How can I find the index of a specific element in a 1D array using Simulink?

I`m working with Matlab Function Block in Simulink. Initially, this block is reading one variable (X) as input. The variable X changes over the time, so I`m capturing six samples and storing such samples in persistent variables. I`m also writing these samples in array format [1x6], that is:

persistent x0 x1 x2 x3 x4 x5
X_save = [x0 x1 x2 x3 x4 x5];

The objective is to compare another variable (y) and determine which elements of X is nearest to it. Suppose X = [0 10 20 30 40 50] while y = 7. I need Matlab Function to return as result the information w0 = 10 and w1 = 0, because x1 is closer to y than x0 is. I can easily implement this in workspace writing the functions below:

d = sort(abs(y - U(1,:)),'ascend'); % sort abs(y-U) in ascending order
Idx_1st = find(abs(y - U( ,:)) == d(1,1) ); % find index of x1 = 10
Idx_2nd = find(abs(y - U(1,:)) == d(1,2) ); % find index of x2 = 0
w0 = U(1,Idx_1st);
w1 = U(1,Idx_2nd);
W = [w0 w1];

That is, first I calculate the difference between y and all elements of X_save. Then, I sort the difference values (between y and X_save) in ascending order, so I can read the first the second element as the two nearest values. However, this workspace method doesn't work when implemented in Matlab Function Block (Simulink). It seems that the find(x) function can't be used properly. The diagnostic viewer is returning the following messages:

(1) 'W' is inferred as a variable-size matrix, but its size is specified as inherited or fixed. Verify 'y' is defined in terms of non-tunable parameters, or select the 'Variable Size' check box and specify the upper bounds in the Size box.

(2) Simulink cannot determine sizes and/or types of the outputs for block 'untitled/MATLAB Function' due to errors in the block body, or limitations of the underlying analysis. The errors might be inaccurate. Fix the indicated errors, or explicitly specify sizes and/or types for all block outputs.

Does anyone know why I`m having these issues? I appreciate any help. Thank you!

[Eric Bernard Dilger]

**I could easily do this using if and else for a small number of samples X, but that`s unsuitable for a large number os samples.

Answer 1

[SANKALP DEV]

Hi Eric,

I understand that you are working on implementing a MATLAB Function Block in Simulink to find the two nearest values in the array 'X' concerning a given variable 'Y.'

The error messages you have encountered seem to be related to the inferred size of the output variable 'W,' causing issues as Simulink is unable to determine the sizes and/or types of the outputs.

To address these issues, you may consider making modifications to the MATLAB Function Block code. One effective approach is to replace the 'find' function with alternative methods that do not rely on it.

Here is an example of a code snippet that calculates the absolute differences between 'Y' and each element of 'X,' sorts them in ascending order, and retrieves the two nearest values from 'X' without the utilizing the 'find' function.

Implementing this approach should resolve the issues and enable the MATLAB Function Block to function correctly within Simulink.

function W = findNearestValues(y, X) 
    X = [0 10 20 30 40 50]; 
    y = 7; 
    % Calculate the absolute differences between y and all elements of X 
    diff = abs(y - X); 
    % Sort the differences in ascending order 
    [~, sortedIdx] = sort(diff); 
    % Find the indices of the two nearest values 
    idx1 = sortedIdx(1); 
    idx2 = sortedIdx(2); 
   % Retrieve the corresponding values from X 
    w0 = X(idx1); 
    w1 = X(idx2); 
    % Return the result as an array 
    W = [w0, w1]; 
end

I hope this provides a solution to your question.

Best regards,

Sankalp