Unrecognized function or variable 'x'. Error (line 65)

am computing and I get an error on line 65

x_v=[1 2 5 6 7 8 10 13 17];   % The corresponding x values 
y_v = [3 3.7 3.9 4.2 5.7 6.6 7.7 6.7 4.5];   % The corresponding y values 
dy_v= [1 NaN  NaN  -0.67];   % The slopes/clamps at the end 
n=length(x_v) -1;  % Find the n for size of the matrix  
h_v = diff(x_v);  % Find the h values (step size) from x values 
 
%% Create Matrix A
for i=1:n+1  % Start row loop  
     
    for j=1:n+1  % Start column loop  
        
        if i==1&&j==1   % Condition for first row and first column  
            A(i,j) = 2*h_v(i);  % First Entry  
        elseif i==n+1&&j==n+1  % Condition for last row and last column  
            A(i,j) = 2*h_v(i-1);  % Last Entry  
        elseif j==i-1   % Condition for element jsut left of the diagonal  
            A(i,j) = h_v(i-1);
         elseif j==i+1   % Condition for element jsut right of the diagonal  
             A(i,j) = h_v(i);
        elseif i==j  % Condition for element in diagonal  
            A(i,j) = 2*(h_v(i-1)+h_v(i));
        else
            A(i,j) = 0;  % Condition for all other elements in matrix  
        end
    end
end
 
%% Create Vector B 
for i=1:n+1
    
   if i==1   % Condition for first row 
       B(i) = 3*((1/h_v(i))*(y_v(i+1) - y_v(i)) - dy_v(i));  
   elseif i==n+1   % Condition for last row 
       B(i) = 3*(dy_v(i) - (1/h_v(i-1))*(y_v(i) - y_v(i-1)) );
   else   % Condition for all other elements 
        B(i) = 3*((1/h_v(i))*(y_v(i+1) - y_v(i)) - (1/h_v(i-1))*(y_v(i) - y_v(i-1)));
   end
end
 
%% Find the Matrix C 
c = (A^-1)*B';    % Find the product of inverse of A and B 
for i=1:n  % Loop for remaining constants (bs) and (ds) 
   
    b(i) = (1/h_v(i))*(y_v(i+1) - y_v(i)) - (h_v(i)/3)*(2*c(i) + c(i+1));
    d(i) =  (1/(3*h_v(i)))*(c(i+1) - c(i));
    
end
%% Find the splines 
for i=1:n
    
   
    S{i} = num2str(y_v(i),8) + num2str(b(i),8)*(x-  num2str(x_v(i),8) ) + num2str(c(i),8)*(x- num2str(x_v(i),8)).^2 +   num2str(d(i),8)*(x- num2str(x_v(i),8)).^3;
    SS{i} =['$$' latex(S{i}) '$$'];
    SS2{i} =[ latex(S{i}) ];
    SSD{i} =[ latex(diff(S{i})) ];
    
end

[the cyclist]

I don't think you posted 65 lines of code, so that's not very helpful reference. When I run the code you posted, I get an error on this line:

B(i) = 3*(dy_v(i) - (1/h_v(i-1))*(y_v(i) - y_v(i-1)) );

but not the error you report.

Assuming you are getting an error from this line:

S{i} = num2str(y_v(i),8) + num2str(b(i),8)*(x-  num2str(x_v(i),8) ) + num2str(c(i),8)*(x- num2str(x_v(i),8)).^2 +   num2str(d(i),8)*(x- num2str(x_v(i),8)).^3;

it seems as though you try to use the variable x, which has not been defined.

[camilo jose]

help me for this wrong

Answer 1

[Walter Roberson]

num2str returns a character vector. You are trying to add and subtract character vectors, which MATLAB considers the same thing as creating a vector of double of the character codes needed to represent the characters, and attempting to add or subtract those. That is not going to work because the vectors are different lengths.

You look like perhaps you are trying to concatenate strings.You can use + to concatenate string objects but not character vectors. You would need to use string() around each of the num2str. But even if you did that, x minus a string object is not defined for any MATLAB object classes that I can think of.

If you are trying to do string concatenation please see sprintf() or compose()

Meanwhile you need to figure out what your x variable is.

Note also that since r2018b it has not been permitted to use diff() with a character vector. If you are attempting to take the derivative of a symbolic formula then build up the formula symbolically instead of by characters. I suspect that your x is intended to be a symbolic variable

syms x