how can i solve the error "Undefined function 'sign' for input arguments of type 'sym'."

i have the following function

function [S] = Signfunciton(f,a,b)
v=-a:0.5:b;
S= f(v);
S=sign(S);

then i give it inputs

>>syms x
>>f(x)=3*x+2
>>Signfunction(f,4,5)

but i always get the error "Undefined function 'sign' for input arguments of type 'sym'."

how can i solve this?? thank you

Answer 1

[Wayne King]

Do you have the Symbolic Toolbox?

And if you do, why aren't you just doing

    syms x;
    f = 3*x+2;
    sign(subs(f,-4:0.5:5))

[Paolo]

i don't know what a symbolic toolbox is but i used your code and it works great. but i still don't understand what was wrong with my code.

Answer 2

[Wayne King]

Yours works for me:

function [S] = Signfunction(f,a,b)  %you misspelled this above
v=-a:0.5:b;
S= f(v);
S=sign(S);

Then execute:

syms x;
f(x) = 3*x+2;
Signfunction(f,4,5);

Note if you don't want a symbolic output

   clearvars x;
   f = @(x) 3*x+2;
   S =  Signfunction(f,4,5);

Now S is a double-precision vector.