Looping over lists

[Tommy Grav]

I have a list:

a = [1., 2., 3., 4., 5.]

I want to loop over a and then
loop over the elements in a
that is to the right of the current
element of the first loop

In C this would be equivalent to:

for(i = 0; i < n; i++) {
for(j=i+1; j < n; j++) {
print a[i], a[j]

and should yield:
1. 2.
1. 3.
1. 4.
1. 5.
2. 3.
2. 4.
2. 5.
3. 4.
3. 5.
4. 5.

Can anyone help me with the right approach for this
in python?

Cheers
Tommy

[Alex Martelli]

Re: Looping over lists

Tommy Grav <tgrav@mac.comw rote:

I have a list:
>
a = [1., 2., 3., 4., 5.]
>
I want to loop over a and then
loop over the elements in a
that is to the right of the current
element of the first loop
>
In C this would be equivalent to:
>
for(i = 0; i < n; i++) {
for(j=i+1; j < n; j++) {
print a[i], a[j]
>
and should yield:
1. 2.
1. 3.
1. 4.
1. 5.
2. 3.
2. 4.
2. 5.
3. 4.
3. 5.
4. 5.
>
Can anyone help me with the right approach for this
in python?

Closes to the C++ code would be:

for i in range(n):
for j in range(i+1, n):
print a[i], a[j]


Alex

[Peter Otten]

Re: Looping over lists

Tommy Grav wrote:

I have a list:
>
a = [1., 2., 3., 4., 5.]
>
I want to loop over a and then
loop over the elements in a
that is to the right of the current
element of the first loop
>
In C this would be equivalent to:
>
for(i = 0; i < n; i++) {
for(j=i+1; j < n; j++) {
print a[i], a[j]
>
and should yield:
1. 2.
1. 3.
1. 4.
1. 5.
2. 3.
2. 4.
2. 5.
3. 4.
3. 5.
4. 5.
>
Can anyone help me with the right approach for this
in python?

Two more options:

def pop_combine(ite ms):
items = list(items)
while items:
a = items.pop(0)
for b in items:
print a, b

def enumerate_combi ne(items):
for i, a in enumerate(items ):
for b in items[i+1:]:
print a, b

Peter

[Paul Rubin]

Re: Looping over lists

Tommy Grav <tgrav@mac.comw rites:

In C this would be equivalent to:
for(i = 0; i < n; i++) {
for(j=i+1; j < n; j++) {
print a[i], a[j]

for i in xrange(n):
for j in xrange(i+1, n):
print a[i], a[j]

[kaens]

Re: Looping over lists

I think the for i in range() is more readable (Maybe because I'm
coming from a c-style syntax language background) - but what would
the benefits of using enumerate be (other that being more . . .
pythonesque?)

On 5/5/07, Dennis Lee Bieber <wlfraed@ix.net com.comwrote:

On Fri, 4 May 2007 19:26:17 -0700, aleax@mac.com (Alex Martelli)
declaimed the following in comp.lang.pytho n:
>

for i in range(n):
for j in range(i+1, n):
print a[i], a[j]

Ah, but wouldn't the cleaner Python be something like
>
>

>a = [1, 2, 3, 4, 5, 3, 6] #just to confuse matters
>for pos, val in enumerate(a):

... for v2 in a[pos+1:]:
... print val, v2
...
1 2
1 3
1 4
1 5
1 3
1 6
2 3
2 4
2 5
2 3
2 6
3 4
3 5
3 3
3 6
4 5
4 3
4 6
5 3
5 6
3 6

>>

--
Wulfraed Dennis Lee Bieber KD6MOG
wlfraed@ix.netc om.com wulfraed@bestia ria.com

HTTP://wlfraed.home.netcom.com/

(Bestiaria Support Staff: web-asst@bestiaria. com)

HTTP://www.bestiaria.com/

--

http://mail.python.org/mailman/listinfo/python-list

>

[Dustan]

Re: Looping over lists

On May 5, 3:15 am, kaens <apatheticagnos ...@gmail.comwr ote:

I think the for i in range() is more readable (Maybe because I'm
coming from a c-style syntax language background) - but what would
the benefits of using enumerate be (other that being more . . .
pythonesque?)

It doesn't create a whole new list just for iterating.

