weird calculating result from symbolic equation when using parenthesis

Hi, I am getting a very weird result when using the symbolic library to generate some equations.

apc6 is calculated incorrectly when i use parenthesis: apc6 = (-CC(1:N)-(AC(1:N)-FC(1:N))); and correctly when I dont: apc6 = (-CC(1:N)-AC(1:N)-FC(1:N));

could someone please clarify why this occurs?

Many thanks, Jesse

clear

N=1;

FC = sym('FC',[1,N]);     % coal fuel cost €/MWh
CC = sym('CC',[1,N]);     % CO2 fee €/MWh
SC = sym('SC',[1,N]);     % Startup costs
AC = sym('AC',[1,N]);     % Additional costs

%The start costs vector
ep = ones(1,N)*6.17;

%The start costs vector
sc6 = ones(1,N)*5000;
% sc(1)=0; %%Enable if block is off at t=0 

%The fixed costs vector
ac6 = ones(1,N)*1.9;

%The fuel costs vector
fc = ones(1,N)*10.28;

%CO2 cost vector
cc = ones(1,N).*6.17';

%%Sanity Check

ap6 = (-AC(1:N)-FC(1:N));
apc6 = (-CC(1:N)-(AC(1:N)-FC(1:N))); %%INCORRECT RESULT
%apc6 = (-CC(1:N)-AC(1:N)-FC(1:N)); %%CORRECT RESULT

ap6 = subs(ap6,[FC;AC],[fc';ac6']);
apc6 = subs(apc6,[CC;FC;AC],[cc';fc';ac6']);

double(ap6)
double(apc6)
double(ap6) - double(apc6)

Answer 1

[Titus Edelhofer]

Hi Jesse,

these are two different things. Slightly simpler example:

x1 = a - (b - c)

and

x2 = a - b - c

are two different things. You need to take the "-" in front of the parenthesis into account

x2 = a - b - (-c) = a - b + c

would be correct ...

Titus