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looking for a neat solution to a nested loop problem

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  looking for a neat solution to a nested loop problem Tom P <werotizy@freent.dd> - 2012-08-06 17:52 +0200
    Re: looking for a neat solution to a nested loop problem John Gordon <gordon@panix.com> - 2012-08-06 16:03 +0000
      Re: looking for a neat solution to a nested loop problem Tom P <werotizy@freent.dd> - 2012-08-06 19:16 +0200
        Re: looking for a neat solution to a nested loop problem Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-08-07 01:27 +0000
    Re: looking for a neat solution to a nested loop problem Ian Foote <ian@feete.org> - 2012-08-06 17:07 +0100
    Re: looking for a neat solution to a nested loop problem Ethan Furman <ethan@stoneleaf.us> - 2012-08-06 09:15 -0700
    Re: looking for a neat solution to a nested loop problem Nobody <nobody@nowhere.com> - 2012-08-06 17:18 +0100
      Re: looking for a neat solution to a nested loop problem Tom P <werotizy@freent.dd> - 2012-08-06 19:14 +0200
        Re: looking for a neat solution to a nested loop problem Emile van Sebille <emile@fenx.com> - 2012-08-06 11:11 -0700
          Re: looking for a neat solution to a nested loop problem Larry Hudson <orgnut@yahoo.com> - 2012-08-06 21:02 -0700
            Re: looking for a neat solution to a nested loop problem Nobody <nobody@nowhere.com> - 2012-08-07 16:32 +0100
              Re: looking for a neat solution to a nested loop problem 88888 Dihedral <dihedral88888@googlemail.com> - 2012-08-07 18:42 -0700
              Re: looking for a neat solution to a nested loop problem 88888 Dihedral <dihedral88888@googlemail.com> - 2012-08-09 11:46 -0700
              Re: looking for a neat solution to a nested loop problem 88888 Dihedral <dihedral88888@googlemail.com> - 2012-08-09 11:39 -0700
              Re: looking for a neat solution to a nested loop problem 88888 Dihedral <dihedral88888@googlemail.com> - 2012-08-14 14:08 -0700
        Re: looking for a neat solution to a nested loop problem Grant Edwards <invalid@invalid.invalid> - 2012-08-06 18:25 +0000
          Re: looking for a neat solution to a nested loop problem Grant Edwards <invalid@invalid.invalid> - 2012-08-06 18:29 +0000
            Re: looking for a neat solution to a nested loop problem Tom P <werotizy@freent.dd> - 2012-08-06 21:03 +0200
              Re: looking for a neat solution to a nested loop problem Grant Edwards <invalid@invalid.invalid> - 2012-08-06 19:22 +0000
                Re: looking for a neat solution to a nested loop problem Emile van Sebille <emile@fenx.com> - 2012-08-06 12:52 -0700
    Re: looking for a neat solution to a nested loop problem André Malo <ndparker@gmail.com> - 2012-08-06 20:19 +0200
    Re: looking for a neat solution to a nested loop problem Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-08-06 15:31 -0400
    Re: looking for a neat solution to a nested loop problem Arnaud Delobelle <arnodel@gmail.com> - 2012-08-06 21:14 +0100

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#26625 — looking for a neat solution to a nested loop problem

consider a nested loop algorithm -

for i in range(100):
     for j in range(100):
         do_something(i,j)

Now, suppose I don't want to use i = 0 and j = 0 as initial values, but 
some other values i = N and j = M, and I want to iterate through all 
10,000 values in sequence - is there a neat python-like way to this? I 
realize I can do things like use a variable for k in range(10000): and 
then derive values for i and j from k, but I'm wondering if there's 
something less clunky.

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#26627

In <a8a7hvF8crU1@mid.individual.net> Tom P <werotizy@freent.dd> writes:

> consider a nested loop algorithm -

> for i in range(100):
>      for j in range(100):
>          do_something(i,j)

> Now, suppose I don't want to use i = 0 and j = 0 as initial values, but 
> some other values i = N and j = M, and I want to iterate through all 
> 10,000 values in sequence - is there a neat python-like way to this? I 
> realize I can do things like use a variable for k in range(10000): and 
> then derive values for i and j from k, but I'm wondering if there's 
> something less clunky.

