# loops

Discussion in 'Python' started by Gandalf, Oct 18, 2008.

1. ### GandalfGuest

how can I do width python a normal for loop width tree conditions like
for example :

for x=1;x<=100;x+x:
print x

thanks

Gandalf, Oct 18, 2008

2. ### GandalfGuest

On Oct 18, 12:39 pm, Duncan Booth <>
wrote:
> Gandalf <> wrote:
> > how can I do width python a normal for loop width tree conditions like
> > for example :

>
> > for x=1;x<=100;x+x:
> >     print x

>
> What you wrote would appear to be an infinite loop so I'll assume you meant
> to assign something to x each time round the loop as well. The simple
> Python translation of what I think you meant would be:
>
> x = 1
> while x <= 100:
>    print x
>    x += x
>
> If you really insist on doing it with a for loop:
>
> def doubling(start, limit):
>     x = start
>     while x <= limit:
>         yield x
>         x += x
>
> ...
>
> for x in doubling(1, 100):
>     print x

thanks

Gandalf, Oct 18, 2008

3. ### GandalfGuest

On Oct 18, 12:39 pm, Duncan Booth <>
wrote:
> Gandalf <> wrote:
> > how can I do width python a normal for loop width tree conditions like
> > for example :

>
> > for x=1;x<=100;x+x:
> >     print x

>
> What you wrote would appear to be an infinite loop so I'll assume you meant
> to assign something to x each time round the loop as well. The simple
> Python translation of what I think you meant would be:
>
> x = 1
> while x <= 100:
>    print x
>    x += x
>
> If you really insist on doing it with a for loop:
>
> def doubling(start, limit):
>     x = start
>     while x <= limit:
>         yield x
>         x += x
>
> ...
>
> for x in doubling(1, 100):
>     print x

I was hopping to describe it with only one command. most of the
languages I know use this.
It seems weird to me their is no such thing in python. it's not that I
can't fined a solution it's all about saving code

Gandalf, Oct 18, 2008
4. ### robertGuest

Gandalf wrote:
> On Oct 18, 12:39 pm, Duncan Booth <>
> wrote:
>> Gandalf <> wrote:
>>> how can I do width python a normal for loop width tree conditions like
>>> for example :
>>> for x=1;x<=100;x+x:
>>> print x

>> What you wrote would appear to be an infinite loop so I'll assume you meant
>> to assign something to x each time round the loop as well. The simple
>> Python translation of what I think you meant would be:
>>
>> x = 1
>> while x <= 100:
>> print x
>> x += x
>>

...
>
> I was hopping to describe it with only one command. most of the
> languages I know use this.
> It seems weird to me their is no such thing in python. it's not that I
> can't fined a solution it's all about saving code

You'd not save code, but only lines (and clearness). You'd also
need more (non-space) characters

Python saves confusion and arbitrariness => you'll usually code
faster, because you don't think so much about voluptuous
multimulti..possibilites, not worth the play: one-ness of mind

If insistent, you could sometimes save lines like this ;-)

x=1
while x<=100: print x; x+=x

Robert

robert, Oct 18, 2008

Gandalf wrote:

> On Oct 18, 12:39 pm, Duncan Booth <>
> wrote:
>> Gandalf <> wrote:
>> > how can I do width python a normal for loop width tree conditions like
>> > for example :

>>
>> > for x=1;x<=100;x+x:
>> >     print x

>>
>> What you wrote would appear to be an infinite loop so I'll assume you meant
>> to assign something to x each time round the loop as well. The simple
>> Python translation of what I think you meant would be:
>>
>> x = 1
>> while x <= 100:
>>    print x
>>    x += x
>>
>> If you really insist on doing it with a for loop:
>>
>> def doubling(start, limit):
>>     x = start
>>     while x <= limit:
>>         yield x
>>         x += x
>>
>> ...
>>
>> for x in doubling(1, 100):
>>     print x

>
> I was hopping to describe it with only one command. most of the
> languages I know use this.
> It seems weird to me their is no such thing in python. it's not that I
> can't fined a solution it's all about saving code

Do you anticipate reusing it? You could make something a little more
extendable.

for x in iexpression( 'x', 1, 100, 'x+x' ):
print x

or

for x in iexpression( lambda x: x+x, 1, 100 ):
print x

I'm assuming you don't want or have a closed form, in this case x= 2**
_x.

