using this as Explicit is better
than Implicit
P.P.S. It is strongly undesirable for us humans to use anything
starting with __ :)
On May 15, 5:10*pm, "Geoffrey Clements"
<geoffrey.clement...@SPAMbaesystems.comwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message
>news:51**********************************@m44g200 0hsc.googlegroups.com....
On May 15, 10:06 am, "Terry Reedy" <tjre...@udel.eduwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message
news:f8**********************************@w7g2000h sa.googlegroups.com....
| It seems that I rather frequently need a list or iterator of the form
| [x for x in <while <>]
I can think of two ways to interpret that.
I mean like [x for x in <Aif <B>], only that it breaks the loop when
the expression <Bis false.
def gen(a):
* * for x in a:
* * * * if B: break
* * * * yield x
a_gen = gen(A)
# now iterate over a_gen
--
Geoff- Hide quoted text -
- Show quoted text -

In your original, you have:
l = range(6)
[1 if b!=4 else l.__delslice__(0,len(l)) for b in l][:-1]

You may be hyperextending the use of '..if..else..', which is one of
my fears regarding 'with x as y'. *"l.__delslice__(0,len(l))" is not
an expression.
Jun 27 '08 #9
l.__delslice__(0,len(l)) is an expression as it returns None which is
a value
On May 16, 4:23*am, castironpi <castiro...@gmail.comwrote:
On May 15, 6:07*pm, afrobeard <afrobe...@gmail.comwrote:
The following proposed solution is not intended to be a solution, it
goes completely against the zen of python. [Type import this into the
python command interpreter]
I brought it down to two lines:-
l = range(6)
[1 if b!=4 else l.__delslice__(0,len(l)) for b in l][:-1]
itertools would still be a better approach in my opinion.
Just because I'm curious to know, can anyone bring it shorter[even if
its cryptic] than this without invoking any Python Library.
P.S. Once again I would not recommend using this as Explicit is better
than Implicit
P.P.S. It is strongly undesirable for us humans to use anything
starting with __ :)
On May 15, 5:10*pm, "Geoffrey Clements"
<geoffrey.clement...@SPAMbaesystems.comwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message
>news:51**********************************@m44g200 0hsc.googlegroups.com....
On May 15, 10:06 am, "Terry Reedy" <tjre...@udel.eduwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message
news:f8**********************************@w7g2000h sa.googlegroups.com....
| It seems that I rather frequently need a list or iterator of the form
| [x for x in <while <>]
I can think of two ways to interpret that.
I mean like [x for x in <Aif <B>], only that it breaks the loop when
the expression <Bis false.
def gen(a):
* * for x in a:
* * * * if B: break
* * * * yield x
a_gen = gen(A)
# now iterate over a_gen
--
Geoff- Hide quoted text -
- Show quoted text -

In your original, you have:
l = range(6)
[1 if b!=4 else l.__delslice__(0,len(l)) for b in l][:-1]

You may be hyperextending the use of '..if..else..', which is one of
my fears regarding 'with x as y'. *"l.__delslice__(0,len(l))" is not
an expression.
Jun 27 '08 #10
On May 15, 6:52*pm, afrobeard <afrobe...@gmail.comwrote:
l.__delslice__(0,len(l)) is an expression as it returns None which is
a value