You could define your own generator function that yields values
in whatever order you want:

def my_generator():
    yield 9
    yield 100
    for i in range(200, 250):
        yield i
    yield 5


-- 
John Gordon                   A is for Amy, who fell down the stairs
gordon@panix.com              B is for Basil, assaulted by bears
                                -- Edward Gorey, "The Gashlycrumb Tinies"

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#26641

On 08/06/2012 06:03 PM, John Gordon wrote:
> In <a8a7hvF8crU1@mid.individual.net> Tom P <werotizy@freent.dd> writes:
>
>> consider a nested loop algorithm -
>
>> for i in range(100):
>>       for j in range(100):
>>           do_something(i,j)
>
>> Now, suppose I don't want to use i = 0 and j = 0 as initial values, but
>> some other values i = N and j = M, and I want to iterate through all
>> 10,000 values in sequence - is there a neat python-like way to this? I
>> realize I can do things like use a variable for k in range(10000): and
>> then derive values for i and j from k, but I'm wondering if there's
>> something less clunky.
>
> You could define your own generator function that yields values
> in whatever order you want:
>
> def my_generator():
>      yield 9
>      yield 100
>      for i in range(200, 250):
>          yield i
>      yield 5
>
>
Thanks, I'll look at that but I think it just moves the clunkiness from 
one place in the code to another.

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#26672

On Mon, 06 Aug 2012 19:16:45 +0200, Tom P wrote:

>> def my_generator():
>>      yield 9
>>      yield 100
>>      for i in range(200, 250):
>>          yield i
>>      yield 5
>>
>>
> Thanks, I'll look at that but I think it just moves the clunkiness from
> one place in the code to another.

And if there was a built-in command that did exactly what you wanted, it 
too would also move the clunkiness from one place in the code to another.

What you are asking for is clunky:

    [quote]
    j runs through range(M, 100) and then range(0,M), and 
    i runs through range(N,100) and then range(0,N)
    [end quote]


There's no magic pixie dust that you can sprinkle on it to make it 
elegant. Assuming M and N are small (under 100), you can do this:

values = range(100)  # or list(range(100)) in Python 3.
for j in (values[M:] + values[:M]):
    for i in (values[N:] + values[:N]):
        ...

which isn't too bad. If you have to deal with much large ranges, you can 
use itertools to chain them together:

import itertools
jvalues = itertools.chain(xrange(M, 1000000), xrange(M))  # or just range
ivalues = itertools.chain(xrange(N, 2500000), xrange(N))  # in Python 3
for j in jvalues:
    for i in ivalues:
        ...



-- 
Steven

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#26629

The function range can be called with more than one argument. For example:

for i in range(N, N + 10):
     for j in range(M, M + 100):
         do_something(i, j)

You can also call range with 3 arguments, if want a step size different 
to 1:

for k in range(2, 11, 3):
     print(k)

2
5
8

Hope this is clear,
Ian

On 06/08/12 16:52, Tom P wrote:
> consider a nested loop algorithm -
>
> for i in range(100):
>     for j in range(100):
>         do_something(i,j)
>
> Now, suppose I don't want to use i = 0 and j = 0 as initial values, 
> but some other values i = N and j = M, and I want to iterate through 
> all 10,000 values in sequence - is there a neat python-like way to 
> this? I realize I can do things like use a variable for k in 
> range(10000): and then derive values for i and j from k, but I'm 
> wondering if there's something less clunky.

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#26630

Tom P wrote:
> consider a nested loop algorithm -
> 
> for i in range(100):
>     for j in range(100):
>         do_something(i,j)
> 
> Now, suppose I don't want to use i = 0 and j = 0 as initial values, but 
> some other values i = N and j = M, and I want to iterate through all 
> 10,000 values in sequence - is there a neat python-like way to this? I 
> realize I can do things like use a variable for k in range(10000): and 
> then derive values for i and j from k, but I'm wondering if there's 
> something less clunky.

for i in range(N, N+100):
     for j in range(M, M+100):
         do_something(i, j)


~Ethan~

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#26634

On Mon, 06 Aug 2012 17:52:31 +0200, Tom P wrote:

> consider a nested loop algorithm -
> 
> for i in range(100):
>      for j in range(100):
>          do_something(i,j)
> 
> Now, suppose I don't want to use i = 0 and j = 0 as initial values, but 
> some other values i = N and j = M, and I want to iterate through all 
> 10,000 values in sequence - is there a neat python-like way to this?

	for i in range(N,N+100):
	    for j in range(M,M+100):
	        do_something(i,j)

Or did you mean something else?

