Symbolic declaration of variables in Matlab

Question

I would like to write my variables as operations between other variables.

For instance if I put a = c + b then value that a keeps inside is the numeric result of the operation of a sum between c and b.

If c = 4 and b = 2 then, the value that a keeps is 6.

But I would like that a keeps the symbolic expression instead of the numeric value. and every time I write a in the command windows, matlab cacht the numeric value of c and the numeric value of b of the worspace variable and a sum them.

Normally if you write a, matlab displays the numeric value that is in this variable. Does anyone know how to do this?

Answer 1

You can do this using the symbolic toolbox. Here's an example:

syms a b c %# declare a b c to be symbolic variables
a = b + c;

b=3;c=4; %# now set values for b and c
eval(a)  %# evaluate the expression in a

ans =

    7

b=5;c=9; %# change the values of b and c
eval(a)

ans =

    14

So the definition of a is still b + c (you can check this by typing a at the command window) and when you evaluate it using eval, it uses the current value of b and c to calculate a. Note that b and c are no longer symbolic variables and are converted to doubles. However a still is, and the definition holds because by default, the expressions in symbolic variables are held unevaluated.

Answer 2

If you don't have the symbolic toolbox, you could use an anonymous function to achieve a similar result.

b=2; c=4; 
a=@()(evalin('caller','b+c')); 
a(), 

ans =

     6

b=1; 

a()


ans =

     5

Not ideal but may be helpful.

You could make this easier with the following function:

function [ anonFunction ] = AnonEval( expression )
%AnonEval Create an anonymous function that evaluates an expression
   anonFunction = @()(evalin('caller',expression)); 
end

b=2,c=4, 
a=AnonEval('b+c'); 
a(),
b=1; 
a()