MATLAB Answers

How do I use splitapply when the applied function has non-scalar output?

3 views (last 30 days)
clauper
clauper on 6 Dec 2019
Edited: dpb on 7 Dec 2019
I am trying clean variables in a dataset, creating a new table variable, but splitapply does not seam to allow for nonscalar outputs of the function.
As a minimal working example, I have a table
myTable = table();
myTable.Gender = ["Male","Female","Male","Female"]';
myTable.Var1 = [1,2,3,4]';
And I am now trying to create a new table variable where the group mean is subtracted from "Var1" (yielding Var2 = [-1, -1, 1, 1])
When using splitapply(), I get the error
Error using splitapply (line 132)
The function '...' returned a non-scalar value when applied to the 1st group of data.
Is there a way to do this, also for function more involved than demeaning the input variables?
Thanks in advance,
Chris

  0 Comments

Sign in to comment.

Accepted Answer

dpb
dpb on 6 Dec 2019
Edited: dpb on 7 Dec 2019
OK, I reread the Q? and realize there was enough in it to see what it was that was wanted...
>> t=myTable; % shorter name; I'm lazy... :)
>> t.Gender=categorical(t.Gender); % gender is a categorical variable; treat it as such
>> fnMN=@(v) {v-mean(v)}; % have to encapsulate the return values as cell
>> splitapply(fnMN,t.Var1,g) % show what that returns is the cell array by group
ans =
2×1 cell array
{2×1 double}
{2×1 double}
>> cell2mat(ans) % and convert to the array form to see what values are...
ans =
-1
1
-1
1
>>
Now, the cell array is by group so to convert the cell array positioning to position in table :
grpmn=splitapply(fnMN,t.Var1,g); % the group mean-adjusted values
for i=1:numel(grpmn) % number of groups (2 here)
t.Var2(i==g)=grpmn{i}; % spread cell values over rows in table
end
Above results in:
>> t
t =
4×3 table
Gender Var1 Var2
______ ____ ____
Male 1 -1
Female 2 -1
Male 3 1
Female 4 1
>>

  8 Comments

Show 5 older comments
dpb
dpb on 7 Dec 2019
I see I inadvertentedly didn't edit out the cell2mat that I had used initially before it dawned on me the order wasn't correct that way. I originally posted an ERRATUM on that answer but had meeting in town at the time had to go to so didn't have time to fix it then...sorry.
Glad you caught it. Corrected rest of original...
clauper
clauper on 7 Dec 2019
Glad you corrected it!
As a final remark, we should loop over
for i = 1 : numel(grpmn)
since numel(g)==4 is not correct.
Thanks again!
dpb
dpb on 7 Dec 2019
Ah so! I had changed variable names in command window and didn't catch one in the forum code...thanks.

Sign in to comment.

More Answers (1)

dpb
dpb on 6 Dec 2019
[A,B,C,...]=splitapply(func, ...);
applies func to inputs by group and returns the given outputs...
See the doc for details, examples.
The function uses the group inputs for each call and outputs M (scalar) variables. Each output can be a different type/class, but must be consistent from call to call for each output.

  1 Comment

clauper
clauper on 6 Dec 2019
Dear dpb
Thanks for your answer.
Your solution allows for having multiple vectors as inputs. It still does not allow 'func' to have non-scalar output (same error).
Thanks anyways, Chris

Sign in to comment.

Sign in to answer this question.

Products


Release

R2019a