6. ### Terry ReedyGuest

Gandalf wrote:
> On Oct 18, 12:39 pm, Duncan Booth <>
> wrote:
>> Gandalf <> wrote:
>>> how can I do width python a normal for loop width tree conditions like
>>> for example :
>>> for x=1;x<=100;x+x:
>>> print x

>> What you wrote would appear to be an infinite loop so I'll assume you meant
>> to assign something to x each time round the loop as well. The simple
>> Python translation of what I think you meant would be:
>>
>> x = 1
>> while x <= 100:
>> print x
>> x += x
>>
>> If you really insist on doing it with a for loop:
>>
>> def doubling(start, limit):
>> x = start
>> while x <= limit:
>> yield x
>> x += x
>>
>> ...
>>
>> for x in doubling(1, 100):
>> print x

>
> I was hopping to describe it with only one command. most of the
> languages I know use this.
> It seems weird to me their is no such thing in python. it's not that I
> can't fined a solution it's all about saving code

Python: 'makes common things easy and uncommon things possible'.

The large majority of use cases for iteration are iterating though
sequences, actual and virtual, including integers with a constant step
size. Python make that trivial to do and clear to read. Your example is
trivially written as

for i in range(11):
print 2**i

Python provide while loops for more fine-grain control, and a protocol
so *reuseable* iterators can plug into for loops. Duncan showed you
both. If you *need* a doubling loop variable once, you probably need
one more than once, and the cost of the doubling generator is amortized
over all such uses. Any Python proprammer should definitely know how to
write such a thing without hardly thinking. We can squeeze a line out
of this particular example:

def doubling(value, limit):
while value <= limit:
yield value
value += value

Terry Jan Reedy

Terry Reedy, Oct 18, 2008
7. ### wbowersGuest

On Oct 18, 11:31 am, Terry Reedy <> wrote:
> Gandalf wrote:
> > On Oct 18, 12:39 pm, Duncan Booth <>
> > wrote:
> >> Gandalf <> wrote:
> >>> how can I do width python a normal for loop width tree conditions like
> >>> for example :
> >>> for x=1;x<=100;x+x:
> >>>     print x
> >> What you wrote would appear to be an infinite loop so I'll assume you meant
> >> to assign something to x each time round the loop as well. The simple
> >> Python translation of what I think you meant would be:

>
> >> x = 1
> >> while x <= 100:
> >>    print x
> >>    x += x

>
> >> If you really insist on doing it with a for loop:

>
> >> def doubling(start, limit):
> >>     x = start
> >>     while x <= limit:
> >>         yield x
> >>         x += x

>
> >> ...

>
> >> for x in doubling(1, 100):
> >>     print x

>
> > I was hopping to describe it with only one command. most of the
> > languages I know use this.
> > It seems weird to me their is no such thing in python. it's not that I
> > can't fined a solution it's all about saving code

>
> Python: 'makes common things easy and uncommon things possible'.
>
> The large majority of use cases for iteration are iterating though
> sequences, actual and virtual, including integers with a constant step
> size.  Python make that trivial to do and clear to read. Your example is
> trivially written as
>
> for i in range(11):
>    print 2**i
>
> Python provide while loops for more fine-grain control, and a protocol
> so *reuseable* iterators can plug into for loops. Duncan showed you
> both.  If you *need* a doubling loop variable once, you probably need
> one more than once, and the cost of the doubling generator is amortized
> over all such uses.  Any Python proprammer should definitely know how to
> write such a thing without hardly thinking.  We can squeeze a line out
> of this particular example:
>
> def doubling(value, limit):
>    while value <= limit:
>      yield value
>      value += value
>
> Terry Jan Reedy

I agree that using range() for simple iterations is the way to go.
Here are some examples of python expressions you'd use in specific
situations:

# instead of for (i = 0; i < 100; i++)
for i in range(100): pass

# instead of for (i = 10; i < 100; i++)
for i in range(10, 100): pass

# instead of for (i = 1; i < 100; i += 2)
for i in range(1, 100, 2): pass

# instead of for (i = 99; i >= 0; i--)
for i in range(100)[::-1]: pass

There's always a way to do it, and it's almost always really simple :-D

wbowers, Oct 18, 2008
8. ### robertGuest

> Gandalf wrote:
>
>> On Oct 18, 12:39 pm, Duncan Booth <>
>> wrote:
>>> Gandalf <> wrote:
>>>> how can I do width python a normal for loop width tree conditions like
>>>> for example :
>>>> for x=1;x<=100;x+x:
>>>> print x
>>> What you wrote would appear to be an infinite loop so I'll assume you meant
>>> to assign something to x each time round the loop as well. The simple
>>> Python translation of what I think you meant would be:
>>>
>>> x = 1
>>> while x <= 100:
>>> print x
>>> x += x
>>>
>>> If you really insist on doing it with a for loop:
>>>
>>> def doubling(start, limit):
>>> x = start
>>> while x <= limit:
>>> yield x
>>> x += x
>>>
>>> ...
>>>
>>> for x in doubling(1, 100):
>>> print x

>> I was hopping to describe it with only one command. most of the
>> languages I know use this.
>> It seems weird to me their is no such thing in python. it's not that I
>> can't fined a solution it's all about saving code

