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

Here's a solution using stack(), and aggregate(), although it requires the second data.frame contain character vectors, as opposed to factors (could be forced with lapply(df2,as.character)):

df1 <- data.frame(a=c(8,1,2,5), b=c(12,6,5,3), c=c(5,4,4,7), d=c(14,3,11,2) );
df2 <- data.frame(a=c('A','B','C','D'), b=c('B','C','D','E'), c=c('C','D','E','F'), d=c('D','E','F','G'), stringsAsFactors=F );
aggregate(sum~group,data.frame(sum=stack(df1)[,1],group=stack(df2)[,1]),sum);
##   group sum
## 1     A   8
## 2     B  13
## 3     C  13
## 4     D  28
## 5     E  10
## 6     F  18
## 7     G   2