ismember 0×0 empty logical array to logic

Hi All,

I have an array "A = [1 2 3 4 5 6]" which its length reduces in a loop

if ~ismember(A,ID)
    do this
else
    do that
end

This works fine until the A=[], which ismember returns

0×0 empty logical array

However I need a Single Value and not logical array. Note that I cannot use any and all functions to reduce Logical Arrays to Single Value, beacuse I faced with other problems when "A" array is not yet empty!

How can I fix this problem?

[madhan ravi]

First and foremost what is your goal? What is the expected result for given A?

Answer 1

[Walter Roberson]

The obvious answer would seem to be to test

 if isempty(A) || ~ismember(A,ID)

If you were hoping there were a hidden preference you could set that would make ismember(A,ID) return a non-empty value without having to change your test.. sorry, there is no such test.

I would point out that you could also rewrite your code:

 if ismember(A,ID)
     do that
 else
     do this
 end

the ismember() would be empty when A is empty, so the test would fail, and do that would not be done, leaving you to fall through to the do this case.

[Sonima]

I tried this already, but it doesn't work!

>> isempty(A) || ~ismember(A,ID)
Operands to the || and && operators must be convertible to logical scalar values.

Answer 2

[the cyclist]

Making a huge guess here, but are you sure you don't mean

~ismember(ID,A)

rather than

~ismember(A,ID)

Then, assuming ID is a scalar,

~ismember(ID,[])

returns true, which I'm guessing is the result you are intending.

Like I said ... just a guess. (And you might need to adjust your other logic accordingly.)

Part of the reason I am guessing this is that

~ismember(A,ID)

returns a vector of values (if A is a vector and ID is a scalar), which is not typically what you want in an if statement. But maybe it is in your case.

[Sonima]

Hi!

No, of course it is ~ismember(A,ID)

A = [1 2 3 4 5 6 7];
ID =5;
~ismember(A,ID)

ans =

  1×7 logical array

   1   1   1   1   0   1   1

[the cyclist]

OK, so are you aware that

if [true true false]
    do this
else
    do that
end

will "do that", because every element has to be true to get to the "do this" part of the if statement. (The reason I am so insistent is that this is a very common misunderstanding, and people often think that MATLAB somehow process the vector "in parallel".)

It would be good to give a an example of a combination of A and ID where you expect your if statement to "do this". It seems to me that the only way it happens is if none of the elements of A is equal to ID, in which case there are simpler ways to do what you want.

[Sonima]

Good point. thanks!