"For" loop and list comprehension similarity

[s.lipnevich@gmail.com]

Hi All,

I apologize if this was brought up before, I couldn't find any "prior
art" :-).
On more than one occasion, I found myself wanting to use a "conditiona l
loop" like this (with "Invalid syntax" error, of course):

for i in c if <test>:
print i*2

....because it's similar to the list comprehension construct:

[i*2 for i in c if <test>]
---------

Is this the intended difference in constructs? The available equivalent
feels a bit awkward:

for i in c:
if <test>:
print i*2

Just curious. Thanks!

Sergey.

[Mitja Trampus]

Re: &quot;For&qu ot; loop and list comprehension similarity

s.lipnevich@gma il.com wrote:
[color=blue]
> On more than one occasion, I found myself wanting to use a "conditiona l
> loop" like this (with "Invalid syntax" error, of course):
>
> for i in c if <test>:
> print i*2[/color]

Maybe there's been a PEP, don't really know...
Currently, the only sensible alternative is what you've written below:
[color=blue]
> The available equivalent
> feels a bit awkward:
>
> for i in c:
> if <test>:
> print i*2[/color]


This indeed doesn't look nice, especially if you've got lots of code instead of just
print. An alternative which avoids double indentation is

for i in c:
if not <test>: continue
print i*2

[s.lipnevich@gmail.com]

Re: &quot;For&qu ot; loop and list comprehension similarity

Thank you for replying, Mitja! That *is* a nice alternative.

Do you think it's a good idea to ask on comp.python.dev el if they would
be interested in a PEP about this (provided there is none)?

Cheers,
Sergey.

[Grant Edwards]

Re: &quot;For&qu ot; loop and list comprehension similarity

On 2006-03-26, s.lipnevich@gma il.com <s.lipnevich@gm ail.com> wrote:[color=blue]
> Hi All,
>
> I apologize if this was brought up before, I couldn't find any "prior
> art" :-).
> On more than one occasion, I found myself wanting to use a "conditiona l
> loop" like this (with "Invalid syntax" error, of course):
>
> for i in c if <test>:
> print i*2
>
> ...because it's similar to the list comprehension construct:
>
> [i*2 for i in c if <test>]
> ---------
>
> Is this the intended difference in constructs? The available equivalent
> feels a bit awkward:
>
> for i in c:
> if <test>:
> print i*2[/color]

for j in [i*2 for i in c if <test>]:
print j

--
Grant Edwards grante Yow! .. I wonder if I
at ought to tell them about my
visi.com PREVIOUS LIFE as a COMPLETE
STRANGER?

[Ben Finney]

Re: &quot;For&qu ot; loop and list comprehension similarity

s.lipnevich@gma il.com writes:
[color=blue]
> On more than one occasion, I found myself wanting to use a "conditiona l
> loop" like this (with "Invalid syntax" error, of course):
>
> for i in c if <test>:
> print i*2
>
> ...because it's similar to the list comprehension construct:
>
> [i*2 for i in c if <test>][/color]

Why not combine the two:

for i in [j for j in c if <test>]:
print i*2

--
\ "I got food poisoning today. I don't know when I'll use it." |
`\ -- Steven Wright |
_o__) |
Ben Finney

[s.lipnevich@gmail.com]

Re: &quot;For&qu ot; loop and list comprehension similarity

> Why not combine the two:

I guess because (at least in source code) you're doing a loop twice
:-). I don't know what a compiler would do. I think though that the
"for i in c if test:" construct is more readable and maybe can even be
better optimized.
Thanks!

Sergey.

[John Zenger]

Re: &quot;For&qu ot; loop and list comprehension similarity

Rather than a list comprehension, it would be faster and more
memory-efficient to use a generator comprehension. Just change the
square brackets to parentheses:

for j in (i*2 for i in c if <test>):
print j


Grant Edwards wrote:[color=blue]
> On 2006-03-26, s.lipnevich@gma il.com <s.lipnevich@gm ail.com> wrote:
>[color=green]
>>Hi All,
>>
>>I apologize if this was brought up before, I couldn't find any "prior
>>art" :-).
>>On more than one occasion, I found myself wanting to use a "conditiona l
>>loop" like this (with "Invalid syntax" error, of course):
>>
>> for i in c if <test>:
>> print i*2
>>
>>...because it's similar to the list comprehension construct:
>>
>> [i*2 for i in c if <test>]
>> ---------
>>
>>Is this the intended difference in constructs? The available equivalent
>>feels a bit awkward:
>>
>> for i in c:
>> if <test>:
>> print i*2[/color]
>
>
> for j in [i*2 for i in c if <test>]:
> print j
>[/color]

[Terry Reedy]

Re: &quot;For&qu ot; loop and list comprehension similarity


<s.lipnevich@gm ail.com> wrote in message
news:1143414870 .330921.305890@ j33g2000cwa.goo glegroups.com.. .[color=blue][color=green]
>> Why not combine the two:[/color]
>
> I guess because (at least in source code) you're doing a loop twice
> :-). I don't know what a compiler would do. I think though that the
> "for i in c if test:" construct is more readable and maybe can even be
> better optimized.[/color]

There are also the filter and ifilter functions:

for i in filter(testfunc , c):

tjr

[s.lipnevich@gmail.com]

Re: &quot;For&qu ot; loop and list comprehension similarity

I think I like generator comprehension in this case better than either
list comprehension or a filter because both of the latter create a new
full "result list" before the loop even begins. At least I suppose they
do. Also, I think Mitja's suggestion "if not <test>: continue" and
Terry's filter function are more readable than comprehensions.
It's not a contest though :-), all these variants are great, thank you
all!
Do you think this discussion is a proof that the following principle
got violated, or do you think that "loop with condition" is not such an
atomic thing to be subject to this: "There should be one -- and
preferably only one -- obvious way to do it."
Cheers,

Sergey.

[Peter Hansen]

Re: &quot;For&qu ot; loop and list comprehension similarity

s.lipnevich@gma il.com wrote:[color=blue]
> Do you think this discussion is a proof that the following principle
> got violated, or do you think that "loop with condition" is not such an
> atomic thing to be subject to this: "There should be one -- and
> preferably only one -- obvious way to do it."[/color]

Mitja's suggestion was the one obvious way. The others are all
interesting, maybe even preferable in some cases, but I don't think most
experienced Python programmers would be more likely to start with one of
them than with the simple for-loop-with-explicit-test.

-Peter

[Terry Reedy]

Re: &quot;For&qu ot; loop and list comprehension similarity


"Peter Hansen" <peter@engcorp. com> wrote in message
news:e08r7a$4b5 $1@sea.gmane.or g...[color=blue]
> s.lipnevich@gma il.com wrote:[color=green]
>> Do you think this discussion is a proof that the following principle
>> got violated, or do you think that "loop with condition" is not such an
>> atomic thing to be subject to this: "There should be one -- and
>> preferably only one -- obvious way to do it."[/color]
>
> Mitja's suggestion was the one obvious way. The others are all
> interesting, maybe even preferable in some cases, but I don't think most
> experienced Python programmers would be more likely to start with one of
> them than with the simple for-loop-with-explicit-test.[/color]

If by 'explicit-test' you mean a nested if-statement, then I agree. When I
mentioned filter() as one way to avoid the obvious, I was aware that it
creates an intermediate list that is usually not needed. (And if it is
needed, then it should be name-assigned before the loop.)

Terry Jan Reedy