Hello!
I am currently trying to port a C++ code to python, and I think I am stuck
because of the very different behavior of STL iterators vs python
iterators. What I need to do is a simple arithmetic operations on objects
I don't know. In C++, the method doing that was a template, and all that
was required is that the template class has an iterator conforming to the
STL forward iterator definition. Then, the class would look like:
template <class H>
class MyClass
{
public:
MyClass(H& o1, H& o2) : object1(o1), object2(o2) {}
void compute();
private:
H& object1;
H& object2;
};
template <class H>
void MyClass::comput e()
{
typedef typename H::iterator I;
I o1_begin = object1.begin() ;
I o2_begin = object2.begin() ;
I o1_end = object1.end();
for(I io1 = o1_begin, io2 = o2_begin; io1 != o1_end; ++io1, ++io2)
{
// Do something with *io1 and *io2, for instance:
// *io1 += *io2;
}
}
This is all nice: any object having a forward iterator works in there.
Then I discovered python and wanted to use all its goodies. I thought it
would be easy to do the same thing but I can't: the iterator mechanism is
read-only, right? So it does no make sense to write:
io1 = iter(object1)
io2 = iter(object2)
try:
while 1:
io1.next() += io2.next()
except StopIteration:
pass
That won't work:
SyntaxError: can't assign to function call
Here is my question: how could I do that and retain enough generallity?
Thanks!
Pierre 10 1616
I think you are going to need to provide some python minimal code as an
example of what is not working for you.
-Chris
On Wed, Jul 26, 2006 at 11:39:36AM -0400, Pierre Thibault wrote:
Hello!
I am currently trying to port a C++ code to python, and I think I am stuck
because of the very different behavior of STL iterators vs python
iterators. What I need to do is a simple arithmetic operations on objects
I don't know. In C++, the method doing that was a template, and all that
was required is that the template class has an iterator conforming to the
STL forward iterator definition. Then, the class would look like:
template <class H>
class MyClass
{
public:
MyClass(H& o1, H& o2) : object1(o1), object2(o2) {}
void compute();
private:
H& object1;
H& object2;
};
template <class H>
void MyClass::comput e()
{
typedef typename H::iterator I;
I o1_begin = object1.begin() ;
I o2_begin = object2.begin() ;
I o1_end = object1.end();
for(I io1 = o1_begin, io2 = o2_begin; io1 != o1_end; ++io1, ++io2)
{
// Do something with *io1 and *io2, for instance:
// *io1 += *io2;
}
}
This is all nice: any object having a forward iterator works in there.
Then I discovered python and wanted to use all its goodies. I thought it
would be easy to do the same thing but I can't: the iterator mechanism is
read-only, right? So it does no make sense to write:
io1 = iter(object1)
io2 = iter(object2)
try:
while 1:
io1.next() += io2.next()
except StopIteration:
pass
That won't work:
SyntaxError: can't assign to function call
Here is my question: how could I do that and retain enough generallity?
Thanks!
Pierre
-- http://mail.python.org/mailman/listinfo/python-list
Pierre Thibault wrote:
Hello!
I am currently trying to port a C++ code to python, and I think I am stuck
because of the very different behavior of STL iterators vs python
iterators. What I need to do is a simple arithmetic operations on objects
I don't know. In C++, the method doing that was a template, and all that
was required is that the template class has an iterator conforming to the
STL forward iterator definition. Then, the class would look like:
template <class H>
class MyClass
{
public:
MyClass(H& o1, H& o2) : object1(o1), object2(o2) {}
void compute();
private:
H& object1;
H& object2;
};
template <class H>
void MyClass::comput e()
{
typedef typename H::iterator I;
I o1_begin = object1.begin() ;
I o2_begin = object2.begin() ;
I o1_end = object1.end();
for(I io1 = o1_begin, io2 = o2_begin; io1 != o1_end; ++io1, ++io2)
{
// Do something with *io1 and *io2, for instance:
// *io1 += *io2;
}
}
This is all nice: any object having a forward iterator works in there.