On 5/5/07, Dennis Lee Bieber <wlfr...@ix.net com.comwrote:
>

On Fri, 4 May 2007 19:26:17 -0700, a...@mac.com (Alex Martelli)
declaimed the following in comp.lang.pytho n:

>

for i in range(n):
for j in range(i+1, n):
print a[i], a[j]

>

Ah, but wouldn't the cleaner Python be something like

>

>>a = [1, 2, 3, 4, 5, 3, 6] #just to confuse matters
>>for pos, val in enumerate(a):

... for v2 in a[pos+1:]:
... print val, v2
...
1 2
1 3
1 4
1 5
1 3
1 6
2 3
2 4
2 5
2 3
2 6
3 4
3 5
3 3
3 6
4 5
4 3
4 6
5 3
5 6
3 6

>

--
Wulfraed Dennis Lee Bieber KD6MOG
wlfr...@ix.netc om.com wulfr...@bestia ria.com

HTTP://wlfraed.home.netcom.com/

(Bestiaria Support Staff: web-a...@bestiaria. com)

HTTP://www.bestiaria.com/

--
http://mail.python.org/mailman/listinfo/python-list

[Paul Rubin]

Re: Looping over lists

Dustan <DustanGroups@g mail.comwrites:

I think the for i in range() is more readable (Maybe because I'm
coming from a c-style syntax language background) - but what would
the benefits of using enumerate be (other that being more . . .
pythonesque?)

>
It doesn't create a whole new list just for iterating.

That's what xrange is for.

[Alex Martelli]

Re: Looping over lists

Dustan <DustanGroups@g mail.comwrote:

On May 5, 3:15 am, kaens <apatheticagnos ...@gmail.comwr ote:

I think the for i in range() is more readable (Maybe because I'm
coming from a c-style syntax language background) - but what would
the benefits of using enumerate be (other that being more . . .
pythonesque?)

>
It doesn't create a whole new list just for iterating.

As the example list was of length 5, that's not all that important in
this case. In cases where it _is_ crucial, you can use xrange.

The performance of the various ways of looping is substantially the
same:

$ python -mtimeit -s'n=5; a=n*[23]' 'for i in range(n): x=a[i]'
1000000 loops, best of 3: 1.4 usec per loop
$ python -mtimeit -s'n=5; a=n*[23]' 'for i in xrange(n): x=a[i]'
1000000 loops, best of 3: 1.18 usec per loop
$ python -mtimeit -s'n=5; a=n*[23]' 'for i,v in enumerate(a): x=v'
1000000 loops, best of 3: 1.49 usec per loop
$

Here, xrange is minutely faster and enumerate slower, but each speed
difference "in the noise". Focusing on clarity is thus well warranted.

for i in range(n):
for j in range(i+1, n):
print a[i], a[j]

Ah, but wouldn't the cleaner Python be something like

>a = [1, 2, 3, 4, 5, 3, 6] #just to confuse matters
>for pos, val in enumerate(a):
... for v2 in a[pos+1:]:
... print val, v2

This "breaks symmetry", by using enumerate in the outer loop and a slice
in the inner loop; the symmetrical construction of using range in both
loops is a big conceptual/clarity win -- the reader of the code needs to
"grasp" one fewer concept (locally). Using xrange in both loops would
be just as good from this POV.


Alex

[Gabriel Genellina]

Re: Looping over lists

En Sat, 05 May 2007 05:15:32 -0300, kaens <apatheticagnos tic@gmail.com>
escribió:

I think the for i in range() is more readable (Maybe because I'm
coming from a c-style syntax language background) - but what would
the benefits of using enumerate be (other that being more . . .
pythonesque?)

If you want to iterate over a sequence and process each item, you could do
this (as one would do in C):

for i in range(len(value s)):
dosomethingwith (values[i])

Problems: values[i] gets translated into values.__getite m__(i), and that
requires a lookup for the __getitem__ method and a function call; and you
don't actually need the index i, but it's created anyway.
In this case the best (and fastest) way in Python is:

for item in values:
dosomethingwith (item)

No spurious variable, no method lookup, no aditional function call. But
what if you need *also* the index? On earlier Python versions one had to
go back to the first method; enumerate() was made just for this case:

for i, item in enumerate(value s):
dosomethingwith (item, i)

You get all the advantages of the iterator approach plus the index
available.

--
Gabriel Genellina