>
> Do you anticipate reusing it? You could make something a little more
> extendable.
>
> for x in iexpression( 'x', 1, 100, 'x+x' ):
> print x
>
> or
>
> for x in iexpression( lambda x: x+x, 1, 100 ):
> print x
>
> I'm assuming you don't want or have a closed form, in this case x= 2**
> _x.
>

import this # ;-)

robert, Oct 18, 2008
9. ### MRABGuest

On Oct 18, 7:31 pm, Terry Reedy <> wrote:
> Gandalf wrote:
> > On Oct 18, 12:39 pm, Duncan Booth <>
> > wrote:
> >> Gandalf <> wrote:
> >>> how can I do width python a normal for loop width tree conditions like
> >>> for example :
> >>> for x=1;x<=100;x+x:
> >>>     print x
> >> What you wrote would appear to be an infinite loop so I'll assume you meant
> >> to assign something to x each time round the loop as well. The simple
> >> Python translation of what I think you meant would be:

>
> >> x = 1
> >> while x <= 100:
> >>    print x
> >>    x += x

>
> >> If you really insist on doing it with a for loop:

>
> >> def doubling(start, limit):
> >>     x = start
> >>     while x <= limit:
> >>         yield x
> >>         x += x

>
> >> ...

>
> >> for x in doubling(1, 100):
> >>     print x

>
> > I was hopping to describe it with only one command. most of the
> > languages I know use this.
> > It seems weird to me their is no such thing in python. it's not that I
> > can't fined a solution it's all about saving code

>
> Python: 'makes common things easy and uncommon things possible'.
>
> The large majority of use cases for iteration are iterating though
> sequences, actual and virtual, including integers with a constant step
> size.  Python make that trivial to do and clear to read. Your example is
> trivially written as
>
> for i in range(11):
>    print 2**i
>
> Python provide while loops for more fine-grain control, and a protocol
> so *reuseable* iterators can plug into for loops. Duncan showed you
> both.  If you *need* a doubling loop variable once, you probably need
> one more than once, and the cost of the doubling generator is amortized
> over all such uses.  Any Python proprammer should definitely know how to
> write such a thing without hardly thinking.  We can squeeze a line out
> of this particular example:
>
> def doubling(value, limit):
>    while value <= limit:
>      yield value
>      value += value
>

Shouldn't the upper limit be exclusive in order to be Pythonic?

MRAB, Oct 19, 2008
10. ### Terry ReedyGuest

MRAB wrote:
> On Oct 18, 7:31 pm, Terry Reedy <> wrote:

>> Python provide while loops for more fine-grain control, and a protocol
>> so *reuseable* iterators can plug into for loops. Duncan showed you
>> both. If you *need* a doubling loop variable once, you probably need
>> one more than once, and the cost of the doubling generator is amortized
>> over all such uses. Any Python proprammer should definitely know how to
>> write such a thing without hardly thinking. We can squeeze a line out
>> of this particular example:
>>
>> def doubling(value, limit):
>> while value <= limit:
>> yield value
>> value += value
>>

> Shouldn't the upper limit be exclusive in order to be Pythonic?

Yes, and perhaps I could have mentioned that, but the OP wanted a port
of the C construct.

Terry Reedy, Oct 19, 2008
11. ### Steven D'ApranoGuest

On Sat, 18 Oct 2008 03:52:51 -0700, Gandalf wrote:

> I was hopping to describe it with only one command. most of the
> languages I know use this.
> It seems weird to me their is no such thing in python. it's not that I
> can't fined a solution it's all about saving code

It shouldn't be about saving code. There's no shortage of code so that we
have to conserve it. But there is a shortage of time and effort, so
making your code easy to read and easy to maintain is far more important.

for x in (2**i for i in xrange(10)):
print x

will also print 1, 2, 4, 8, ... up to 1000.

--
Steven

Steven D'Aprano, Oct 19, 2008
12. ### James MillsGuest

On Sun, Oct 19, 2008 at 1:30 PM, Steven D'Aprano
<> wrote:
> for x in (2**i for i in xrange(10)):
> print x

This is by far the most concise solution I've seen so far.
And it should never be about conserving code.
Also, Python IS NOT C (to be more specific: Python
is not a C-class language).

--JamesMills

--
--
-- "Problems are solved by method"

James Mills, Oct 19, 2008
13. ### James MillsGuest

On Sun, Oct 19, 2008 at 1:44 PM, James Mills
<> wrote:
> On Sun, Oct 19, 2008 at 1:30 PM, Steven D'Aprano
> <> wrote:
>> for x in (2**i for i in xrange(10)):
>> print x

>
> This is by far the most concise solution I've seen so far.
> And it should never be about conserving code.
> Also, Python IS NOT C (to be more specific: Python
> is not a C-class language).