Then I discovered python and wanted to use all its goodies. I thought it
would be easy to do the same thing but I can't: the iterator mechanism is
read-only, right? So it does no make sense to write:
io1 = iter(object1)
io2 = iter(object2)
try:
while 1:
io1.next() += io2.next()
except StopIteration:
pass
That won't work:
SyntaxError: can't assign to function call
Here is my question: how could I do that and retain enough generallity?
You need a temporary variable (out in the example below):
>>accus = [[] for i in range(3)] ins = ("abc" for i in range(3)) outs = iter(accus) while 1:
.... out = outs.next()
.... out += ins.next()
....
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
StopIteration
In idiomatic Python that becomes
>>accus = [[] for i in range(3)] ins = ("abc" for i in range(3)) outs = iter(accus) from itertools import izip for out, in_ in izip(outs, ins):
.... out += in_
....
>>accus
[['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c']]
Peter
On Wed, 26 Jul 2006 18:54:39 +0200, Peter Otten wrote:
Pierre Thibault wrote:
>Hello!
I am currently trying to port a C++ code to python, and I think I am stuck because of the very different behavior of STL iterators vs python iterators. What I need to do is a simple arithmetic operations on objects I don't know. In C++, the method doing that was a template, and all that was required is that the template class has an iterator conforming to the STL forward iterator definition. Then, the class would look like:
template <class H> class MyClass { public:
MyClass(H& o1, H& o2) : object1(o1), object2(o2) {}
void compute();
private: H& object1; H& object2;
};
template <class H> void MyClass::comput e() { typedef typename H::iterator I;
I o1_begin = object1.begin() ; I o2_begin = object2.begin() ; I o1_end = object1.end();
for(I io1 = o1_begin, io2 = o2_begin; io1 != o1_end; ++io1, ++io2) { // Do something with *io1 and *io2, for instance: // *io1 += *io2; } }
This is all nice: any object having a forward iterator works in there.
Then I discovered python and wanted to use all its goodies. I thought it would be easy to do the same thing but I can't: the iterator mechanism is read-only, right? So it does no make sense to write:
io1 = iter(object1) io2 = iter(object2)
try: while 1: io1.next() += io2.next() except StopIteration: pass
That won't work: SyntaxError: can't assign to function call
Here is my question: how could I do that and retain enough generallity?
You need a temporary variable (out in the example below):
>>>accus = [[] for i in range(3)] ins = ("abc" for i in range(3)) outs = iter(accus) while 1:
... out = outs.next()
... out += ins.next()
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
StopIteration
In idiomatic Python that becomes
>>>accus = [[] for i in range(3)] ins = ("abc" for i in range(3)) outs = iter(accus) from itertools import izip for out, in_ in izip(outs, ins):
... out += in_
...
>>>accus
[['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c']]
Peter
Hum, this example seems like a special case not really appropriate for my
needs. Let me make my problem a little more precise. The objects I'll want
to iterate through will always contain some floats. Very often, I guess
they will be numpy.ndarray instances, but they can be something else as
well. For instance, I will have to deal with arrays having internal
symmetries (crystallograph ic datasets). The most efficient thing to do
with those is save only the unique values and keep track of the symmetry
operations needed to extract other values.
A 1d examples: I have a class which represents a unit cell
which has the additional mirror symmetry in the middle of the cell. Say
C is an instance of this class representing data on a 50-long array. Only
25 datum will be stored in C, and the class takes care of giving the value
of C[0] if C[49] is asked, C[1] for C[48], and so on. Since crystals are
periodic, the translational symmetry can also be managed (C[-1] == C[49]
== C[0], C[-2] == C[48] == C[1]...). In any case, the user of this object
need not know how the data is stored: for him, C is just a funny
array-like object defined from -infinity to +infinity on a 1-D lattice.
Now, I want to do simple math operations on the data in C. Doing a loop
from 0 to 49 would loop twice through the actual data. In this
context, an iterator is perfect since it can take care internally of going
through the data only once, so it would be great to access _AND MODIFY_
data over which I iterate.
