Matlab: Use Splitapply to write multiple files - matlab

Question

I have grouped tables by a variable and I am trying to write multiple files based on the grouping variable. But it does not work.
I used findgroups and splitapply, but the splitapply is where I am having problems.
Here is one version of the commands I am using:
load patients;
G=findgroups(Gender);
func=#(x,y) csvwrite(x,y);
splitapply(func,Gender,Weight,G);
I am getting the following error message:
Error using splitapply (line 132)
Applying the function '#(x,y)csvwrite(x,y)' to the 1st group of data generated the following error:
FILENAME must be a character vector or string scalar.
When I figure out how to use this, I will be using it on large datastore tall arrays. Please help !

Answer 1

The problem is that the first parameter of csvwrite must be a file name.
In your code sample, the first parameter to csvwrite is a cell array, and not a string.
You can see it by using the following trick:
func=#(x,y) display(x);
The output of splitapply(func,Gender,Weight,G) is:
x =
53×1 cell array
{'Female'}
{'Female'}
{'Female'}
...
x =
47×1 cell array
{'Male'}
{'Male'}
{'Male'}
Solution:
Use x{1} instead of x:
func=#(x,y) csvwrite(x{1}, y);
It's recommended to add file extension like .txt to file name:
func=#(x,y) csvwrite([x{1}, '.txt'], y);
Remark:
It's possible that the combination of splitapply with csvwrite misses the original intent of splitapply function.
According to documentation it looks like splitapply is better fitted for statistical calculations (and not intended to be used for I/O operations [writing files]).
I am not sure whether the above code pattern is the right way for "using it on large datastore tall arrays".
Complete code sample:
load patients;
G=findgroups(Gender);
%The first parameter of csvwrite must be a file name.
%x{1} = 'Male' for all the Male group, and 'Female' for all the Female group.
%[x{1}, '.txt'] adds a '.txt' extension to the file name.
%
%y will be array of Weight like
%[71
% 69
% 68]
func=#(x,y) csvwrite([x{1}, '.txt'], y);
splitapply(func,Gender,Weight,G)