Contours in Matlab/Octave - matlab

Question

I am having some trouble understanding contours.
What I have understood so far is that contours are a way to represent a 3d figure in a 2d plane. It does so by plotting a function of 2 variables as curves along which the function has same value.
Now if I do:
z=[1 4; 10 7];
contour(z);
I get this:
I read the documentation and it says:
contour(Z) draws a contour plot of matrix Z, where Z is interpreted as
heights with respect to the x-y plane. Z must be at least a 2-by-2
matrix that contains at least two different values. The x values
correspond to the column indices of Z and the y values correspond to
the row indices of Z. The contour levels are chosen automatically.
Thus for x=1,y=1: z=1, x=2,y=1: z=4 and so on. However I can't understand how to interpret this as the contour plot shown above.
And if I write:
contour(X1, X2, vals, [0.5 0.5], 'b'); where X1, X2 and vals are equal sized matrices and vals is a matrix of only 0s and 1s. I can't understand what does the argument [0.5 0.5] do. I read the documentation which states:
contour(Z,v) draws a contour plot of matrix Z with contour lines at
the data values specified in the monotonically increasing vector v. To
display a single contour line at a particular value, define v as a
two-element vector with both elements equal to the desired contour
level.
and I am unable to understand this statement.

Answer 1

The problem of the first contour is that there are just 4 values. Try something like
x = 0:0.1:10;
y = 0:0.1:10;
z = sin(x') * cos(y);
contour(z)
For the second thing, this means that if you want to see just particular contours, input them as vector v. In the example above:
contour(z, [0.1, 0.2, 0.3])
will show contour lines of 0.1, 0.2 and 0.3.
To have a single contour line, you can't have just (z, 0) but require (z, [0,0])