I now realize I don't know how to do that with numpy.ndarray either. It
looks like the only way I can modify the content of an array is by using
the [] notation (that is, in random access). For instance, the
following will not change the state of a:
import numpy
a = numpy.array([[1.,2.,3.],[4.,5.,6.],[7.,8.,9.]])
for c in a.flat
c += 2.
Of course, I know that a += .2 would be alright, but this is not general
enough for more complicated classes.
So I guess my problem is worst than I thought.
Thanks anyway for your help!
Pierre
Pierre Thibault wrote:
Hum, this example seems like a special case not really appropriate for my
needs. Let me make my problem a little more precise. The objects I'll want
to iterate through will always contain some floats. Very often, I guess
they will be numpy.ndarray instances, but they can be something else as
well. For instance, I will have to deal with arrays having internal
symmetries (crystallograph ic datasets). The most efficient thing to do
with those is save only the unique values and keep track of the symmetry
operations needed to extract other values.
A 1d examples: I have a class which represents a unit cell
which has the additional mirror symmetry in the middle of the cell. Say
C is an instance of this class representing data on a 50-long array. Only
25 datum will be stored in C, and the class takes care of giving the value
of C[0] if C[49] is asked, C[1] for C[48], and so on. Since crystals are
periodic, the translational symmetry can also be managed (C[-1] == C[49]
== C[0], C[-2] == C[48] == C[1]...). In any case, the user of this object
need not know how the data is stored: for him, C is just a funny
array-like object defined from -infinity to +infinity on a 1-D lattice.
Now, I want to do simple math operations on the data in C. Doing a loop
from 0 to 49 would loop twice through the actual data. In this
context, an iterator is perfect since it can take care internally of going
through the data only once, so it would be great to access _AND MODIFY_
data over which I iterate.
That would not only be nice but crucial to the integrity of your data. You'd
have to explicitly forbid operations on individual cells as applying an
operation to a single cell would change two or more cells, depending of the
kind of symmetry.
I now realize I don't know how to do that with numpy.ndarray either. It
looks like the only way I can modify the content of an array is by using
the [] notation (that is, in random access). For instance, the
following will not change the state of a:
import numpy
a = numpy.array([[1.,2.,3.],[4.,5.,6.],[7.,8.,9.]])
for c in a.flat
c += 2.
Of course, I know that a += .2 would be alright, but this is not general
enough for more complicated classes.
So I guess my problem is worst than I thought.
Indeed, but I don't think it would be easier in C++...
Peter
Pierre Thibault, some people here surely know enough Python (and C++)
to solve your problem, but often the problem is understanding the
problem. I have understood your problem just partially, so the
following are just ideas.
First of all I suggest you to use a normal Python list to keep the
data, and later modify the code to use an array if it is too much slow
or if you need arrays to do some special things, like plotting, etc.
Developing using normal Python lists is a bit simpler.
With python you can redefine the __getitem__ and __setitem__ of an
object according to its crystal symmetries. Then you can define some
method like iterkeys that yields (with yield) the only indexes that the
symmetry allows to be set.
Then you can iterate on those indexes and update the values in the list
attribute.
Bye,
bearophile
Pierre Thibault wrote:
Hello!
I am currently trying to port a C++ code to python, and I think I am stuck
because of the very different behavior of STL iterators vs python
iterators. What I need to do is a simple arithmetic operations on objects
I don't know. In C++, the method doing that was a template, and all that
was required is that the template class has an iterator conforming to the
STL forward iterator definition. Then, the class would look like:
<SNIP>
Then I discovered python and wanted to use all its goodies. I thought it
would be easy to do the same thing but I can't: the iterator mechanism is
read-only, right? So it does no make sense to write:
io1 = iter(object1)
io2 = iter(object2)
try:
while 1:
io1.next() += io2.next()
except StopIteration:
pass
That won't work:
SyntaxError: can't assign to function call
Here is my question: how could I do that and retain enough generallity?
Thanks!
Pierre
Pierre,
You should be able to write
io1.next().para m += io2.next().para m
If iter(object1) and iter(object2) both return classes or instances
with the appropriate parameter.
