symbolic logical operation can't get right answer

Hello,

I have

syms a b c
aa=(a==b)+c
bb=eval(subs(aa,[a,b,c], [1/5, 0, 1051/10000]))

My expected result is

aa=(a==b)+c
bb=
    0.1051

However, the actual result is

aa =
 
  a + c == b + c
 

bb =

  logical

   0

In my opinion, (a==b) should be computed first. The logical result then is added to c. Finally I can get a double number 0.1051. However, the actual result is not what I want.

How did that happen? What can I do to solve this problem?

Answer 1

[Walter Roberson]

The comparison operators do not return 0 or 1 for the symbolic toolbox: they return constants TRUE or FALSE from DOM::Logical if I recall correctly.

If you want the relational operators to return 0 or 1 then you need to use piecewise(Condition, 1, 0)

Also, using eval() with symbolic expressions has undefined results.

MATLAB has three different programming languages. One is ordinary MATLAB. One is the MuPad engine language. The third is symbolic expressions, which are a hybrid between the other two and have incompatibilities with both of them. eval() applied to a symbolic expression will produce results that are not either MATLAB or MuPad in some situations.

[alice W]

Thank you for your reply. I will check that later.

The other problem is why

(a==b)+c

is equal to

a + c == b + c

Could you give me some pointers?

[Walter Roberson]

Why not? You have an equation and you add c to both sides of it. It is a common algebraic operation when solving equations, adding something to both sides of the equation, or multiplying or dividing both sides of the equation by a value.

Remember, (a==b) in symbolic toolbox is not going to give you a logical value.

[alice W]

I got that. Many thanks.