# Generator expressions are not useful for extracting matrix colums
import copy

def plus(v,w):
    return [x+y for (x,y) in zip(v,w)]

def getcol_list(M,j):
    return [m[j] for m in M]

def getcol_gen(M,j):
    return (m[j] for m in M)

def getcol_map(M,j):
    return map(lambda m: m[j], M)


# Define a matrix (list of lists)
A = [[i*j for j in range(1,6)] for i in range(1,11)]

# Extract column 2 as a list and do stuff with it: OK
print("with list:")
c = getcol_list(A,2)
print(plus([0]*10,c))
print(plus([1]*10,c))

# Again, but with generator expression - does not work
print("with generator object:")
c = getcol_gen(A,2)
print(plus([0]*10,c))
print(plus([1]*10,c))

# Again, but with map - does not work
print("with map:")
c = getcol_map(A,2)
print(plus([0]*10,c))
print(plus([1]*10,c))

# A reusable generator
class ReusableGenerator():
    def __init__(self,g): self.g = g
    def __iter__(self):   return self.g()
print("with a reusable generator:")
c = ReusableGenerator(lambda: (a[2] for a in A))
print(plus([0]*10,c))
print(plus([1]*10,c))


# Trying another reusable generator
# this does not work, cannot copy a generator object. Why not?
class ReusableGenerator2():
    def __init__(self,genexp):
        print("ReusableGenerator got a",genexp)
        self.g = genexp
    def __iter__(self):
        return copy.deepcopy(self.g) # stupid?
print("with another reusable generator attempt, it does not work:")
c = ReusableGenerator2(a[2] for a in A)
print(plus([0]*10,c))
print(plus([1]*10,c))
# Strange error message:
# TypeError: zip argument #2 must support iteration
# even though __iter__() is defined.
# Running it by hand results in a differnt error message:
# TypeError: object.__new__(generator) is not safe, use generator.__new__()