'Subscript indices must either be real positive integers or logicals' on calculating rank(A) of a numeric matrix.

Here are two errors I'm getting when using the rank function.

B = [0 0 0 0 0 0; 0 0 0 0 0 0; 0 0 0 0 0 0; 0 0 0 0 0 0; 1 0 0 0 0 0; 0 1 0 0 0 0]
D = [z y x 0 0 0; y z y x 0 0; x y z y x 0; 0 x y z y x; 0 0 x y z y; 0 0 0 x y z]

rank(B)
rank(D)

For rank(B) or other numeric matrices I get:

 Subscript indices must either be real positive integers or logicals.

For rank(D) or other symbolic matrices I get:

Error using sym/subsindex (line 766)
Invalid indexing or function definition. When defining a function, ensure that the arguments are symbolic variables and the body of the function is a
SYM expression. When indexing, the input must be numeric, logical, or ':'.

I've googled and tried to figure both out but am really struggling.

Answer 1

[John D'Errico]

Don't define a variable named rank, certainly not if you ever intend to use the FUNCTION named rank. This is why you get the subscript indices problem.

Similarly, when you use rank(D), MATLAB sees the result is symbolic array. But you cannot index using a symbolic variable. Again the problem is DON'T NAME A VARIABLE RANK. The same applies to variables named sum, mean, etc.

Try this:

which rank -all

What does it tell you?

[Will Bolton]

As far as I can tell I haven't done that. This is the output I get:

>> which rank -all

rank is a variable.

C:\Program Files\MATLAB\R2016a\toolbox\symbolic\symbolic\@sym\rank.m  % Shadowed sym method
C:\Program Files\MATLAB\R2016a\toolbox\comm\comm\@gf\rank.m           % Shadowed gf method
C:\Program Files\MATLAB\R2016a\toolbox\matlab\matfun\rank.m           % Shadowed

I haven't defined a variable anywhere called rank, is it possible to do this implicitly somehow?

[Will Bolton]

Ok I've restarted MATLAB, suspect I unintentionally mistyped something in the command window hours ago and didn't realise. Sorry for the trouble!

[John D'Errico]

You can also look in the list of workspace variables. rank would have been listed there too.

whos rank

Restarting from a clean session will make the rank variable fall into the bit bucket of course, as long as you did not save that workspace and then reload it. Just wondering though, would a rank variable smell badly? ;-) Sorry. Could not resist.