r - Adding a column to a ragged array matrix by grouping variables -

in r:

i not sure proper title question is, maybe can me out. appreciated. i'm sorry if called searchable.

so have ragged array matrix (multiple upcs)

     [upc]    [quantity]          [day]    [daysum] [1]  123         11                 1         na      [2]  123          2                 1         na  [3]  789          5                 1         na  [4]  456         10                 2         na  [5]  789          6                 2         na  

i want matrix summed upc each day, example:

    [upc]    [quantity]          [day]    [daysum] [1]  123         11                 1         13 [2]  123          2                 1         13 [3]  789          5                 1         5 [4]  456         10                 2         10 [5]  789          6                 2         6 

thank time , help.

you have not described supposed happen "clean matrix" code create "column" larger matrix suitable binding on row-aligned basis quite simple:

 b <- cbind(b, daysum=ave(b[, 'quantity'],   # analysis variable                           b[, 'upc'], b[ , 'day'], # strata variables                           fun=sum)  )     # function applied in strata 

this of course assumes b has column names indicated. should work if dataframe, although output not suggest have r objects yet. ave function replicate sums rows same stratification variables.