Writing IF STATEMENT that omits empty fields in different arrays of a structure

I need writing an if statement that would exclude empty fields in a structured array (pictured attached). I am to make use of non-empty fields of M.id, M.xd and M.yd. I came up with this but not working:

for i=1:1:n % n is 100

if ~isempty M(i) % this is where my problem lies

for z=1:t % t is 5

min_dis_local=inf;

distanceL=sqrt( (S(i).xd-(M(z).xd) )^2

end

end

end

I got this error instead:

Error using isempty

Not enough input arguments.

[Ayobami Meadows]

Please can anyone help with a solution to this stated problem?

Answer 1

[Dyuman Joshi]

You need to use () with isempty, otherwise it would assume the input to be a string array

y = [];
isempty(y)

ans = logical
   1

isempty y

ans = logical
   0

%even though x is not defined, it returns 0 when checked with isempty
isempty x

ans = logical
   0

When you add ~ operator before isempty without (), it will through you an error, which is what you encountered

~isempty y

[Ayobami Meadows]

I have enclosed the M(i) in a bracket as you corrected, it worked only to some extent, but brought up another error before completing the iteration:

Index exceeds the number of array elements (169).

for i=1:1:n % n is 100
    if ~isempty(M(i)) % I have enclosed it in a bracket as you corrected
       
       for z=1:t % t is 5
            
            min_dis_local=inf;
            distanceL=sqrt(S(i).xd-(M(z).xd) )^2 
       end
    end
end

[Dyuman Joshi]

Please copy paste the full error message.

What is S? And what is the use of min_dis_local?

It would be better you provide all the code that is relevant to your issue.

[Ayobami Meadows]

This is a simple demo of the code

xm=1000;
ym=1000;
n=200;

S(n+1).xd=200;
S(n+1).yd=400;
for i=1:1:n
    S(i).xd=rand(1,1)*xm;
    XR(i)=S(i).xd;
    S(i).yd=rand(1,1)*ym;
    YR(i)=S(i).yd;
    S(i).id=i;

end 
Wmin=30;

for i=1:1:n

    theta_m(i)=atan(abs(S(i+1).yd-S(i).yd))/S(i+1).xd-(S(i).xd);
                 
     S(i).theta_m=theta_m(i);

        if (0 <= abs(theta_m(i))) && (abs(theta_m(i)) <= 360)  
                                      
            S(i).Wm=Wmin/theta_m(i); 
                    
             if ~isempty(S(i).Wm)
                 S(i).type='M';
             end
                          
        end               
end
tau=sum(arrayfun(@(S) ~isempty(S.Wm),S));

for i=1:1:n
    
    if S(i).type=='M'
      if ~isempty(S(i))  
        M(i).xd=S(i).xd;
        M(i).yd=S(i).yd;
        M(i).id=S(i).id;
      end
        
    end
end

for i=1:1:n

       if ~isempty(M(i)) % to calculate for only the nonempty fields in struct M
        for z=1:tau
            
            min_local=inf;

            distanceL=sqrt( (S(i).xd-(M(z).xd) )^2 + (S(i).yd-(M(z).yd) )^2 );

                if distanceL < min_local
                    min_local=distanceL;        
                end
            S(i).d_distance=min_local;
        end
       end
end

[Dyuman Joshi]

What is the full error you get when you run this code?

[Ayobami Meadows]

Index exceeds the number of array elements (127)

Error in Demo_code (line 54) if ~isempty(M(i))