I know this should not be difficult, probably I've been too lazy to figure it out. When I execute the python script below, I get this message: <bound method Forces.Sums of <main.Forces instance at 0x7fcecfb56a28>>. What is wrong?

import numpy

class Forces:
    """ Forces between particles"""

    def __init__(self, l1, l2):
        self.l1 = l1
        self.l2 = l2
        N = len(l1)
        k = list(xrange(N*N))
        result = numpy.zeros(N*N) #No repetition N*(N-1)
    def Sums(self): 
        l1, l2 = self.l1, self.l2 
        for i in range(0, (len(l1)-1)):
            for j in range(0, (len(l2)-1)):
                result[i+j] += l1[i]*l2[j]
                k += 1
        return result

#Demo 
if __name__ == '__main__':
    list1 = [1,2,3,4,5]
    list2 = [6,7,8,9,10]
    p = Forces(list1, list2)
    print p.Sums

Thank you Griboullis. I wanted the code to look something like this:

import numpy

class Particles:
    """ Forces between particles"""

    def __init__(self, l1, l2):
        self.l1 = l1
        self.l2 = l2
        N = len(l1)
        self.result = numpy.zeros(N) #No repetition N*(N-1)
        self.Total = 0
    def First_sums(self): 
        l1, l2, ith_sums, Total = self.l1, self.l2, self.result, self.Total
        for i in range(0, (len(l1))):
            for j in range(0, len(l2)):
                ith_sums[i] += l1[i]*l2[j]
        for k in range(0, len(ith_sums)):  # We can use Total = sum(ith_sums) here
            Total += ith_sums[k]
        return Total

    def Second_sums(self):                                               
        l1, l2, ith_sums, Total = self.l1, self.l2, self.result, self.Total        
        for i in range(0, (len(l1))):
            for j in range(0, len(l2)):
                if i == j:
                    ith_sums[i] += l1[i]*l2[j]
        for k in range(0, len(ith_sums)):
            Total += ith_sums[k]
        return Total

    #def Differences(self): You can create another method to compute the difference
#Demo 
if __name__ == '__main__':
    N = 1000000 #Define N
    list1 = list2 = numpy.random.randn(N) #list = [1,2,3,4,5], Try something simple                   
    p = Particles(list1, list2)
    y = Particles(list1, list2)
    sum1 = p.First_sums()
    sum2 = y.Second_sums()
    diff = sum1 - sum2
    print "The difference between the two sums:", diff  

#Try the following, and see what happens:

    #print "The difference between the two sums:", p.First_sums(), p.Second_sums() 

With numpy, it is very mportant to vectorize the computations. I think the following code does the same thing but avoids a 10**6 * 10**6 pure python loop which takes an infinite amount of time

import numpy

class Particles:
    """ Forces between particles"""

    def __init__(self, l1, l2):
        self.l1 = l1
        self.l2 = l2
        N = len(l1)
        self.result = numpy.zeros(N) #No repetition N*(N-1)
        self.Total = 0
    def First_sums(self): 
        l1, l2, ith_sums, Total = self.l1, self.l2, self.result, self.Total
        ith_sums += l1 * numpy.sum(l2)
        Total = numpy.sum(ith_sums)
        return Total

    def Second_sums(self):                                               
        l1, l2, ith_sums, Total = self.l1, self.l2, self.result, self.Total        
        ith_sums += l1 * l2
        Total = numpy.sum(ith_sums)
        return Total

    #def Differences(self): You can create another method to compute the difference
#Demo 
if __name__ == '__main__':
    N = 1000000 #Define N
    list1 = list2 = numpy.random.randn(N) #list = [1,2,3,4,5], Try something simple                   
    p = Particles(list1, list2)
    y = Particles(list1, list2)
    sum1 = p.First_sums()
    sum2 = y.Second_sums()
    diff = sum1 - sum2
    print "The difference between the two sums:", diff

Edited 5 Months Ago by Gribouillis