Alternatively:

	import itertools

	for i, j in itertools.product(range(N,N+100),range(M,M+100)):
	    do_something(i,j)

This can be preferable to deeply-nested loops.

Also: in 2.x, use xrange() in preference to range().

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#26640

On 08/06/2012 06:18 PM, Nobody wrote:
> On Mon, 06 Aug 2012 17:52:31 +0200, Tom P wrote:
>
>> consider a nested loop algorithm -
>>
>> for i in range(100):
>>       for j in range(100):
>>           do_something(i,j)
>>
>> Now, suppose I don't want to use i = 0 and j = 0 as initial values, but
>> some other values i = N and j = M, and I want to iterate through all
>> 10,000 values in sequence - is there a neat python-like way to this?
>
> 	for i in range(N,N+100):
> 	    for j in range(M,M+100):
> 	        do_something(i,j)
>
> Or did you mean something else?

no, I meant something else ..

   j runs through range(M, 100) and then range(0,M), and i runs through 
range(N,100) and then range(0,N)

.. apologies if I didn't make that clear enough.

>
> Alternatively:
>
> 	import itertools
>
> 	for i, j in itertools.product(range(N,N+100),range(M,M+100)):
> 	    do_something(i,j)
>
> This can be preferable to deeply-nested loops.
>
> Also: in 2.x, use xrange() in preference to range().
>

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#26643

On 8/6/2012 10:14 AM Tom P said...
> On 08/06/2012 06:18 PM, Nobody wrote:
>> On Mon, 06 Aug 2012 17:52:31 +0200, Tom P wrote:
>>
>>> consider a nested loop algorithm -
>>>
>>> for i in range(100):
>>>       for j in range(100):
>>>           do_something(i,j)
>>>
>>> Now, suppose I don't want to use i = 0 and j = 0 as initial values, but
>>> some other values i = N and j = M, and I want to iterate through all
>>> 10,000 values in sequence - is there a neat python-like way to this?
>>
>>     for i in range(N,N+100):
>>         for j in range(M,M+100):
>>             do_something(i,j)
>>
>> Or did you mean something else?
>
> no, I meant something else ..
>
>    j runs through range(M, 100) and then range(0,M), and i runs through
> range(N,100) and then range(0,N)
>
> .. apologies if I didn't make that clear enough.
>

      for i in range(N,N+100):
          for j in range(M,M+100):
              do_something(i % 100 ,j % 100)

Emile

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#26678

On 08/06/2012 11:11 AM, Emile van Sebille wrote:
    <snip>
>
>       for i in range(N,N+100):
>           for j in range(M,M+100):
>               do_something(i % 100 ,j % 100)
>
> Emile

How about...

for i in range(100):
     for j in range(100):
         do_something((i + N) % 100, (j + M) % 100)

      -=- Larry -=-

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#26721

On Mon, 06 Aug 2012 21:02:33 -0700, Larry Hudson wrote:

>>       for i in range(N,N+100):
>>           for j in range(M,M+100):
>>               do_something(i % 100 ,j % 100)
>>
>> Emile
> 
> How about...
> 
> for i in range(100):
>      for j in range(100):
>          do_something((i + N) % 100, (j + M) % 100)

Both of these approaches move the modifications to the sequence into the
body of the loop. It may be preferable to iterate over the desired
sequence directly. E.g.

	for i in ((N + ii) % 100 for ii in xrange(100)):
	    for j in ((M + jj) % 100 for jj in xrange(100)):
	        do_something(i, j)

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#26746

Nobody於 2012年8月7日星期二UTC+8下午11時32分55秒寫道：
> On Mon, 06 Aug 2012 21:02:33 -0700, Larry Hudson wrote:
> 
> 
> 
> >>       for i in range(N,N+100):
> 
> >>           for j in range(M,M+100):
> 
> >>               do_something(i % 100 ,j % 100)
> 
> >>
> 
> >> Emile
> 
> > 
> 
> > How about...
> 
> > 
> 
> > for i in range(100):
> 
> >      for j in range(100):
> 
> >          do_something((i + N) % 100, (j + M) % 100)
> 
> 
> 
> Both of these approaches move the modifications to the sequence into the
> 
> body of the loop. It may be preferable to iterate over the desired
> 
> sequence directly. E.g.
> 
> 
> 
> 	for i in ((N + ii) % 100 for ii in xrange(100)):
> 
> 	    for j in ((M + jj) % 100 for jj in xrange(100)):
> 
> 	        do_something(i, j)



