Why do I get the error "Unable to use a value of type '...' as an index."?

Why do I get error messages such as:

Unable to use a value of type cell as an index.
Unable to use a value of type idnlarx as an index.

Answer 1

[MathWorks Support Team]

This error can occur if you accidentally used a function name as a variable name. The solution is to rename your variable to something else.

See the following documentation to learn more about the Function Precedence Order in MATLAB:

https://www.mathworks.com/help/matlab/matlab_prog/function-precedence-order.html

Example:

load twotankdata;
z = iddata(y, u, 1);
sys1 = nlarx(z(1:300),[15 15 1]);
forecast = 6;
figure();
compare(z(1:800), sys1, forecast);
Y_Pred = forecast(sys1, z(1:500), 6, z.u(501:506));

The variable defined by "forecast = 6;" shadows the function "forecast" of the same name.

Solution:

Rename the variable "forecast" to "forecast1":

load twotankdata;
z = iddata(y, u, 1);
sys1 = nlarx(z(1:300),[15 15 1]);
forecast1 = 6;
figure();
compare(z(1:800), sys1, forecast1);
Y_Pred = forecast(sys1, z(1:500), 6, z.u(501:506));