# Could someone explain how this code works?

2 views (last 30 days)
IBM watson on 23 Oct 2018
Commented: Torsten on 24 Oct 2018
S=[1 3 0 2 0 0];
C=[0 2];
[~,col] = ismember(C,S);
S(col) = [];
S =
1 3 0 0
This code is for : exclude C from S with repetition while keeping the same order of elements in S.
But how it works?
[~,col] = ismember(C,S);

Guillaume on 23 Oct 2018
Edited: Guillaume on 23 Oct 2018
I'm not sure what there is to explain. As documented, col tells you where each element corresponding element of C is found in S. If the element of C is found in more than one position, you'll get the first one of these.
The code is also flawed and will error if any element of C is not found at all in S. The safe version of the code would be:
S = [1 3 0 2 0 0];
C = [0 2 4]; %note that the original code would error because 4 is not present is S
[found, where] = ismember(C, S);
S(where(found)) = [] %this will not error

Show 1 older comment
Guillaume on 23 Oct 2018
Oh! the ~ in a function output just means ignore that output as the author was only interested in the 2nd output.
However, as I've shown you can't ignore this output in case an element of C is not in S.
You'll find that ~ is often used when calling min or max when you're just interested in knowing where the min or max is, but not its value:
value = max(somearray); %want to know what the max is
[value, where] = max(somearray); %wamt to know what the max is and where it is
[~, where] = max(somearray); %don't care about the value of the max, just want to know where it is
IBM watson on 23 Oct 2018
Many thanks to you!
Torsten on 24 Oct 2018
Note that the code from above assumes that the elements in C are distinct.
So setting
S=[1 3 0 2 0 0];
C=[0 0 2];
will not produce
S=[1 3 0]
but also
S=[1 3 0 0]
Best wishes
Torsten.