How to sum over diagonals of data frame

Question

Say that I have this data frame:

     1   2   3   4      
100  8   12  5   14 
99   1   6   4   3   
98   2   5   4   11  
97   5   3   7   2   

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Answer 1

You can use row() and col() to identify row/column relationships.

m <- read.table(text="
    1   2   3   4      
100  8   12  5   14 
99   1   6   4   3   
98   2   5   4   11  
97   5   3   7   2")

vals <- sapply(2:8,
       function(j) sum(m[row(m)+col(m)==j]))

or (as suggested in comments by ?@thelatemail)

vals <- sapply(split(as.matrix(m), row(m) + col(m)), sum)
data.frame(group=LETTERS[seq_along(vals)],sum=vals)

or (@Frank)

data.frame(vals = tapply(as.matrix(m), 
       (LETTERS[row(m) + col(m)-1]), sum))

as.matrix() is required to make split() work correctly ...