[Tutor] Multiple for and if/else statements into a single list comprehension
Is it possible to get two nested for statements followed by a nested
if/else statement all into a single list comprehension ie. the equivalent
of the below:
for i in xrange(1,20):
for j in xrange(1,10):
if j<6:
j=int("8"+str(j))
else:
j=int("9"+str(j))
print "%(i)02d_%(j)02d" % locals()
# double for statement without if/else works
print "\n".join(["%(i)02d_%(j)02d" % locals() for i in xrange(1,20) for j
in xrange(1,10)])
#now try to incorporate if/else part
#failed attempt 1
print "\n".join(["%(i)02d_%(j)02d" % locals() for i in xrange(1,20) for j
in xrange(1,10) j=int("8"+str(j)) if j<6 else int("9"+str(j))])
#failed attempt 2
print "\n".join(["%(i)02d_%(j)02d" % locals() for i in xrange(1,20)
j=int("8"+str(j)) if j<6 else int("9"+str(j)) for j in xrange(1,10)])
#failed attempt 3
print "\n".join(["%(i)02d_%(j)02d" % locals() j=int("8"+str(j)) if j<6 else
int("9"+str(j)) for i in xrange(1,20) for j in xrange(1,10)])
Many thanks in advance.
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Re: [Tutor] Multiple for and if/else statements into a single list comprehension
Jignesh Sutar wrote:
> Is it possible to get two nested for statements followed by a nested
> if/else statement all into a single list comprehension ie. the equivalent
> of the below:
>
>
> for i in xrange(1,20):
> for j in xrange(1,10):
> if j<6:
> j=int("8"+str(j))
> else:
> j=int("9"+str(j))
> print "%(i)02d_%(j)02d" % locals()
> Many thanks in advance.
Need I say that it is a bad idea to build overly complex list
comprehensions? Under that proviso:
The first step is always to ensure that there is a single expression in the
inner loop. I'm keeping as similar as possible to your loops:
for i in xrange(1, 20):
for j in xrange(1, 10):
print "%02d_%02d" % (i,
int(("8" if j < 6 else "9") + str(j)))
Now the translation is mechanical
for x in a:
for y in b:
expr
becomes
[expr for x in a for y in b]
or (using a genexp rather than a listcomp as you are going to print it
anyway)
print "\n".join(
"%02d_%02d" % (i, int(("8" if j < 6 else "9") + str(j)))
for i in xrange(1, 20)
for j in xrange(1, 10))
But again: don't do that ;)
By the way, using locals() is another bad idea, plus it does not capture the
loop vars of genexps (all pythons) and listcomps (python3).
PS: a simple alternative:
print "\n".join(
"%02d_%02d" % (i, k)
for i in range(1, 20)
for k in range(81, 86) + range(96, 100))
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Re: [Tutor] Multiple for and if/else statements into a single list comprehension
Peter Otten wrote: > [locals()] does not capture > the loop vars of genexps (all pythons) and listcomps (python3). Sorry, I was totally wrong on that one. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Multiple for and if/else statements into a single list comprehension
On 03/17/2014 11:22 AM, Jignesh Sutar wrote:
Is it possible to get two nested for statements followed by a nested
if/else statement all into a single list comprehension ie. the equivalent
of the below:
for i in xrange(1,20):
for j in xrange(1,10):
if j<6:
j=int("8"+str(j))
else:
j=int("9"+str(j))
print "%(i)02d_%(j)02d" % locals()
You can do it by reformulating your inner block into an expression (here, using
a ternary if expression), which will then become the expression part of the
comprehension. However, a few remarks:
* don't do that: the only advantage is to make your code unreadable
* you may reformulate using 2 comprehensions; if you don't want intermediate
lists, use a generator expression for the inner one
* above, the inner j is a new variable with a distinct meaning: why do you call
it j?
* do you really need string concat to perform arithmetic?
d
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[Tutor] Multiple for and if/else statements into a single list comprehension
Is it possible to get two nested for statements followed by a nested
if/else statement all into a single list comprehension ie. the equivalent
of the below:
for i in xrange(1,20):
for j in xrange(1,10):
if j<6:
j=int("8"+str(j))
else:
j=int("9"+str(j))
print "%(i)02d_%(j)02d" % locals()
# double for statement without if/else works
print "\n".join(["%(i)02d_%(j)02d" % locals() for i in xrange(1,20) for j
in xrange(1,10)])
#now try to incorporate if/else part
#failed attempt 1
print "\n".join(["%(i)02d_%(j)02d" % locals() for i in xrange(1,20) for j
in xrange(1,10) j=int("8"+str(j)) if j<6 else int("9"+str(j))])
#failed attempt 2
print "\n".join(["%(i)02d_%(j)02d" % locals() for i in xrange(1,20)
j=int("8"+str(j)) if j<6 else int("9"+str(j)) for j in xrange(1,10)])
#failed attempt 3
print "\n".join(["%(i)02d_%(j)02d" % locals() j=int("8"+str(j)) if j<6 else
int("9"+str(j)) for i in xrange(1,20) for j in xrange(1,10)])
Many thanks in advance.
Jignesh
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Re: [Tutor] Tutor Digest, Vol 121, Issue 42 nested "for"
python executes the first "for" and its condititions then the next "for". What i'm saying is that you should first deal with one as nested block codes ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Multiple for and if/else statements into a single list comprehension
Thanks Peter/Denis. I wasn't aware of genexp. I see how you have adapted
the code to make it work, I'll adapt the same in my program. Good point
about duplicating j , Denis, I guess I was happy to override the outer j as
it was intermediate.
On 17 March 2014 12:36, spir wrote:
> On 03/17/2014 11:22 AM, Jignesh Sutar wrote:
>
>> Is it possible to get two nested for statements followed by a nested
>> if/else statement all into a single list comprehension ie. the equivalent
>> of the below:
>>
>>
>> for i in xrange(1,20):
>> for j in xrange(1,10):
>> if j<6:
>> j=int("8"+str(j))
>> else:
>> j=int("9"+str(j))
>> print "%(i)02d_%(j)02d" % locals()
>>
>
> You can do it by reformulating your inner block into an expression (here,
> using a ternary if expression), which will then become the expression part
> of the comprehension. However, a few remarks:
>
> * don't do that: the only advantage is to make your code unreadable
> * you may reformulate using 2 comprehensions; if you don't want
> intermediate lists, use a generator expression for the inner one
> * above, the inner j is a new variable with a distinct meaning: why do you
> call it j?
> * do you really need string concat to perform arithmetic?
>
>
> d
> ___
> Tutor maillist - Tutor@python.org
> To unsubscribe or change subscription options:
> https://mail.python.org/mailman/listinfo/tutor
>
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