Get an array of results of a function using as input an array of values (in Matlab)

Question

I want to get the array of results of a function using as input an array of values. The function receives two variables (x1, x2) and a constant x3, so I'm trying to input all combination of it in a range using mesh.

The result is incorrect, I'm missing something.

Sample:

fun = @(x1,x2,x3) (x2-x1^2)^2+(1-x1)^2 + x3;
x3 = 7;
fun2 = @(x) fun(x(1,1),x(1,2),x3);

x0 = [2 3];
min = fminsearch(fun2, x0);
disp(min);

x = min(1)-10:1:min(1)+10;
y = min(2)-10:1:min(2)+10;

[X,Y] = meshgrid(x,y);

% I'm getting strange values here, like z < 0, how it is possible if everything is squared in the function.
Z = fun(X,Y,x3);

My guess: you should be using element-wise operations, e.g. .^ instead of the matrix operations, e.g., ^, when defining fun. — mikkola
– mikkola, Commented Apr 1, 2016 at 16:52
LOL, done... Thanks. I think you should write it as an answer. — Pedro77
– Pedro77, Commented Apr 1, 2016 at 16:57

mikkola · Accepted Answer · 2016-04-01 17:09:37Z

It's important to note that there is a difference between matrix and element-wise operations in MATLAB.

Matrix operations are defined via plain operators, such as *, or ^. So for example, A*B performs a matrix multiplication between A and B.

Element-wise operators make use of the dot . before the operator, i.e., .*, .^, and so on. Thus, A.*B performs an element-wise multiplication of A and B. The end result of this operation is an array of the same size as A and B (whose sizes must be equal), where the jj'th element of the array is equal to A(jj)*B(jj).

Now, consider your definition of fun:

fun = @(x1,x2,x3) (x2-x1^2)^2+(1-x1)^2 + x3;

What happens when MATLAB evaluates this expression is that it applies the matrix operations, such as ^ to the input arrays. However, to obtain your desired result of applying the operation to every individual element in your input arrays x1, x2, you should be using element-wise operations.

A new definition

fun = @(x1,x2,x3) (x2-x1.^2).^2+(1-x1).^2 + x3;

should provide the desired result.

Collectives™ on Stack Overflow

Get an array of results of a function using as input an array of values (in Matlab)

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related