Reputation: 28062
This seems to be a very common problem of mine:
data = [1 2 3; 4 5 6];
mask = [true false true];
mask = repmat(mask, 2, 1);
data(mask) ==> [1; 4; 3; 6]
What I wanted was [1 3; 4 6]
.
Yes I can just reshape
it to the right size, but that seems the wrong way to do it. Is there a better way? Why doesn't data(mask)
return a matrix when it is actually rectangular? I understand in the general case it may not be, but in my case since my original mask is an array it always will be.
Corollary
Thanks for the answer, I just also wanted to point out this also works with anything that returns a numeric index like ismember
, sort
, or unique
.
I used to take the second return value from sort
and apply it to every column manually when you can use this notion to do it one shot.
Upvotes: 4
Views: 5364
Reputation: 125854
This will give you what you want:
>> data = [1 2 3; 4 5 6];
>> mask = [true false true];
>> data(:,mask)
ans =
1 3
4 6
This works because you can simply apply the logical index mask
to the columns, selecting all the rows with :
.
Even when a 2-D logical array is used for an input, the output will be a column array of indexed values. This is because there is no guarantee that the indexed elements can be organized into a 2-D (i.e. rectangular) output. Consider if your 2-D mask were the following:
mask = [true false true; true false false];
This would index 3 values, which can't be organized into anything but a row or column vector for the output. Here's another example:
mask = [true true true; true false false];
This would index 4 values, but 3 are from the first row and 1 is from the second row. How should these values be shaped into a rectangular output matrix? Since there's no clear way to do this in general for an arbitrary 2-D index matrix, a column vector of indexed values is returned.
Upvotes: 12