question about for cycle

[fdu.xiaojf]

Hi all,

I have the following code:

for i in generator_a: # the first "for" cycle
for j in generator_b:
if something_happen:
# do something here ..., I want the outer cycle to break
break

What should I do if I want the outer "for" cycle to continue or break ? If I
put a "continue" or "break" in the inner cycle, it has no effect on the outer
cycle.

And I have another question. Which is the most efficient way to check if there
are duplicate items in a list ? The items in the list may cannot be hashed, so
set() may not work on the list.

Regards,

[Ant]

On Sep 29, 11:04 am, "[e-mail]" <[e-mail]>
wrote:
....

What should I do if I want the outer "for" cycle to continue or break ? If I
put a "continue" or "break" in the inner cycle, it has no effect on the outer
cycle.

I'd also be interested in the idiomatic solution to this one. I can
see a number of solutions, from the ugly:

for i in range(10):
do_break = True
for j in range(10):
if j == 6:
break
else:
do_break = False

if do_break:
break

This will break the outer loop if the inner loop exited with a break.

Using exceptions:

for i in range(10):
try:
for j in range(10):
print i, j
if j == 6:
raise MyException
except MyException, e:
break # or continue or whatever.

Encapsulating in a function and using return:

def get_value():
for i in range(10):
for j in range(10):
print i, j
if j == 6:
return fn(i, j)

I guess to an extent it would depend on the exact situation as to
which of these is more suitable. Are there any other recommended
solutions to this?

--
Ant...

[Peter Otten]

Ant wrote:

On Sep 29, 11:04 am, "[e-mail]" <[e-mail]>
wrote:
...

>What should I do if I want the outer "for" cycle to continue or break ? If I
put a "continue" or "break" in the inner cycle, it has no effect on the outer
cycle.

I'd also be interested in the idiomatic solution to this one. I can
see a number of solutions, from the ugly:

for i in range(10):
do_break = True
for j in range(10):
if j == 6:
break
else:
do_break = False

if do_break:
break

Here's a variant that doesn't need the flag

>>inner = "abc"
outer = "xbz"
for i in outer:

.... for k in inner:
.... if i == k:
.... print "found", i
.... break
.... else:
.... print i, "not found"
.... continue
.... break
....
x not found
found b

but I usually prefer a helper function like this

def get_value():
for i in range(10):
for j in range(10):
print i, j
if j == 6:
return fn(i, j)

or this:

>>def f(i, inner):

.... for k in inner:
.... if i == k:
.... print "found", i
.... return True
....

>>for i in outer:

.... if f(i, inner):
.... break
.... print i, "not found"
....
x not found
found b

Peter

[Duncan Booth]

Ant <an****@gmail.comwrote:

On Sep 29, 11:04 am, "[e-mail]" <[e-mail]>
wrote:
...

>What should I do if I want the outer "for" cycle to continue or break
? If I put a "continue" or "break" in the inner cycle, it has no
effect on the outer cycle.

....

I guess to an extent it would depend on the exact situation as to
which of these is more suitable. Are there any other recommended
solutions to this?

I think the other Pythonic option you have missed is to convert the nested
for loops into a single loop by writing a generator.

def ranges(limit1, limit2):
range1, range2 = range(limit1), range(limit2)
for i in range1:
for j in range2:
yield i,j

....
for i, j in ranges(10, 10):
... whatever ...
if j==6:
break

[John Machin]

On Sep 29, 8:04 pm, "[e-mail]" <[e-mail]>
wrote:
[snip]

And I have another question. Which is the most efficient way to check if there
are duplicate items in a list ? The items in the list may cannot be hashed, so
set() may not work on the list.

The following classic by Tim Peters answers the question "How do I
remove duplicate items from a list".

http://aspn.activestate.com/ASPN/Coo...n/Recipe/52560

It should help -- provided of course that you mean "duplicates" in the
sense that two Python objects a and b are duplicates iff a == b.
If you mean duplicates in a fuzzier sense e.g. "Mao Zedong" and "Mao
Tse-Tung", the problem gets harder ...

