Wrong index acquired or wrong value taken from the index

Hello, I am currently doing some data analysis and need to get certain data points with the use of the "max" and "min" functions. That seems to be fine on its own, however I am going through a for loop and have to gather that single point in each case in the loop and store it in a matrix. That again, works fine. However, half of the numbers acquired seem to be wrong. E.g. I have xmx (x_maximum), from which I acquire the index to find the y value ymn at that point. According to all the graphs in ALL x_maximum cases ymn should be positive. The opposite applied to ymn. However for some reason ymx matrix has some negative values and ymn has some positive values.

I have tested each of those cases individually outside of the for loop and that gives the correct answers (positive for ymx and negative for ymn). So something may be wrong in the code itself, but I can't seem to find it.

If there is anyone who could help I would really appreciate it.

Kind regards, Edvardas

Code given below:

     %%Displacement and Velocity Front 1 Hz
          load('Front_Damper_1Hz');  %B(n)R(m)_F1Hz (notation)

          ymx_big=[];

          xmx_big=[];

          ymn_big=[];

          xmn_big=[];

          for n=1:11;

              figure(1)

              Fbump1=cat(3, -B0R0_F1Hz, -B2R0_F1Hz, -B4R0_F1Hz, -B6R0_F1Hz, -B8R0_F1Hz, -B10R0_F1Hz, -B12R0_F1Hz, -B14R0_F1Hz, -B16R0_F1Hz, -B18R0_F1Hz, -B20R0_F1Hz);

              Frebound1= cat(3, -B20R0_F1Hz,-B20R2_F1Hz, -B20R4_F1Hz, -B20R6_F1Hz, -B20R8_F1Hz, -B20R10_F1Hz, -B20R12_F1Hz, -B20R14_F1Hz, -B20R16_F1Hz, -B20R18_F1Hz, -B20R20_F1Hz);

            subplot(2,2,1) %bump displacement
            hold all
            plot(Fbump1(:,2,n), Fbump1(:,1,n)); % displacement
            %     set(gca,'Ydir','reverse')
            %     set(gca,'Xdir','reverse')

            subplot(2,2,2) %rebound displacement
            hold all
            plot(Frebound1(:,2,n), Frebound1(:,1,n)); % displacement
            %     set(gca,'Ydir','reverse')
            %     set(gca,'Xdir','reverse')
            subplot(2,2,3) %bump velociy
            hold all
            plot(Fbump1(:,3,n), Fbump1(:,1,n)); % displacement
            %     set(gca,'Ydir','reverse')
            %     set(gca,'Xdir','reverse')
            [xmx,loc] = max(Fbump1(:,3,n));
            ymx = Fbump1(loc);
            xmx_big(n)=xmx;
            ymx_big(n)=ymx;

            [xmn,loc1] = min(Fbump1(:,3,n));
            ymn = Fbump1(loc1);
            xmn_big(n)=xmn;
            ymn_big(n)=ymn;

            subplot(2,2,4) %rebound velocity
            hold all
            plot(Frebound1(:,3,n), Frebound1(:,1,n)); % displacement
            %     set(gca,'Ydir','reverse')
            %     set(gca,'Xdir','reverse')
        end

[dpb]

Nothing much we can do w/o the data that reproduces the problem.

Have you stepped through with the debugger to see where you code logic fails?

[Edvardas]

Yes, I have identified the points in the for loop at which the ymx and ymn values seem to be chosen incorrectly (even though the xmx and xmn values are correct).

It's at n = 5; 6; 8; 9; 10

I am attaching the data for this loop in case someone wished to try it out. I've tried using the individual data outside of the "cat" 3d matrix, and in that case managed to get the correct values for ymx and ymn.

Answer 1

[Roger Stafford]

In the line

   [xmx,loc] = max(Fbump1(:,3,n));

the index 'loc' refers to the row number of the maximum value in the third column of the n-th "page" of Fbump1. However in the next line

   ymx = Fbump1(loc);

this same 'loc' index is being used as a linear index and an entirely different element of Fbump is being referenced. I suspect you wanted to reference a different column of the same row and same n-th page. If that column is 4, for example, you should write:

   ymx = Fbump1(loc,4,n);

I have the same objection to the two lines:

   [xmn,loc1] = min(Fbump1(:,3,n));
   ymn = Fbump1(loc1);

[Edvardas]

Yes, that indeed was the problem. Thank you for pointing out, seems to be doing as expected now!