Array indices must be positive integers or logical values.

Hi everyone! I'm writing a code of a constraint for the fmincon function but i don't understand what i'm doing wrong with the index. This is my code:

-lcon.m

function [c,ceq] = lcon(u, x_i, y_i, theta_i, x_f, y_f, theta_f, alphamin, alphamax)

c = zeros([51*2, 1])
 for (s=0:0.02:1)

  k_i = u(1)
  k_f = u(2)

  c((2*s/0.02+1)) = alphamax * ((9*s^4*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i))^2 + 4*s^2*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i))^2 + k_i^2*cos(theta_i)^2 + k_i^2*sin(theta_i)^2 + 9*s^4*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i))^2 + 4*s^2*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i))^2 - 12*s^3*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i))*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i)) - 12*s^3*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i))*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i)) - 4*k_i*s*cos(theta_i)*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i)) - 4*k_i*s*sin(theta_i)*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i)) + 6*k_i*s^2*cos(theta_i)*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i)) + 6*k_i*s^2*sin(theta_i)*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i)))^(1/2)) -((6*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i))*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i)) - 6*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i))*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i)))*s^2 + (6*k_i*sin(theta_i)*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i)) - 6*k_i*cos(theta_i)*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i)))*s - 2*k_i*sin(theta_i)*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i)) + 2*k_i*cos(theta_i)*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i)))/(k_i^2*cos(theta_i)^2 - s*(4*k_i*sin(theta_i)*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i)) + 4*k_i*cos(theta_i)*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i))) + k_i^2*sin(theta_i)^2 + s^4*(9*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i))^2 + 9*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i))^2) - s^3*(12*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i))*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i)) + 12*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i))*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i))) + s^2*(4*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i))^2 + 4*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i))^2 + 6*k_i*sin(theta_i)*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i)) + 6*k_i*cos(theta_i)*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i))))
  c((2*s/0.02+2)) = alphamin * ((9*s^4*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i))^2 + 4*s^2*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i))^2 + k_i^2*cos(theta_i)^2 + k_i^2*sin(theta_i)^2 + 9*s^4*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i))^2 + 4*s^2*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i))^2 - 12*s^3*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i))*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i)) - 12*s^3*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i))*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i)) - 4*k_i*s*cos(theta_i)*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i)) - 4*k_i*s*sin(theta_i)*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i)) + 6*k_i*s^2*cos(theta_i)*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i)) + 6*k_i*s^2*sin(theta_i)*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i)))^(1/2)) - (-((6*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i))*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i)) - 6*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i))*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i)))*s^2 + (6*k_i*sin(theta_i)*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i)) - 6*k_i*cos(theta_i)*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i)))*s - 2*k_i*sin(theta_i)*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i)) + 2*k_i*cos(theta_i)*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i)))/(k_i^2*cos(theta_i)^2 - s*(4*k_i*sin(theta_i)*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i)) + 4*k_i*cos(theta_i)*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i))) + k_i^2*sin(theta_i)^2 + s^4*(9*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i))^2 + 9*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i))^2) - s^3*(12*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i))*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i)) + 12*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i))*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i))) + s^2*(4*(3*x_i - 3*x_f + k_f*cos(theta_f) + 2*k_i*cos(theta_i))^2 + 4*(3*y_i - 3*y_f + k_f*sin(theta_f) + 2*k_i*sin(theta_i))^2 + 6*k_i*sin(theta_i)*(2*y_i - 2*y_f + k_f*sin(theta_f) + k_i*sin(theta_i)) + 6*k_i*cos(theta_i)*(2*x_i - 2*x_f + k_f*cos(theta_f) + k_i*cos(theta_i)))))
 end

ceq=[];
end

The error i get:

Array indices must be positive integers or logical values.

What can i do?

Answer 1

[Daniel M]

Here is a minimal working example of your problem

x = rand(1,15);
ind = 2*0.14/0.02+1; % occurs when s = 8
x(ind) 
Error: Array indices must be positive integers or logical values

It would seem that the value of ind is 15, but it is not.

ind - 15
% ans = 1.77635683940025e-15

So you are trying to access x at a non-integer position. The easiest solution is to round (in some manner) to an integer.

x(round(ind))
% and this works!

Also see fix, ceil, and floor for other ways to round.

[Simone Costi]

Thank you, it works!