Array construction from object members

[MKoool]

Hi everyone,

I am doing several operations on lists and I am wondering if python has
anything built in to get every member of several objects that are in an
array, for example, if i have a class like the following:

class myClass:
a = 0.0

And lets say I populate the "a" element in an array of objects of
myClass. If I want to retrieve all items in this and perhaps give it
to a mean function, I would need to make a loop now:

mySimpleArray = []
for i in range(0,len(myArray)):
mySimpleArray.append(myArray.a)

There must be some more efficient way to do this, can someone point me
to the right direction so that I can review some documentation and get
things a little more efficient?

thanks!

 

[Gerard Flanagan]

Hi everyone,

I am doing several operations on lists and I am wondering if python has
anything built in to get every member of several objects that are in an
array, for example, if i have a class like the following:

class myClass:
a = 0.0

And lets say I populate the "a" element in an array of objects of
myClass. If I want to retrieve all items in this and perhaps give it
to a mean function, I would need to make a loop now:

mySimpleArray = []
for i in range(0,len(myArray)):
mySimpleArray.append(myArray.a)

There must be some more efficient way to do this, can someone point me
to the right direction so that I can review some documentation and get
things a little more efficient?

thanks!

not sure if this is what you want to do, but does this help:

class myclass(object):
def __init__(self, a):
self.a = a

mylist = [ myclass('one'), myclass('two'), myclass('three'),
myclass('four') ]

alist = [ A.a for A in mylist ] #this is called a 'list comprehension'

print alist

output: ['one', 'two', 'three', 'four']

Gerard

 

[Paul McGuire]

Hi everyone,

I am doing several operations on lists and I am wondering if python has
anything built in to get every member of several objects that are in an
array,

Here's some sample code to show you how list comprehensions and generator
expressions do what you want.

-- Paul


class A(object):
def __init__(self,val):
self.a = val

# define _repr_ to make it easy to print list of A's
def __repr__(self):
return "A(%s)" % str(self.a)

Alist = [ A(i*1.5) for i in range(5) ]

print Alist

# create new list of .a attributes, print, and sum
Alist_avals = [ x.a for x in Alist ]
print Alist_avals
print sum(Alist_avals)

# if original list is much longer...
Alist = [ A(i*1.5) for i in range(500000) ]

# ... creating list comprehension will take a while...
print sum( [ x.a for x in Alist ] )

# ... instead use generator expression - avoids creation of new list
print sum( x.a for x in Alist )

 

[Steven D'Aprano]

Hi everyone,

I am doing several operations on lists and I am wondering if python has
anything built in to get every member of several objects that are in an
array, for example, if i have a class like the following:

class myClass:
a = 0.0

Did you mean for a to be a class attribute? You possibly want something
like this:

class myClass:
def __init__(self, value=0.0):
self.a = value # "a" for attribute

Then you can create new instances:

fred = myClass(2.7)
wilma = myClass() # just use the default
betty = myClass(1.3)
barney = myClass(0.9)

And lets say I populate the "a" element in an array of objects of
myClass. If I want to retrieve all items in this and perhaps give it
to a mean function, I would need to make a loop now:

mySimpleArray = []
for i in range(0,len(myArray)):
mySimpleArray.append(myArray.a)

There must be some more efficient way to do this, can someone point me
to the right direction so that I can review some documentation and get
things a little more efficient?

myArray = [fred, wilma, betty, barney]

mySimpleArray = []
for person in myArray:
mySimpleArray.append(person.a)


Or try this:

mySimpleArray = [person.a for person in myArray]

Or this:

def mean_value(*people):
"""Takes a list of people and returns the mean of their attributes"""
total = 0.0
for person in people:
total += person.a
return total/len(people)

mean_value(fred, betty, barney, wilma)

That last version is not recommended, because it requires a separate
function for everything you want to calculate. For instance, if you add a
second attribute "height" to myClass, and want to work out the mean
height, you would need a second function to do it.