Nobody於 2012年8月7日星期二UTC+8下午11時32分55秒寫道：
> On Mon, 06 Aug 2012 21:02:33 -0700, Larry Hudson wrote:
> 
> 
> 
> >>       for i in range(N,N+100):
> 
> >>           for j in range(M,M+100):
> 
> >>               do_something(i % 100 ,j % 100)
> 
> >>
> 
> >> Emile
> 
> > 
> 
> > How about...
> 
> > 
> 
> > for i in range(100):
> 
> >      for j in range(100):
> 
> >          do_something((i + N) % 100, (j + M) % 100)
> 
> 
> 
> Both of these approaches move the modifications to the sequence into the
> 
> body of the loop. It may be preferable to iterate over the desired
> 
> sequence directly. E.g.
> 
> 
> 
> 	for i in ((N + ii) % 100 for ii in xrange(100)):
> 
> 	    for j in ((M + jj) % 100 for jj in xrange(100)):
> 
> 	        do_something(i, j)

This is a good example to be tuned into some example 
 such that  this kind of loops by iterators of parameters in python 
wich do not use any division at all.

But I am getting lazy recently for non-critical parts.

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#26797

Nobody於 2012年8月7日星期二UTC+8下午11時32分55秒寫道：
> On Mon, 06 Aug 2012 21:02:33 -0700, Larry Hudson wrote:
> 
> 
> 
> >>       for i in range(N,N+100):
> 
> >>           for j in range(M,M+100):
> 
> >>               do_something(i % 100 ,j % 100)
> 
> >>
> 
> >> Emile
> 
> > 
> 
> > How about...
> 
> > 
> 
> > for i in range(100):
> 
> >      for j in range(100):
> 
> >          do_something((i + N) % 100, (j + M) % 100)
> 
> 
> 
> Both of these approaches move the modifications to the sequence into the
> 
> body of the loop. It may be preferable to iterate over the desired
> 
> sequence directly. E.g.
> 
> 
> 
> 	for i in ((N + ii) % 100 for ii in xrange(100)):
> 
> 	    for j in ((M + jj) % 100 for jj in xrange(100)):
> 
> 	        do_something(i, j)

list(range(N,100))+ list(range(0,N))  //good for j

Well, this is the way!

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#26799

Nobody於 2012年8月7日星期二UTC+8下午11時32分55秒寫道：
> On Mon, 06 Aug 2012 21:02:33 -0700, Larry Hudson wrote:
> 
> 
> 
> >>       for i in range(N,N+100):
> 
> >>           for j in range(M,M+100):
> 
> >>               do_something(i % 100 ,j % 100)
> 
> >>
> 
> >> Emile
> 
> > 
> 
> > How about...
> 
> > 
> 
> > for i in range(100):
> 
> >      for j in range(100):
> 
> >          do_something((i + N) % 100, (j + M) % 100)
> 
> 
> 
> Both of these approaches move the modifications to the sequence into the
> 
> body of the loop. It may be preferable to iterate over the desired
> 
> sequence directly. E.g.
> 
> 
> 
> 	for i in ((N + ii) % 100 for ii in xrange(100)):
> 
> 	    for j in ((M + jj) % 100 for jj in xrange(100)):
> 
> 	        do_something(i, j)



Nobody於 2012年8月7日星期二UTC+8下午11時32分55秒寫道：
> On Mon, 06 Aug 2012 21:02:33 -0700, Larry Hudson wrote:
> 
> 
> 
> >>       for i in range(N,N+100):
> 
> >>           for j in range(M,M+100):
> 
> >>               do_something(i % 100 ,j % 100)
> 
> >>
> 
> >> Emile
> 
> > 
> 
> > How about...
> 
> > 
> 
> > for i in range(100):
> 
> >      for j in range(100):
> 
> >          do_something((i + N) % 100, (j + M) % 100)
> 
> 
> 
> Both of these approaches move the modifications to the sequence into the
> 
> body of the loop. It may be preferable to iterate over the desired
> 
> sequence directly. E.g.
> 
> 
> 
> 	for i in ((N + ii) % 100 for ii in xrange(100)):
> 
> 	    for j in ((M + jj) % 100 for jj in xrange(100))

list(range(N,100))+list(range(1,N)) //good for  i

I contribute my python code here to avoid any divison.

