Connecting an element of array to another Component's input - openmdao

Question

I am trying to connect the last element of an output of one component to the input of another component. An example is shown below:
import numpy as np
from openmdao.api import Component, Problem, Group
class C1(Component):
def __init__(self):
super(C1, self).__init__()
self.add_param('fin', val=1.0)
self.add_output('arr', val=np.zeros(5))
def solve_nonlinear(self, params, unknowns, resids):
fin = params['fin']
unknowns['arr'] = np.array([2*fin])
class C2(Component):
def __init__(self):
super(C2, self).__init__()
self.add_param('flt', val=0.0)
self.add_output('fout', val=0.0)
def solve_nonlinear(self, params, unknowns, resids):
flt = params['flt']
unknowns['fout'] = 2*flt
class A(Group):
def __init__(self):
super(A, self).__init__()
self.add('c1', C1())
self.add('c2', C2())
self.connect('c1.arr[-1]', 'c2.flt')
if __name__ == '__main__':
a = Problem()
a.root = A()
a.setup()
a.run()
print a.root.c2.unknowns['fout']
I am given the error:
openmdao.core.checks.ConnectError: Source 'c1.arr[-1]' cannot be connected to target 'c2.flt': 'c1.arr[-1]' does not exist.
Is there a way to do this? I know it worked in the old version of OpenMDAO.

Answer 1

OpenMDAO supports connection to specific indices of a source by using the 'src_indices' arg. For example:
self.connect('c1.arr', 'c2.flt', src_indices=[4])
Negative indices are not currently supported.

Answer 2

There are a number of small issues here. First the solve_nonlinear method of C1 has the wrong size for its array computation. It ends up working out, but you should really set the array to the right size (length 5).
To part of an array (see docs and more advanced docs) , you specify src_indices argument to connect.
import numpy as np
from openmdao.api import Component, Problem, Group
class C1(Component):
def __init__(self):
super(C1, self).__init__()
self.add_param('fin', val=1.0)
self.add_output('arr', val=np.zeros(5))
def solve_nonlinear(self, params, unknowns, resids):
fin = params['fin']
unknowns['arr'] = fin*np.arange(5)
class C2(Component):
def __init__(self):
super(C2, self).__init__()
self.add_param('flt', val=0.0)
self.add_output('fout', val=0.0)
def solve_nonlinear(self, params, unknowns, resids):
flt = params['flt']
unknowns['fout'] = 2*flt
class A(Group):
def __init__(self):
super(A, self).__init__()
self.add('c1', C1())
self.add('c2', C2())
self.connect('c1.arr', 'c2.flt', src_indices=[4,])
if __name__ == '__main__':
a = Problem()
a.root = A()
a.setup()
a.run()
print a.root.c2.unknowns['fout']