help! my variable entry is not displaying as entered.

%loop that will have 2000 iterations (each iteration
%representing a rotation of the plane by 0.5 degrees)
for t= 1:2000

Output=zeros(1,2000);

    format('long')

    %imagine points in 2-dimensional euclidean space.  These three vectors
    %represent the three vertices of an equilateral triangle cetered at the
    %origin
A = [-0.5; 0.5];
B = [-0.5; -0.5];
C = [0.5; -0.5];
D = [0.5; 0.5];

    %T is designed to be a 0.5 degree counter-clockwise rotation matrix, sending each of the
    %previous vectors to their 0.5 degree rotated counterparts

    T= [ 0.99996 -0.00873;0.00873 0.99996];

    %represent the rotated images of A,B,C, for some iteration 't'
    tA=(T^t)*A;
    tB=(T^t)*B;
    tC=(T^t)*C;
    tD=(T^t)*D;
    %tE=(T^t)*E
    %tF=(T^t)*F
    %tG=(T^t)*G
    %tH=(T^t)*H
    %tI=(T^t)*I

    %P extracts the x values of each vector above, discarding the rest
    P= [1 0];

    %represent the projection of tA,tB,tC onto the x-axis. 
    imA=P*tA;
    imB=P*tB;
    imC=P*tC;
    imD=P*tD;

    %imE=P*tE;
    %imF=P*tF;
    %imG=P*tG;
    %imH=P*tH;
    %imI=P*tI;
    %imJ=P*tJ;
    %distances between the respective projections
    distAB=abs(imA-imB);
    distBC=abs(imB-imC);
    distCD=abs(imC-imD);
    distDA=abs(imD-imA);
    distAC=abs(imC-imA);
    distBD=abs(imB-imD);
    %making a vector V with elements 'distAB' 'distBC' 'distCA'
    X1=[1 0 0 0 0 0];
    X2=[0 1 0 0 0 0];
    X3=[0 0 1 0 0 0];
    X4=[0 0 0 1 0 0];
    X5=[0 0 0 0 1 0];
    X6=[0 0 0 0 0 1];

    dX1=distAB.*X1;
    dX2=distBC.*X2;
    dX3=distCD.*X3;
    dX4=distDA.*X4;
    dX5=distAC.*X5;
    dX6=distBD.*X6;

    V= dX1+dX2+dX3+dX4+dX5+dX6;

    % D = max element in V is the width of the 'shadow' of the triangle on the
    %x-axis
    D=max(V);

    Output(t)=D;

end

When I run the script, my vector 'D', which, as far as I can tell is very clearly defined as a 2x1 vector, is assigned the value '1.1597'.. seemingly without my consent or permission!

Can anyone figure out where this anomalous value is coming from?

Answer 1

[Guillaume]

First to answer your question, in the line before last of your script D is very clearly defined as the max of V, overwriting whatever was stored previously in it. Morale: Use variable names that have more than one letter and more importantly have meaning, such as shadowwidth or some such and it'll be a lot more difficult to reuse them by mistake.

On another note, you certainly like to write the same code over and over. This would achieve the same in so much fewer lines:

vertices = [A, B, C, D, E, F, G, H, I];  %these variables should never have been created in the first place
rotation = [0.99996 -0.00873;0.00873 0.99996];
numrotations = 2000;
shadowwidth = zeros(1, numrotations);
for rotationstep = 1:numrotations
   rotatedvertices = (rotation ^ rotationstep) * vertices; %all your tx = ... all at once!
   xprojection = rotatedvertices(1, [1 4]);  %hardly worth a matrix multiplication, but you could do it with P*rotatedvertices
   distances = abs(xprojection - xprojection.'); %only works in R2016b and later
   %in previous versions, replace with:
   %distances = abs(bsxfun(@minus, xprojection, xprojection.');  
   distances = distances(triu(true(size(distances)), 1)); %only keep upper triangle of distance matrix
   shadowwidth(t) = max(distances);
end

[Erik Anderson]

how embarrassing. I saw an error at line 71 and assumed that the error must have been upstream.. foolish.

I really appreciate you showing me how to write more efficiently. It's not necessarily that I like to write the same thing redundantly, but that I only barely understand the matlab syntax enough to get my code to run, and so I write code the same way that I would calculate by hand, on paper. I'll review what you wrote and try to learn from it, as my redundancy frustrates me endlessly.

Thank you for your answer :)