On May 16, 4:23*am, castironpi <castiro...@gmail.comwrote:
On May 15, 6:07*pm, afrobeard <afrobe...@gmail.comwrote:
The following proposed solution is not intended to be a solution, it
goes completely against the zen of python. [Type import this into the
python command interpreter]
I brought it down to two lines:-
l = range(6)
[1 if b!=4 else l.__delslice__(0,len(l)) for b in l][:-1]
itertools would still be a better approach in my opinion.
Just because I'm curious to know, can anyone bring it shorter[even if
its cryptic] than this without invoking any Python Library.
P.S. Once again I would not recommend using this as Explicit is better
than Implicit
P.P.S. It is strongly undesirable for us humans to use anything
starting with __ :)
On May 15, 5:10*pm, "Geoffrey Clements"
<geoffrey.clement...@SPAMbaesystems.comwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message
news:51**********************************@m44g2000 hsc.googlegroups.com...
On May 15, 10:06 am, "Terry Reedy" <tjre...@udel.eduwrote:
"urikaluzhny" <ukaluz...@nds.comwrote in message
>news:f8**********************************@w7g2000 hsa.googlegroups.com...
| It seems that I rather frequently need a list or iterator of theform
| [x for x in <while <>]
I can think of two ways to interpret that.
>I mean like [x for x in <Aif <B>], only that it breaks the loopwhen
>the expression <Bis false.
def gen(a):
* * for x in a:
* * * * if B: break
* * * * yield x
a_gen = gen(A)
# now iterate over a_gen
--
Geoff- Hide quoted text -
- Show quoted text -
In your original, you have:
l = range(6)
[1 if b!=4 else l.__delslice__(0,len(l)) for b in l][:-1]
You may be hyperextending the use of '..if..else..', which is one of
my fears regarding 'with x as y'. *"l.__delslice__(0,len(l))" is not
an expression.- Hide quoted text -

- Show quoted text -
Functional programming is really important to a former professor of
me. I like to say that None returns as a value. I think you can call
functional "evaluational".
Jun 27 '08 #11
urikaluzhny wrote:
>| It seems that I rather frequently need a list or iterator of the form
| [x for x in <while <>]
I can think of two ways to interpret that.
I mean like [x for x in <Aif <B>], only that it breaks the loop when
the expression <Bis false.
How do you plan to modify B during iteration?
May be
[x for x in itertools.takewhile(<B>, <A>)]
when <Bfunction accept element <Aand return True or False
Jun 27 '08 #12
urikaluzhny wrote:
It seems that I rather frequently need a list or iterator of the form
[x for x in <while <>]
And there is no one like this.
May be there is another short way to write it (not as a loop). Is
there?
Thanks
I usually have the same problem and i came up with an solution like that:

from operator import ne
def test(iterable, value, op=ne):
_n = iter(iterable).next
while True:
_x = _n()
if op(_x, value):
yield _x
else:
raise StopIteration

l = range(6)
print [x for x in test(l, 4)]

r@linux:~/tmppython test18.py
[0, 1, 2, 3]

Jun 27 '08 #13
On May 17, 11:08*am, Ruediger <larud...@freenet.dewrote:
urikaluzhny wrote:
It seems that I rather frequently need a list or iterator of the form
[x for x in <while <>]
And there is no one like this.
May be there is another short way to write it (not as a loop). Is
there?
Thanks

I usually have the same problem and i came up with an solution like that:

from operator import ne
def test(iterable, value, op=ne):
* * _n = iter(iterable).next
* * while True:
* * * * _x = _n()
* * * * if op(_x, value):
* * * * * * yield _x
* * * * else:
* * * * * * raise StopIteration
This is better written using takewhile...
itertools.takewhile(lambda x: x != value, iterable)

But if you really need to reinvent the wheel, perhaps this is simpler?

def test(iterable, value, op=operator.ne):
for x in iterable:
if not op(x, value):
return
yield x

--
Paul Hankin
Jun 27 '08 #14
Paul Hankin wrote:

This is better written using takewhile...
itertools.takewhile(lambda x: x != value, iterable)

But if you really need to reinvent the wheel, perhaps this is simpler?

def test(iterable, value, op=operator.ne):
for x in iterable:
if not op(x, value):
return
yield x

yes you are right it is. However as i mentioned in my post i came up with an
solution 'like' that. In fact my original code was to complex to post.
While simplifying it, i've overseen the obvious solution.

For special cases where you need to do more complex tests, the best solution
is IMHO to hide it in an generator function like above.

Jun 27 '08 #15

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