split, apply and combine on 2 columns of data

Question

I've got a dataframe consisting of a group and 2 value columns, as such:

group  val1  val2
    A     5     3
    A     2     4
    A     3     1
    B     3     6
    B     2     1
    B     0     2

I want to work out the number of rows where val1 > val2, split by subset. Initially I hardcoded this per subgroup with:

number_a <- nrow(subset(df, group=="A" & val1 > val2))
number_b <- nrow(subset(df, group=="B" & val1 > val2))

What's the proper way of automating this? I tried using the split() function but I couldn't work out how to pass in both val1 and val2 column.

Answer 1

Pretty straight forward using data.table

If you want the number of rows

library(data.table)
setDT(df)[, .(RowsNum = sum(val1 > val2)), by = group]
#    group RowsNum
# 1:     A       2
# 2:     B       1

If you looking for split, apply combinations in base R, could also try

sapply(split(df[-1], df[1]), function(x) sum(x[1] > x[2]))
# A B 
# 2 1 

Or using tapply (also from base R)

tapply(with(df, val1 > val2), df[1], sum)
# group
# A B 
# 2 1 

---

If you want the rows themselves

setDT(df)[, .SD[val1 > val2]]
#    group val1 val2
# 1:     A    5    3
# 2:     A    3    1
# 3:     B    2    1

Or very simple with base R too

df[with(df, val1 > val2), ]
#    group val1 val2
# 1     A    5    3
# 3     A    3    1
# 5     B    2    1

Or

subset(df, val1 > val2)
#   group val1 val2
# 1     A    5    3
# 3     A    3    1
# 5     B    2    1