How can I use an empty matrix in a logical if statement?

Hello,

I have a program that evaluates mode shapes using the null space of an 8X8 matrix, "K". It works great when the null space exists, and I get an 8X1 vector for "u". However, for some values of "K" there is no null space, so for that "u" I get "Empty matrix: 8-by-0" and need to use a different method.

I tried to use

if eval(['u',num2str(ii),'==[]'])

to test if u is an empty matrix, but I get an error that tells me that "Matrix dimensions must agree". Is there any way to use an if statement that will run only if u is an empty 8x0 matrix?

(And yes, I know that eval + num2str is frowned upon, I didn't write that part! Apologies in advance!)

Thanks! Kaitlin

Answer 1

[Bruno Pop-Stefanov]

Use the isempty function to check if an array is empty:

if isempty(u)
  % do something
else
  % do something else
end

[K]

Thank you! That worked- thanks for directing me to the right command!

Answer 2

[Jackson]

When given a numeric array, if statements look at the first element and return true if it is non-zero. If the array is empty then this appears to be interpreted as false. Run the following:

if  []
  disp  TEST1
end

if  0 : 10
  disp  TEST2
end

if  1 : 10
  disp  TEST3
end

a = [] ;
b = 1 : 10 ;

if  a
  disp  TEST4
elseif  b
  disp  TEST5
end

And only TEST3 and TEST5 are printed on the command line.