Here is what I was thinking of:
class ParamHolder(obj ect):
def __init__(self, n):
self.param = n
class C1(object):
def __init__(self,m ):
self.value = [ParamHolder(n) for n in range(m)]
def __getitem__(sel f, p):
return self.value[p]
obj1 = C1(5)
obj2 = C1(5)
io1 = iter(obj1)
io2 = iter(obj2)
print "obj1 pre loop",[r.param for r in obj1.value]
try:
while 1:
io1.next().para m += io2.next().para m
except StopIteration:
pass
print "obj1 post loop",[r.param for r in obj1.value]
- Paddy.
Paddy wrote:
Pierre Thibault wrote:
Hello!
I am currently trying to port a C++ code to python, and I think I am stuck
because of the very different behavior of STL iterators vs python
iterators. What I need to do is a simple arithmetic operations on objects
I don't know. In C++, the method doing that was a template, and all that
was required is that the template class has an iterator conforming to the
STL forward iterator definition. Then, the class would look like:
<SNIP>
Then I discovered python and wanted to use all its goodies. I thought it
would be easy to do the same thing but I can't: the iterator mechanism is
read-only, right? So it does no make sense to write:
io1 = iter(object1)
io2 = iter(object2)
try:
while 1:
io1.next() += io2.next()
except StopIteration:
pass
That won't work:
SyntaxError: can't assign to function call
Here is my question: how could I do that and retain enough generallity?
Thanks!
Pierre
Pierre,
You should be able to write
io1.next().para m += io2.next().para m
If iter(object1) and iter(object2) both return classes or instances
with the appropriate parameter.
Here is what I was thinking of:
class ParamHolder(obj ect):
def __init__(self, n):
self.param = n
class C1(object):
def __init__(self,m ):
self.value = [ParamHolder(n) for n in range(m)]
def __getitem__(sel f, p):
return self.value[p]
obj1 = C1(5)
obj2 = C1(5)
io1 = iter(obj1)
io2 = iter(obj2)
print "obj1 pre loop",[r.param for r in obj1.value]
try:
while 1:
io1.next().para m += io2.next().para m
except StopIteration:
pass
print "obj1 post loop",[r.param for r in obj1.value]
- Paddy.
I don't like the try/except code and would write something like the
following:
>>obj1 = C1(5) obj2 = C1(5) from itertools import izip for x,y in izip(obj1, obj2):
.... x.param += y.param
....
>>print "obj1 post for loop",[r.param for r in obj1.value]
obj1 post for loop [0, 2, 4, 6, 8]
>>>
- Paddy.
On Wed, 26 Jul 2006 16:11:48 -0700, Paddy wrote:
>
Paddy wrote:
>Pierre Thibault wrote:
Hello!
I am currently trying to port a C++ code to python, and I think I am stuck
because of the very different behavior of STL iterators vs python
iterators. What I need to do is a simple arithmetic operations on objects
I don't know. In C++, the method doing that was a template, and all that
was required is that the template class has an iterator conforming to the
STL forward iterator definition. Then, the class would look like:
<SNIP>
Then I discovered python and wanted to use all its goodies. I thought it
would be easy to do the same thing but I can't: the iterator mechanism is
read-only, right? So it does no make sense to write:
io1 = iter(object1)
io2 = iter(object2)
try:
while 1:
io1.next() += io2.next()
except StopIteration:
pass
That won't work:
SyntaxError: can't assign to function call
Here is my question: how could I do that and retain enough generallity?
Thanks!
Pierre
Pierre, You should be able to write io1.next().para m += io2.next().para m If iter(object1) and iter(object2) both return classes or instances with the appropriate parameter. Here is what I was thinking of:
class ParamHolder(obj ect): def __init__(self, n): self.param = n
class C1(object): def __init__(self,m ): self.value = [ParamHolder(n) for n in range(m)] def __getitem__(sel f, p): return self.value[p]
obj1 = C1(5) obj2 = C1(5)
io1 = iter(obj1) io2 = iter(obj2)
print "obj1 pre loop",[r.param for r in obj1.value]
try: while 1: io1.next().para m += io2.next().para m except StopIteration: pass
print "obj1 post loop",[r.param for r in obj1.value]
- Paddy.
