ismember(a, b) function problem

Hello,

In brief, I am trying to compare two seperate arrays that decrease/increase till they are equal one another.

a = [1, 1, 0];
b = [0, 1, 0];

logic = ismember(a, b); % or ismembertol(a, b, 0.001);
if all(logic(:))
    % Do somthing
end

However, I seem to be getting all true, [1, 1, 1], when I expect a [0, 1, 1] for this particular set of logical comaprison using ismember(). I am not good at coding so will appreciate if anyone could explain why this is happening.

Thank you!

Answer 1

[Voss]

ismember(a,b) tells you whether each element of a exists somewhere in b

ismember([1 2 3 4 5],[2 4 6]) % 2 and 4 exist in [2 4 6], but 1, 3, and 5 do not

ans = 1×5 logical array
   0   1   0   1   0

With your a and b:

a = [1, 1, 0];
b = [0, 1, 0];

logic = ismember(a,b)

ans = 1×3 logical array
   1   1   1

you get logic is all true because all elements of a (i.e., 0 and 1) appear in b.

It seems that you actually want to compare each element of a and b, which you can do using ==

logic = a == b

logic = 1×3 logical array
   0   1   1

[Jeffrey DG]

Wow, thanks. I did not know you could also compare arrays just like that. You learn something new everyday!

[Voss]

You're welcome!

FYI, here are some examples of using == with arrays:

x = [1 2 3 4]; % 1-by-4 array
y = [1 2 1 2]; % 1-by-4 array
x == y % compares each element of x to corresponding element of y

ans = 1×4 logical array
   1   1   0   0

x = [1 2 3 4]; % 1-by-4 array
y = 2; % scalar
x == y % compares each element of x to y

ans = 1×4 logical array
   0   1   0   0

x = [1 2 3 4]; % 1-by-4 array (row vector)
y = [2; 3; 5]; % 3-by-1 array (column vector)
x == y % also works! (compares each element of x to every element of y)

ans = 3×4 logical array
   0   1   0   0
   0   0   1   0
   0   0   0   0

x = [1 2 3 4]; % 1-by-4 array
y = [2 5 8]; % 1-by-3 array
x == y % error

Arrays have incompatible sizes for this operation.

Answer 2

[Bjorn Gustavsson]

The ismember function checks if elements in a are found in b, not that each element match. In your case both "1" in a have a value found in b and the same is true for the "0". Maybe you want your conditional to be:

if all(a==b)
    % Do somthing
end

or perhaps

your_tol = 0.01; 
if all(abs(a-b)<=your_tol)
    % Do somthing
end

HTH