How do I set a parameter based on individual values in an existing array? Should I use a for loop or something else?

I need to have 'maxDistLine' be different for different 'L' values (i.e., if L<300, then maxDistLine should be 70, or else if L is between 300 and 1000 maxDistLine should be 50, else maxDistLine should be 20. I've tried using a 'for loop', 'while', and 'if/else' statements and nothing seems to work. Please see attached section of code. Thanks in advance for any help you could offer!

Section of Code:

classdef TripletGraph2
    
    properties
        
        C; %triplet center points
        T; %triplet endpoints
        Cgraph    %The undirected graph of the center points
        
        ArcPoints %2x4xN list of arc points CenterA-EndpointA-EndpointB-CenterB
    end
    
    methods
        function obj=TripletGraph2(fpep)
            
            %             maxDistLine=20;
            maxLineLength=1800;
            CTDist=35;
            %             maxDistLineDiff=7;
            
            l2norm=@(z,varargin) vecnorm(z,2,varargin{:});
            
            C=fpep(:,7:8);                         %Central points
            T=reshape( fpep(:,1:6)  ,[],2,3);      %Triplet end-points  Nx2x3
            T=(T-C)./l2norm(T-C,2)*CTDist + C;     %Normalize center to endpoint distances
            
            DT=delaunayTriangulation(C);   %Delaunay triangulation of centers
            E=edges(DT);                   %Edge list
            
            C1=C(E(:,1),:); %Center points listed for all DT edges
            C2=C(E(:,2),:);
            L=l2norm(C1-C2,2); %Center-to_center lengths for DT edges
            if L<500
                maxDistLine=21;    
                maxDistLineDiff=7;
            elseif (500<L)&(L<1000)
                maxDistLine=18;    
                maxDistLineDiff=7;                    
            else
                maxDistLine=15;
                maxDistLineDiff=7;
            end

Answer 1

[the cyclist]

My best guess here is that L is a vector, and you are expecting the if-else logic to work on the entire vector simultaneously. That's not how it works. For example

if L < 500

will only be true if all elements are less than 500. (The code won't check each element one-by-one.) This is described in the documentation for if, elseif, else.

So, you could loop over the elements, or you could vectorize explicitly, such as

maxDistLine = 21*(L<500) + 10*((L>=500)&(L<1000)) + 15*(L>1000)

[Steven D]

Thanks for your input! However, I tried running your suggestion but was given the following error:

Unable to perform assignment because the size of the left side is 1-by-2772 and the size of the right side is 1-by-2688.

Error in TripletGraph2 (line 103)

EID2(:,:,j)=I;

Error in combined_foam_code2all (line 158)

obj=TripletGraph2(fpep);

Attached is the whole code (TripletGraph2) for all to view.

Answer 2

[dpb]

The above if...elseif...else...end construct works; there are other ways -- one way w/ MATLAB is

fnDL=@(L)interp1([500-eps(500),1000-eps(1000),realmax],L,'next','extrap');

using 1D lookup with builtin interp1. The eps() fixes up so the <= condition is met for the breakpoints.

An alternate that's a little more computationally efficient is a recast of the IF construct into a one-liner...

fnDL=@(L)21-3*(L>=500)-3*(L>=1000);

There are any number of ways to rewrite the above as well.

And, of course, the SWITCH construct could be used as well.

I'm sure others will think of many other similar implementations.

[Steven D]

Thanks so much for your input. This looks very promising. Could you give me an idea of how this all needs to be placed in the existing code?

Attached above (in the previous comment), is the whole code (TripletGraph2) for everyone to view.

[dpb]

I dunno...what's the input L? and the output to be?

I assumed a vector although an array will work as well with array --

>> L=[0:250:2500];
>> fnLD(L)
ans =
    21    21    18    18    15    15    15    15    15    15    15
>> maxDL=fnLD(L)
maxDL =
    21    21    18    18    15    15    15    15    15    15    15
>> 

If L is a vector going in, maxDL is a vector of same length coming out.

I don't know your application's needs/expectations; that's up to you.

[Steven D]

L is shown in the snippet of code in my original post (L=l2norm(C1-C2,2);). C1 and C2 are points (x,y coordinates) of the Delaunay Triangulation edges.

[Steven D]

Thanks again for your input! Attached are two images of the MATLAB figure outputs from the code. The first image is with maxDistLine hard-set to 30. The second, is with maxDistLine hard-set to 70. I need to be able to have the value for maxDistLine change automatically in the code depending on the length of the edges; so that all the 'polygons' are closed and none have extra edges.

[dpb]

Well, if you have an array for L, the above code will give you a matching distance to go with it.

Pass it in place of the constant you're now using if the input to whatever does the drawing is vectorized -- otherwise, you'll have to pass the element in a loop construct, I suppose.

But, your function

 function obj=TripletGraph2(fpep)

doesn't define its output variable obj at all in order to somehow get the results back to the caller...

[Steven D]

Thanks again for your continued input; it's very much appreciated. Attached is the mother code that calls the function TripletGraph2 (as well as the input image). I'm still not clear as to exactly where to place the snippet of code you've provided. If you have a different suggestion as to how to have the polygons close up with no extra/erroneous edges, I would be very interested in seeing that too.

[dpb]

That just replaces a loop and the IF construct to produce the requested distance value in the code you posted...but your function doesn't return anything it calculates for anything else to be done with it, so it doesn't matter at all what it does until you decide what it is you want/need elsewhere and set up the return values to return the needed-elsewhere variables you're calculating.

[Steven D]

Thanks again for your response. Where would you set up return values elsewhere--in the attached code (i.e., 'combined_foam_code2all.m')? If so, where would you put it? Is there a better way to close the polygonal cells (in the images above), then the current approach?