Also, if the OP is finding himself writing such manual
and mundane looking loops, he/she should reconsider
what it is he/she is doing. You would normally want
to iterate (vs. loop) over a sequence of items.

--JamesMills

--
--
-- "Problems are solved by method"

James Mills, Oct 19, 2008
14. ### John MachinGuest

On Oct 19, 2:30 pm, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.au> wrote:
[snip]
> making your code easy to read and easy to maintain is far more important.
>
> for x in (2**i for i in xrange(10)):
>     print x
>
> will also print 1, 2, 4, 8, ... up to 1000.

I would say up to 512; perhaps your understanding of "up to" differs
from mine.

Easy to read? I'd suggest this:

for i in xrange(10):
print 2 ** i

Cheers,
John

John Machin, Oct 19, 2008
15. ### Paul RubinGuest

"James Mills" <> writes:
> > for x in (2**i for i in xrange(10)):
> > print x

> This is by far the most concise solution I've seen so far.

print '\n'.join(str(2**i) for i in xrange(10))

Paul Rubin, Oct 19, 2008
16. ### Steven D'ApranoGuest

On Sat, 18 Oct 2008 20:45:47 -0700, John Machin wrote:

> On Oct 19, 2:30Â pm, Steven D'Aprano <st...@REMOVE-THIS-
> cybersource.com.au> wrote:
> [snip]
>> making your code easy to read and easy to maintain is far more
>> important.
>>
>> for x in (2**i for i in xrange(10)):
>> Â  Â  print x
>>
>> will also print 1, 2, 4, 8, ... up to 1000.

>
> I would say up to 512; perhaps your understanding of "up to" differs
> from mine.

Well, mine is based on Python's half-open semantics: "up to" 1000 doesn't
include 1000, and the highest power of 2 less than 1000 is 512.

Perhaps you meant "up to and including 512".

> Easy to read? I'd suggest this:
>
> for i in xrange(10):
> print 2 ** i

Well, sure, if you want to do it the right way *wink*.

But seriously, no, that doesn't answer the OP's question. Look at his
original code (which I assume is C-like pseudo-code):

for x=1;x<=100;x+x:
print x

The loop variable i takes the values 1, 2, 4, 8, etc. That's what my code
would be appropriate:

for x=1;x<=100;x++:
print 2**x

--
Steven

Steven D'Aprano, Oct 19, 2008
17. ### John MachinGuest

Steven D'Aprano wrote:

> On Sat, 18 Oct 2008 20:45:47 -0700, John Machin wrote:
>
> > On Oct 19, 2:30 pm, Steven D'Aprano <st...@REMOVE-THIS-
> > cybersource.com.au> wrote:
> > [snip]
> >> making your code easy to read and easy to maintain is far more
> >> important.
> >>
> >> for x in (2**i for i in xrange(10)):
> >>     print x
> >>
> >> will also print 1, 2, 4, 8, ... up to 1000.

> >
> > I would say up to 512; perhaps your understanding of "up to" differs
> > from mine.

>
> Well, mine is based on Python's half-open semantics: "up to" 1000 doesn't
> include 1000, and the highest power of 2 less than 1000 is 512.

We're talking about an English sentence, not a piece of Python code.
When you say "I'm taking the train to X", do you get off at the
station before X, as in "getting off at Redfern"?

>
> Perhaps you meant "up to and including 512".
>

John Machin, Oct 19, 2008
18. ### Steven D'ApranoGuest

On Sun, 19 Oct 2008 03:17:51 -0700, John Machin wrote:

> Steven D'Aprano wrote:
>
>> On Sat, 18 Oct 2008 20:45:47 -0700, John Machin wrote:
>>
>> > On Oct 19, 2:30Â pm, Steven D'Aprano <st...@REMOVE-THIS-
>> > cybersource.com.au> wrote:
>> > [snip]
>> >> making your code easy to read and easy to maintain is far more
>> >> important.
>> >>
>> >> for x in (2**i for i in xrange(10)):
>> >> Â  Â  print x
>> >>
>> >> will also print 1, 2, 4, 8, ... up to 1000.
>> >
>> > I would say up to 512; perhaps your understanding of "up to" differs
>> > from mine.

>>
>> Well, mine is based on Python's half-open semantics: "up to" 1000
>> doesn't include 1000, and the highest power of 2 less than 1000 is 512.

>
> We're talking about an English sentence, not a piece of Python code.
> When you say "I'm taking the train to X", do you get off at the station
> before X, as in "getting off at Redfern"?

But I don't say "I'm taking the train UP TO X".

Intervals in English are often ambiguous, which is why people often
explicitly say "up to and including...". But in this specific case, I
don't see why you're having difficulty. Whether 1000 was included or not
makes no difference, because 1000 is not a power of 2.

--
Steven

Steven D'Aprano, Oct 19, 2008