Complex matlab-like data structure in python (numpy/scipy)

Question

I have data currently structured as following in Matlab

item{i}.attribute1(2,j)

Where item is a cell from i = 1 .. n each containing the data structure of multiple attributes each a matrix of size 2,j where j = 1 .. m. The number of attributes is not fixed.

I have to translate this data structure to python, but I am new to numpy and python lists. What is the best way of structuring this data in python with numpy/scipy?

Thanks.

Answer 1

I've often seen the following conversion approaches:

matlab array -> python numpy array

matlab cell array -> python list

matlab structure -> python dict

So in your case that would correspond to a python list containing dicts, which themselves contain numpy arrays as entries

item[i]['attribute1'][2,j]

Note

Don't forget the 0-indexing in python!

[Update]

Additional: Use of classes

Further to the simple conversion given above, you could also define a dummy class, e.g.

class structtype():
    pass

This allows the following type of usage:

>> s1 = structtype()
>> print s1.a
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-40-7734865fddd4> in <module>()
----> 1 print s1.a
AttributeError: structtype instance has no attribute 'a'
>> s1.a=10
>> print s1.a
10

Your example in this case becomes, e.g.

>> item = [ structtype() for i in range(10)]
>> item[9].a = numpy.array([1,2,3])
>> item[9].a[1]
2

Answer 2

A simple version of the answer by @dbouz , using the idea by @jmetz

class structtype():
    def __init__(self,**kwargs):
        self.Set(**kwargs)
    def Set(self,**kwargs):
        self.__dict__.update(kwargs)
    def SetAttr(self,lab,val):
        self.__dict__[lab] = val

then you can do

myst = structtype(a=1,b=2,c=3)

or

myst = structtype()
myst.Set(a=1,b=2,c=3)

and still do

myst.d = 4 # here, myst.a=1, myst.b=2, myst.c=3, myst.d=4

or even

myst = structtype(a=1,b=2,c=3)
lab = 'a'
myst.SetAttr(lab,10) # a=10,b=2,c=3 ... equivalent to myst.(lab)=10 in MATLAB

and you get exactly what you'd expect in matlab for myst=struct('a',1,'b',2,'c',3).

The equivalent of a cell of structs would be a list of structtype

mystarr = [ structtype(a=1,b=2) for n in range(10) ]

which would give you

mystarr[0].a # == 1
mystarr[0].b # == 2