[R] Summing data frame columns on identical data

[Steve Murray]

Dear all,

I have 9 data frames, and I'm simply trying to sum the values of column 3 (on a row-by-row basis). However, there are a slightly different number of rows in each data frame, so I'm receiving the following error: "Error in Ops.data.frame(mrunoff_207101[3], mrunoff_207102[3]) : 
  + only defined for equally-sized data frames".

Here is what I'm attempting to do:

> arunoff_2071 <- cbind(mrunoff_207101[1:2], (mrunoff_207101[3] + mrunoff_207102[3] + mrunoff_207103[3] + mrunoff_207104[3] + mrunoff_207105[3] + mrunoff_207106[3] + mrunoff_207107[3] + mrunoff_207108[3] + mrunoff_207109[3]))


Is there an easy way of summing based on congruent values in columns 1 and 2? The only way I can think of would be to use merge, but this would involve doing this for every pair of data frames.

The data for each data frame look like this:

> head(mrunoff_207101)
  Latitude Longitude          FPC
1     5.75      0.25 0.0112384744
2     6.25      0.25 0.0019959067
3     6.75      0.25 0.0003245941
4     7.25      0.25 0.0011973676
5     7.75      0.25 0.0001062602
6     8.25      0.25 0.0451578423


Any suggestions on how to achieve this easily will be very welcome.

Many thanks,

Steve