concatenate numpy arrays that are class instance attributes in python

Question

I am attempting to use a class that strings together several instances of another class as a numpy array of objects. I want to be able to concatenate attributes of the instances that are contained in the numpy array. I figured out a sloppy way to do it with a bunch of for loops, but I think there must be a more elegant, pythonic way of doing this. The following code does what I want, but I want to know if there is a cleaner way to do it:

import numpy as np


class MyClass(object):

    def __init__(self):
        self.a = 37.
        self.arr = np.arange(5)


class MyClasses(object):

    def __init__(self):
        self.N = 5 
        # number of MyClass instances to become attributes of this
    # class 

    def make_subclas_arrays(self):
        self.my_class_inst = np.empty(shape=self.N, dtype="object") 
        for i in range(self.N):
            self.my_class_inst[i] = MyClass()

    def concatenate_attributes(self):
        self.a = np.zeros(self.N)
        self.arr = np.zeros(self.N * self.my_class_inst[0].arr.size)
        for i in range(self.N):
            self.a[i] = self.my_class_inst[i].a
            slice_start = i * self.my_class_inst[i].arr.size
            slice_end = (i + 1.) * self.my_class_inst[i].arr.size
            self.arr[slice_start:slice_end] = ( 
        self.my_class_inst[i].arr )

my_inst = MyClasses()
my_inst.make_subclas_arrays()
my_inst.concatenate_attributes()

Edit: Based on the response from HYRY, here is what the methods look like now:

def make_subclass_arrays(self):
    self.my_class_inst = np.array([MyClass() for i in range(self.N)])

def concatenate_attributes(self):
    self.a = np.hstack([i.a for i in self.my_class_inst])
    self.arr = np.hstack([i.arr for i in self.my_class_inst])

This change cut the necessary number of lines of code in half. — Chad
– Chad, Commented Feb 11, 2012 at 16:13

HYRY · Accepted Answer · 2012-02-10 05:01:43Z

1

you can use numpy.hstack() to concatenate arrays:

def concatenate_attributes(self):
    self.a = np.hstack([o.a for o in self.my_class_inst])
    self.arr = np.hstack([o.arr for o in self.my_class_inst])

1 Comment

Chad Over a year ago

This is beautiful! This greatly improves the pythonicity of my code! Thanks.

Juha · Accepted Answer · 2012-02-12 17:11:43Z

For the latter function I would recommend this:

init = []
ContainerClass.arr = np.array([init + Array(myclass.arr) for myclass in self.my_class_inst])

typecast numpy array to normal array, catenate and typecast it back. Assuming now that you have simple 1D arrays. I don't remember by heart if numpy array has catenation function. You can use that instead of '+' sign and save the trouble of typecasting.

For the first you have the simplest form I can think of, although I usually use normal arrays instead of numpy ones for objects.

If you want to be really clever you can create an __add__ function for both of the classes. Then you can use '+' sign to add classes. a + b calls a.__add__(b). Now you would have to create functions that have following properties

MyClass + MyClass returns new MyClasses instance with a and b inside
MyClasses + MyClass adds MyClass to MyClasses in a way you want

Now if a,b,c,d are myClass instances, a+b+c+d should return MyClasses -class which contains MyClass instances a,b,c and d and their combined arrays. This would be the pythonic way, although its a bit too complicated in my taste.

edit:

Ok, sorry my bad. I did not have python when I wrote the code. This is the correct version:

init = []
my_inst.arr = np.array([init + list(myclass.arr.flat) for myclass in my_inst.my_class_inst]).flatten()

This is what I meant with the __add__ (and the pythonic way... regadless of its complicatedness):

import numpy as np

class MyClass(object):

    def __init__(self):
        self.a = 37.
        self.arr = np.arange(5)

    def __add__(self, classToAdd):
        a = MyClasses() + self + classToAdd
        return a


class MyClasses(object):

    def __init__(self):
        self.N = 0
        self.my_class_inst = np.array([])
        self.a = np.array([])
        self.arr = np.array([])

    def __add__(self, singleClass):
        self.my_class_inst = np.hstack([self.my_class_inst, singleClass])
        self.a = np.hstack([self.a, singleClass.a])
        self.arr = np.hstack([self.arr, singleClass.arr])
        self.N = self.my_class_inst.shape[0]
        return self

#add_test = MyClass() + MyClass()
add_test = np.sum([MyClass() for i in range(5)])

print add_test.a, add_test.arr, add_test.N
print add_test.__class__, add_test.my_class_inst[0].__class__

Can you point to a good reference for what ContainerClass is? I am having trouble understanding what it can do for my purpose.
hups, sorry. ContainerClass is supposed to be instance of MyClasses (my_inst in your example). Also there was also other dimension problems... now fixed.

Collectives™ on Stack Overflow

concatenate numpy arrays that are class instance attributes in python

2 Answers 2

See Also

1 Comment

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

See Also

1 Comment

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related