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#27059

Nobody於 2012年8月7日星期二UTC+8下午11時32分55秒寫道：
> On Mon, 06 Aug 2012 21:02:33 -0700, Larry Hudson wrote:
> 
> 
> 
> >>       for i in range(N,N+100):
> 
> >>           for j in range(M,M+100):
> 
> >>               do_something(i % 100 ,j % 100)
> 
> >>
> 
> >> Emile
> 
> > 
> 
> > How about...
> 
> > 
> 
> > for i in range(100):
> 
> >      for j in range(100):
> 
> >          do_something((i + N) % 100, (j + M) % 100)
> 
> 
> 
> Both of these approaches move the modifications to the sequence into the
> 
> body of the loop. It may be preferable to iterate over the desired
> 
> sequence directly. E.g.
> 
> 
> 
> 	for i in ((N + ii) % 100 for ii in xrange(100)):
> 
> 	    for j in ((M + jj) % 100 for jj in xrange(100)):
> 
> 	        do_something(i, j)

I'll contrubute another version by the xrange function in python.

for i in xrange(N, N+100): # not list based
    if i<100: i2=i else: i2=i-100 # not in the inner most loop
    for j in xrange(M, M+100):
        if j<100: j2=j else: j2=j-100
        do_something(i2,j2)


        

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#26645

On 2012-08-06, Tom P <werotizy@freent.dd> wrote:
> On 08/06/2012 06:18 PM, Nobody wrote:
>> On Mon, 06 Aug 2012 17:52:31 +0200, Tom P wrote:
>>
>>> consider a nested loop algorithm -
>>>
>>> for i in range(100):
>>>       for j in range(100):
>>>           do_something(i,j)
>>>
>>> Now, suppose I don't want to use i = 0 and j = 0 as initial values, but
>>> some other values i = N and j = M, and I want to iterate through all
>>> 10,000 values in sequence - is there a neat python-like way to this?
>>
>> 	for i in range(N,N+100):
>> 	    for j in range(M,M+100):
>> 	        do_something(i,j)
>>
>> Or did you mean something else?
>
> no, I meant something else ..
>
>    j runs through range(M, 100) and then range(0,M), and i runs through 
> range(N,100) and then range(0,N)

In 2.x:

    for i in range(M,100)+range(0,M):
        for j in range(N,100)+range(0,N):
            do_something(i,j)

Dunno if that still works in 3.x.  I doubt it, since I think in 3.x
range returns an iterator, not?

-- 
Grant Edwards               grant.b.edwards        Yow! I wish I was on a
                                  at               Cincinnati street corner
                              gmail.com            holding a clean dog!

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#26646

On 2012-08-06, Grant Edwards <invalid@invalid.invalid> wrote:
> On 2012-08-06, Tom P <werotizy@freent.dd> wrote:
>> On 08/06/2012 06:18 PM, Nobody wrote:
>>> On Mon, 06 Aug 2012 17:52:31 +0200, Tom P wrote:
>>>
>>>> consider a nested loop algorithm -
>>>>
>>>> for i in range(100):
>>>>       for j in range(100):
>>>>           do_something(i,j)
>>>>
>>>> Now, suppose I don't want to use i = 0 and j = 0 as initial values, but
>>>> some other values i = N and j = M, and I want to iterate through all
>>>> 10,000 values in sequence - is there a neat python-like way to this?
>>>
>>> 	for i in range(N,N+100):
>>> 	    for j in range(M,M+100):
>>> 	        do_something(i,j)
>>>
>>> Or did you mean something else?
>>
>> no, I meant something else ..
>>
>>    j runs through range(M, 100) and then range(0,M), and i runs through 
>> range(N,100) and then range(0,N)
>
> In 2.x:
>
>     for i in range(M,100)+range(0,M):
>         for j in range(N,100)+range(0,N):
>             do_something(i,j)
>
> Dunno if that still works in 3.x.  I doubt it, since I think in 3.x
> range returns an iterator, not?

