MATLAB: Use rowfun to sum multiple columns by group

Question

I have a table that looks like this:

Data = 
date            id    flag1    flag2
2019-01-01      x1    1        1
2019-01-01      x1    1        0
2019-01-02      x2    0        0
2019-01-02      x2    0        1
...

The line below will sum "flag1" by "date":

rowfun(@sum, Data, 'groupingVariables','date','inputVariables','flag1')

But how do I apply this to both "flag1" and "flag2"? I tried:

rowfun(@sum, Data, 'goupingVariables','date','inputVariables',{'flag1','flag2'})

…and it throws out the following error:

Dimension argument must be a positive integer scalar within indexing range

Answer 1

rowfun is not the correct function for this. rowfun applies the function by rows and consider the input variables as separate input arguments to the function. Indeed the 2nd argument to sum is the dimension to operate along so you get an error since your 2nd variable does not contain valid dimensions.

You want to apply the same function to the different variables of your table. The function for that is varfun:

varfun(@sum, Data, 'GroupingVariables', 'date', 'InputVariables', {'flag1','flag2'})

However, note that since R2018a we have groupsummary which is even easier to use and allows you to get several statistics at the same time:

groupsummary(Data, 'date', 'sum', {'flag1', 'flag2'})

is the equivalent to the above, but you could also do:

groupsummary(Data, 'date', 'weekly', {'sum', 'mean', 'std'}, {'flag1', 'flag2'})

to get weekly sum, mean and standard deviation at once.