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I'm currently migrating code from R2012a to R2013b.
I noticed that the unique function behavior has changed:
R2012a
>> size(unique([]))
ans =
0 0
R2013b
>> size(unique([]))
ans =
0 1
It seems counter-intuitive to me that a 0x0 matrix would become a 0x1 matrix after removing doublons, which is essentially what the unique function does. Does anybody has a rationale for this?
631
Use unique to find the unique elements in the concatenated vector [x;y] . The unique function performs exact comparisons and determines that some values in x are not exactly equal to values in y . These are the same elements that have a nonzero difference in x-y . Thus, c contains values that appear to be duplicates.
The behavior of accumarray is similar to the functions groupsummary and groupcounts for computing summary statistics by group and counting the number of elements in a group, respectively. For more grouping functionality in MATLAB®, see Preprocessing Data.
Efficient Approach: Using Hashing, we can store the key as a string containing all the characters in that row and its frequency as the value. And traverse all the rows in the map and if its frequency is 1 then count it as unique.
You can find the distinct values in an array using the Distinct function. The Distinct function takes the array as an input parameter and returns another array that consists only of the unique, or non-duplicate, elements. The following example shows how to find the distinct values in an array.
The behaviour has changed with R2013a, if you need the old behaviour use:
size(unique([],'legacy'))
If you need code for both versions, I would recommend to write some function which calls unique(x,'legacy') for new versions and unique(x) for old versions.
btw: same issue with union, intersect, setdiff, setxor and ismember
170
I don't know whether this is the reason, but it does come with an advantage.
Now you will see that unique(M) gives the same output as unique(M(:)), even if M is empty.
Example:
M = magic(5);
isequal(size(unique(M)), size(unique(M(:))));
M = [];
isequal(size(unique(M)), size(unique(M(:))));
The latter returns false on old versions of matlab, this may be confusing.
31
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