Error using trapz (line 47) Dimension argument must be a positive integer scalar within indexing range.

function Length = LengthOfQuadraticSplines(A,X)
Length = 0.0;
for i=1:length(X)-1
    syms x;
    drivative = diff(A(i,1)*x^2+A(i,2)*x+A(i,3));
    Tx = linspace(X(1),X(2),3);
    Ty = sqrt(1+(drivative^2));
    Length = Length + trapz(Tx,Ty);
end

this is my code for this equation and it keep give me an error in trapz function

[KSSV]

What are the inputs which you have tried?

Answer 1

[Walter Roberson]

syms x;

Symbolic

drivative = diff(A(i,1)*x^2+A(i,2)*x+A(i,3));

Symbolic because x is symbolic.

Ty = sqrt(1+(drivative^2));

Symbolic because drivative is symbolic.

    Length = Length + trapz(Tx,Ty);

Tx is a numeric vector of length 3, but Ty is symbolic. There is no symbolic trapz() function.

When x is numeric and equally spaced, then trapz(x,y) mathematically works out as

  (sum(y)-(y(end)-y(1))/2) * (x(2)-x(1))

Your x vector is length 3, so

(y(1)/2 + y(2) + y(3)/2) * (x(2)-x(1)) 

But... your Ty is scalar, not a vector of length 3, do you have a problem.

Perhaps you want to evaluate the expression Ty at the three x locations. If you did that and used double() on the result, you would be able to do a plain trapz()