Matlab sum: The expression to the left of the equals sign is not a valid target for an assignment

Question

When I try to run this code, this error message keeps showing up in matlab:

The expression to the left of the equals sign is not a valid target for an assignment.

a_k and b_k are meant to be row vectors of size (1,m). I can't use the command symsum because it doesn't let me index these vectors within the symsum. The resulting sum has to involve the dirac and heaviside functions. Ideas? :)

prompt = 'Enter m';
m = input(prompt);
prompt = 'Enter x-coordinates of dislocations';
a_k = input(prompt);
prompt = 'Enter y-coordinates of dislocations';
b_k = input(prompt);

syms x_1 x_2 y_1 y_2

F_1(y_1,y_2) = sum(heaviside(y_1-a_k(1,i))*dirac(1,y_2-b_k(1,i)), i=1:m);

Answer 1

You are using an invalid sum syntax, i.e. i=1:m.

sum calculates the sum of all the elements in an array. So, you need to rewrite your formula in vectorised way using elementwise operations as follows:

F_1(y_1, y_2) = sum(heaviside(y_1-a_k(1,1:m)).*dirac(1,y_2-b_k(1,1:m)));