I don't like the try/except code and would write something like the
following:
>>>obj1 = C1(5) obj2 = C1(5) from itertools import izip for x,y in izip(obj1, obj2):
... x.param += y.param
...
>>>print "obj1 post for loop",[r.param for r in obj1.value]
obj1 post for loop [0, 2, 4, 6, 8]
>>>>
- Paddy.
Thanks Paddy,
This looks like the closest thing to what I wanted, though the need for
this "param" makes it not very general to my taste. Besides, this method
would not work with ndarrays anyways.
Thanks again for taking the time to answer,
Pierre
On Wed, 26 Jul 2006 20:59:12 +0200, Peter Otten wrote:
Pierre Thibault wrote:
>Hum, this example seems like a special case not really appropriate for my needs. Let me make my problem a little more precise. The objects I'll want to iterate through will always contain some floats. Very often, I guess they will be numpy.ndarray instances, but they can be something else as well. For instance, I will have to deal with arrays having internal symmetries (crystallograph ic datasets). The most efficient thing to do with those is save only the unique values and keep track of the symmetry operations needed to extract other values.
A 1d examples: I have a class which represents a unit cell which has the additional mirror symmetry in the middle of the cell. Say C is an instance of this class representing data on a 50-long array. Only 25 datum will be stored in C, and the class takes care of giving the value of C[0] if C[49] is asked, C[1] for C[48], and so on. Since crystals are periodic, the translational symmetry can also be managed (C[-1] == C[49] == C[0], C[-2] == C[48] == C[1]...). In any case, the user of this object need not know how the data is stored: for him, C is just a funny array-like object defined from -infinity to +infinity on a 1-D lattice.
Now, I want to do simple math operations on the data in C. Doing a loop from 0 to 49 would loop twice through the actual data. In this context, an iterator is perfect since it can take care internally of going through the data only once, so it would be great to access _AND MODIFY_ data over which I iterate.
That would not only be nice but crucial to the integrity of your data. You'd
have to explicitly forbid operations on individual cells as applying an
operation to a single cell would change two or more cells, depending of the
kind of symmetry.
>I now realize I don't know how to do that with numpy.ndarray either. It looks like the only way I can modify the content of an array is by using the [] notation (that is, in random access). For instance, the following will not change the state of a:
import numpy a = numpy.array([[1.,2.,3.],[4.,5.,6.],[7.,8.,9.]])
for c in a.flat c += 2.
Of course, I know that a += .2 would be alright, but this is not general enough for more complicated classes.
So I guess my problem is worst than I thought.
Indeed, but I don't think it would be easier in C++...
Peter
Well, no, C++ is better in this case because the thing you iterate over is
a generalized pointer, which can be dereferenced to get access to the
memory. So in other word, iterator.next() (in python) is a copy, while
*iterator (in C++) is a reference.
Thanks anyway for your comments!
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I have .net C# application in which I am extracting data from word file and save it in database particularly. To store word all data as it is I am converting the whole word file firstly in HTML and then checking html paragraph one by one.
At the time of converting from word file to html my equations which are in the word document file was convert into image.
Globals.ThisAddIn.Application.ActiveDocument.Select();...
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by: TSSRALBI |
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Hello
I'm a network technician in training and I need your help.
I am currently learning how to create and manage the different types of VPNs and I have a question about LAN-to-LAN VPNs.
The last exercise I practiced was to create a LAN-to-LAN VPN between two Pfsense firewalls, by using IPSEC protocols.
I succeeded, with both firewalls in the same network. But I'm wondering if it's possible to do the same thing, with 2 Pfsense firewalls...
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by: adsilva |
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A Windows Forms form does not have the event Unload, like VB6. What one acts like?
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by: 6302768590 |
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Hai team
i want code for transfer the data from one system to another through IP address by using C# our system has to for every 5mins then we have to update the data what the data is updated we have to send another system
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by: bsmnconsultancy |
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In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence can significantly impact your brand's success. BSMN Consultancy, a leader in Website Development in Toronto offers valuable insights into creating effective websites that not only look great but also perform exceptionally well. In this comprehensive...
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