HTH,
John

[tokland]

On 29 sep, 12:04, "[e-mail]" <[e-mail]:

for i in generator_a: # the first "for" cycle
for j in generator_b:
if something_happen:
# do something here ..., I want the outer cycle to break
break

Do you like this?

generator_ab = ((x, y) for x in generator_a for y in generator_b)
for i, j in generator_ab:
if condition:
# do something
break

[fdu.xiaojf]

to*****@gmail.com wrote:

On 29 sep, 12:04, "[e-mail]" <[e-mail]:

>for i in generator_a: # the first "for" cycle
for j in generator_b:
if something_happen:
# do something here ..., I want the outer cycle to break
break

Do you like this?

generator_ab = ((x, y) for x in generator_a for y in generator_b)
for i, j in generator_ab:
if condition:
# do something
break

In this case, the tuple generator_ab must be generated first. Sometime
it maybe a waste to generate all possible combinations of i,j first.

I have googled and found the PEP-3136(Labeled break and continue),
which has been rejected. I have also read why Guido rejected this PEP(
http://mail.python.org/pipermail/pyt...y/008663.html),
but I still think labeled break and continue is a good feature in my
case.

Regards,

[George Sakkis]

On Sep 29, 10:34 am, "[e-mail]" <[e-mail]>
wrote:

[e-mail] wrote:

On 29 sep, 12:04, "[e-mail]" <[e-mail]:
>

>for i in generator_a: # the first "for" cycle
> for j in generator_b:
> if something_happen:
> # do something here ..., I want the outer cycle to break
> break

>
Do you like this?
>
generator_ab = ((x, y) for x in generator_a for y in generator_b)
for i, j in generator_ab:
if condition:
# do something
break
>

In this case, the tuple generator_ab must be generated first.

It's not a tuple, it is a generator expression that can generate
tuples *lazily*.

Sometime it maybe a waste to generate all possible combinations of i,j first.

It doesn't; read about generator expressions at http://www.python.org/dev/peps/pep-0289/

George

[Zentrader]

On Sep 29, 8:19 am, George Sakkis <[e-mail]:

On Sep 29, 10:34 am, "[e-mail]" <[e-mail]>
wrote:

[e-mail] wrote:

On 29 sep, 12:04, "[e-mail]" <[e-mail]:

>for i in generator_a: # the first "for" cycle
> for j in generator_b:
> if something_happen:
> # do something here ..., I want the outer cycle to break
> break

Do you like this?

generator_ab = ((x, y) for x in generator_a for y in generator_b)
for i, j in generator_ab:
if condition:
# do something
break

In this case, the tuple generator_ab must be generated first.

George

You can get specific break points by expanding the for loop into a
while loop, and this is perhaps why it has never been implemented with
for loops.
ctr_a=0
ctr_b=0
while ctr_a < len(generator_a):
this_el_a = generator_a[ctr_a]

while ctr_b < len(generator_b):
this_el_b = generator_b[ctr_ b]
if something_happen:
ctr_b = len(generator_b) ## break this loop
if something_else:
ctr_a = len(generator_a) ## break outer while loop
ctr_b += 1
ctr_a += 1

[fdu.xiaojf]

George Sakkis wrote:

On Sep 29, 10:34 am, "[e-mail]" <[e-mail]>
wrote:

>[e-mail] wrote:

> On 29 sep, 12:04, "[e-mail]" <[e-mail]:

for i in generator_a: # the first "for" cycle
for j in generator_b:
if something_happen:
# do something here ..., I want the outer cycle to break
break

Do you like this?

generator_ab = ((x, y) for x in generator_a for y in generator_b)
for i, j in generator_ab:
if condition:
# do something
break

In this case, the tuple generator_ab must be generated first.

It's not a tuple, it is a generator expression that can generate
tuples *lazily*.

>Sometime it maybe a waste to generate all possible combinations of i,j first.

It doesn't; read about generator expressions at http://www.python.org/dev/peps/pep-0289/

George

Thanks, I didn't realize that.

However, I still think labeled break and continue is a valuable feature, which
is easier to understand and to use.

[Duncan Booth]

"fd********@gmail.com" <fd********@gmail.comwrote:

>>Sometime it maybe a waste to generate all possible combinations of
i,j first.

It doesn't; read about generator expressions at
http://www.python.org/dev/peps/pep-0289/

George

Thanks, I didn't realize that.

However, I still think labeled break and continue is a valuable
feature, which is easier to understand and to use.

Can you come up with a realistic example where you think a labelled break
would be easier to understand and use? A concrete example would really help
if you hope to persuade anyone that your way is better.

I find that Python's ability to extract the control part of a loop into a
generator is an extremely powerful way to make loops much clearer for
several reasons: you separate completely the 'what we are looping over',
and 'when do we stop' from the 'what do we then do with each item'; also
you have an opportunity to give the code controlling the loop a separate
(and meaningful) name.