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

Question

K 2014-1-22

2
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/113325-how-can-i-use-an-empty-matrix-in-a-logical-if-statement

回答： Jackson 2017-7-20

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

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Bruno Pop-Stefanov 2014-1-22

15
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/113325-how-can-i-use-an-empty-matrix-in-a-logical-if-statement#answer_121839

编辑：Bruno Pop-Stefanov 2014-1-22

在 MATLAB Online 中打开

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

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

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

K 2014-1-22

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

请先登录，再进行评论。

Answer 2

Jackson 2017-7-20

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/113325-how-can-i-use-an-empty-matrix-in-a-logical-if-statement#answer_274859

在 MATLAB Online 中打开

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