how i can change the variable? pleasee

Look at the first code, if you realize and = (x. ^ 3) it is the same as E. ^ 3 that I want to do but in a general way in the code of my gui but what happens is that I do not know. The user for example x ^ 2 but I want to read my program to change it to E. ^ 2 and so it can work. My program asks for this to get the area under the curve and graph it

%FIRST CODE
a=input('Since: ');
b=input('until: ');
x=linspace(a,b,500);
y=(x.^3);

syms K;
syms n;

D=(b-a)/n;
E= a+((K-1)*D)+(D/2);
F= (E.^3)*D;

f= symsum (F,K,1,n);

A= limit(f,n,inf)

area(x,y);
grid on;

%SECOND CODE 
a=str2double(get(handles.edit7,'string'));
b=str2double(get(handles.edit6,'string'));
x=linspace(a,b,500);
y=(get(handles.edit4,'string'));
j=char(y);
syms K;
syms n;

D=(b-a)/n;
E= a+((K-1)*D)+(D/2);
F=(j)*D;

f= symsum (F,K,1,n);

A= limit(f,n,inf)

axes(handles.axes3);
area(x,y);
grid on;

Answer 1

[Walter Roberson]

General matching to determine whether change of variable is appropriate requires subexpression pattern matching that is not available at the MATLAB level. It is possible in theory using the facilities inside the symbolic engine, MuPAD

For example you cannot detect that there is something to the 3rd power in any easy way. You can symvar() to extract the variables and then coeffs() with two outputs and look through the second output to find the cube of the variable and match it with its coefficient, and keep doing that until you find a variable is being cubed. However, this is not able to match a simple (x+1)^3 except by expanding out to x^3 + 3*x^2 + 3*x + 1, which is not what you might hope.

You can convert the expression to a character vector and do text processing... watch out for bracket nesting, though, which is something that no pure regular expression parser can handle.

At the MATLAB level there is no way to ask "does the expression contain something cubed?" .

In Maple you would be able to ask to indets(A, anything^3) or indets(A, `^`(anything,3)) (which are the same thing to Maple) or indets(A, specop(`^`)) to find all of the exponent operators (but you do have to be a bit careful in Maple, as negative exponents are sometimes constructed as 1 divided by the expression to the positive.)

[Erwin Avendaño]

Thank you. Do you know how I could graph the area under a curve of any function in the matlab guide? to graph it in an axe

[Walter Roberson]

You can do anything in GUIDE that you can do in MATLAB (but you will probably make an especially big mess if you "clear all")

Have a look at fill()