R summing factors

Question

I split some data according to factors like:

 a <- factor(data$fact)
 b <- split(data,a)

Now I would like to add some of the factor together e.g.

tot <- b$A+b$B

but I'm getting the following error,

"sum" not meaningful for factors

Any help would be great.

aaa val1 val2 ...
aaa
bbb
bbb
ccc
ccc

Now if I split into factors I have three. But I want for example aaa and ccc to be considered together. This meas that the value in the other column should be summed up.

Thanks

Answer 1

Create a new factor variable before splitting:

# Make up some data
df = data.frame(Cases = sample(LETTERS[1:5], 10, replace=TRUE),
                Set1 = 1:10, Set2 = 11:20)
# Duplicate your cases column
df$Cases_2 = df$Cases
# Create a new set of factor levels
levels(df$Cases_2) <- ifelse(levels(df$Cases_2) %in% c("A","B"), 
                             "AB", levels(df$Cases_2))
temp = split(df[-c(1, 4)], df$Cases_2)
temp
# $AB
#   Set1 Set2
# 3    3   13
# 5    5   15
# 6    6   16
# 8    8   18
# 
# $C
#   Set1 Set2
# 4    4   14
# 9    9   19
# 
# $D
#    Set1 Set2
# 2     2   12
# 7     7   17
# 10   10   20
# 
# $E
#   Set1 Set2
# 1    1   11

Then use lapply to calculate colSums:

lapply(temp, colSums)
# $AB
# Set1 Set2 
#   22   62 
# 
# $C
# Set1 Set2 
#   13   33 
# 
# $D
# Set1 Set2 
#   19   49 
# 
# $E
# Set1 Set2 
#    1   11