Sum columns by group (row names) in a matrix

Question

Let\'s say I have a matrix called x.

x <- structure(c(1, 0, 1, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1), 
.Dim = c(5L, 4L), .Dimname         


        

Answer 1

An approach using plyr:

ldply(split(df, df$Name), function(u) colSums(u[-1]))
#   .id Mon Tue Wed Thurs
#1 Cake   2   1   1     2
#2  Pie   0   0   3     3

Data:

df = structure(list(Name = structure(c(1L, 2L, 1L, 2L, 2L), .Label = c("Cake", 
"Pie"), class = "factor"), Mon = c(1L, 0L, 1L, 0L, 0L), Tue = c(0L, 
0L, 1L, 0L, 0L), Wed = c(1L, 1L, 0L, 1L, 1L), Thurs = c(1L, 1L, 
1L, 1L, 1L)), .Names = c("Name", "Mon", "Tue", "Wed", "Thurs"
), row.names = c(NA, -5L), class = "data.frame")