Indeed it doesn't work in 3.x, but this does:

    from itertools import chain

    for i in chain(range(M,100),range(0,M)):
        for j in chain(range(N,100),range(0,N)):
            do_something(i,j)


-- 
Grant Edwards               grant.b.edwards        Yow! People humiliating
                                  at               a salami!
                              gmail.com            

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#26647

On 08/06/2012 08:29 PM, Grant Edwards wrote:
> On 2012-08-06, Grant Edwards <invalid@invalid.invalid> wrote:
>> On 2012-08-06, Tom P <werotizy@freent.dd> wrote:
>>> On 08/06/2012 06:18 PM, Nobody wrote:
>>>> On Mon, 06 Aug 2012 17:52:31 +0200, Tom P wrote:
>>>>
>>>>> consider a nested loop algorithm -
>>>>>
>>>>> for i in range(100):
>>>>>        for j in range(100):
>>>>>            do_something(i,j)
>>>>>
>>>>> Now, suppose I don't want to use i = 0 and j = 0 as initial values, but
>>>>> some other values i = N and j = M, and I want to iterate through all
>>>>> 10,000 values in sequence - is there a neat python-like way to this?
>>>>
>>>> 	for i in range(N,N+100):
>>>> 	    for j in range(M,M+100):
>>>> 	        do_something(i,j)
>>>>
>>>> Or did you mean something else?
>>>
>>> no, I meant something else ..
>>>
>>>     j runs through range(M, 100) and then range(0,M), and i runs through
>>> range(N,100) and then range(0,N)
>>
>> In 2.x:
>>
>>      for i in range(M,100)+range(0,M):
>>          for j in range(N,100)+range(0,N):
>>              do_something(i,j)
>>
>> Dunno if that still works in 3.x.  I doubt it, since I think in 3.x
>> range returns an iterator, not?
>
> Indeed it doesn't work in 3.x, but this does:
>
>      from itertools import chain
>
>      for i in chain(range(M,100),range(0,M)):
>          for j in chain(range(N,100),range(0,N)):
>              do_something(i,j)
>
>
  ah, that looks good - I guess it works in 2.x as well?

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#26649

On 2012-08-06, Tom P <werotizy@freent.dd> wrote:

>>>> no, I meant something else ..
>>>>
>>>>     j runs through range(M, 100) and then range(0,M), and i runs through
>>>> range(N,100) and then range(0,N)
>>>
>>> In 2.x:
>>>
>>>      for i in range(M,100)+range(0,M):
>>>          for j in range(N,100)+range(0,N):
>>>              do_something(i,j)
>>>
>>> Dunno if that still works in 3.x.  I doubt it, since I think in 3.x
>>> range returns an iterator, not?
>>
>> Indeed it doesn't work in 3.x, but this does:
>>
>>      from itertools import chain
>>
>>      for i in chain(range(M,100),range(0,M)):
>>          for j in chain(range(N,100),range(0,N)):
>>              do_something(i,j)
>
>   ah, that looks good - I guess it works in 2.x as well?

I don't know.  Let me test that for you...

   $ python
   Python 2.6.8 (unknown, May 18 2012, 11:56:26) 
   [GCC 4.5.3] on linux2
   Type "help", "copyright", "credits" or "license" for more information.
   >>> from itertools import chain
   >>> for i in chain(range(0,5),range(5,10)):
   ...   print i
   ... 
   0
   1
   2
   3
   4
   5
   6
   7
   8
   9
   >>> 

Yes, it works in 2.x as well.

-- 
Grant Edwards               grant.b.edwards        Yow! ... bleakness
                                  at               ... desolation ... plastic
                              gmail.com            forks ...

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#26653

On 8/6/2012 12:22 PM Grant Edwards said...
> On 2012-08-06, Tom P <werotizy@freent.dd> wrote:
<snip>
>>    ah, that looks good - I guess it works in 2.x as well?
>
> I don't know.  Let me test that for you...
>
<snip>
>
> Yes, it works in 2.x as well.
>

:)

And from the docs, all the way back to 2.3!

9.7. itertools  Functions creating iterators for efficient looping
New in version 2.3.

Emile

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