Problem with Symbolic Functions

I would like to create a simple "FunctionAnalyzer" in which the user inserts a function expression in general form with some parameters ('f(x)=a*x^2+b*x+c') and the graph is plotted for specific values of parameters chosen from user. And this is done in this way:

syms y(x,a,b,c,...)
y(x,a,b,c,...)=str2sym('a*x^2+b*x+c');
fplot(y(x,1,2,3,...));

My problem raises in the absense of a summation function, so i dediced to create my own function: sumk(f,a,b). Now i can write the function 'sumk(cos(c*k*x)),a,b)' which, in theory, should perform the summation for k=a to b of cos(c*k*x)) with a,b,c parameters. I tryed almost everything but I came to nothing. In particular i wrote:

syms y(x)
a=0;
b=2;
c=2;
y(x)=subs(str2sym('sumk(cos(c*k*x),a,b)'))

str2sym() is the only function i found that allow me to evaluate my local function, but it doesn't work without the actual values of a and b (since they appear in "for k=a:b") so i should substitute them with numbers using subs():

y(x)=subs('sumk(cos(c*k*x),a,b)')

but now the result is y(x)=sumk(cos(2*k*x),0,2) and all I need is evaluate the my local function, maybe using str2sym():

y(x)=str2sym(subs('sumk(cos(c*k*x),a,b)'))

but I get an error since the output of subs() is Sym and str2sym() requires a string.

I tried also to plot directly sumk(cos(2*k*x),0,2) with fplot() but I get an error due to the presence of 2 indipendent variables.

What can i do? How can I tell MatLab to evaluete the function sumk in the symbolic expression sumk(cos(2*k*x),0,2)? Is there a completly different approach i don't know? Please, remenber that this algorithm should be really really fast because I plot many functions with different parameters values in sequence.

Thanks for you help, Federico Spada

Answer 1

[madhan ravi]

help str2func