convert string to double

If I have a string like : `A = cos(2*pi*f1*t) + 4*sin(2*pi*f2*t)`

How I can convert it to double ?

I tried

    B = str2num(A) % I obtain an empty matrix
or 

    B = str2double(A) % I obtain a Nan response...

How can I fixe it ?

[Jos (10584)]

What do you expect to get? Do you want to evaluate the formula?

Answer 1

[Walter Roberson]

MATLAB 5.1 and later, no special toolboxes:

S = 'A = cos(2*pi*f1*t) + 4*sin(2*pi*f2*t)';
temp = vectorize(regexp(S, '(?<==\s*)\S.*', 'match', 'once'));
vars = symvar(temp);
F = str2func(['@(', strjoin(vars,','), ') ', temp ]);

Now execute the function handle F, passing in the variables named in vars, in order, which in this case would be f1, f2, t.

But I suspect this is not the answer you are looking for. An answer closer to what you are looking for would be:

R2017b and later with Symbolic Toolbox

S = 'A = cos(2*pi*f1*t) + 4*sin(2*pi*f2*t)';
temp = regexp(S, '(?<==\s*)\S.*', 'match', 'once');
tempsym = str2sym(temp);
A = double( subs(tempsym) );

R2017a and earlier with Symbolic Toolbox

S = 'A = cos(2*pi*f1*t) + 4*sin(2*pi*f2*t)';
temp = regexp(S, '(?<==\s*)\S.*', 'match', 'once');
tempsym = sym(temp);    %will probably give a warning message
A = double( subs(tempsym) );

But I suspect those are not the answers you are looking for either.

The answer I suspect you are looking for is:

S = 'A = cos(2*pi*f1*t) + 4*sin(2*pi*f2*t)';
eval(S);
%the result will be in A

We recommend against using eval() !!