flipped indexes in 3D array

I'm defining a constant value for a grid point in a 3D matrix, when I plot it it takes the x value as y and the y value as x:

%define the axis

x=[0:10]

y=[0:10]

a=length(x)

%create 3D array of zeros

T=zeros(a,a,a)

%T(x=4,y=8,k=1)=100 a cosntant value for a grid point with position 4,8 in the slice 1 of T

T(4,8,1)=100

%plotting all values of x ,y and the 1st slice of T

surf(x,y,T(:,:,1))

xlabel('x')

ylabel('y')

%%END OF CODE

the graph is the following:

the value 100 is being given to a point in x=8 and y=4, meaning a flip indexing of T, T(y,x,k) why could this be happen?

Also, the matrix is:

Answer 1

[Image Analyst]

Arrays are indexed at M(row, column, slice), or M(y, x, slice). A common mistake is to put x first in the index list like M(x, y, slice). That is not correct. Row is y, NOT x, so row/y comes first, not x.

Also you might experiment around with calling

axis xy

or

axis ij

after you call a display or plotting function. This will flip the vertical axis so that the origin is at the top or bottom - whichever place you want it at.

Answer 2

[Voss]

Notice the documentation for surf states that input argument X is "x-coordinates, specified as a matrix the same size as Z, or as a vector with length n, where [m,n] = size(Z)" and Y is "y-coordinates, specified as a matrix the same size as Z or as a vector with length m, where [m,n] = size(Z)".

In other words, when using vector inputs as X and Y, the number of elements of X must be the same as the number of columns of Z and the number of elements of Y must be the same as the number of rows of Z. So surf interprets Z as having Y along the first dimension (down) and X along the second dimension (across).

In this case, the number of elements of X and Y are the same, so it's not obvious, but if you try it with say, x = [0:5]; y = [0:10]; surf(x,y,zeros(6,11)); you'll get an error because the third argument must be of size 11-by-6, i.e., you must do surf(x,y,zeros(11,6)); in that case.

All that is to say: transpose your matrix in the call to surf, and it'll work:

surf(x,y,T(:,:,1).')