R how can I calculate difference between rows in a data frame

Question

Here is a simple example of my problem:

> df <- data.frame(ID=1:10,Score=4*10:1)
> df
       ID Score
    1   1    40
    2   2    36
    3   3    3         


        

Answer 1

Because df works on vector or matrix. You can use apply to apply the function across columns like so:

 apply( df , 2 , diff )
   ID Score
2   1    -4
3   1    -4
4   1    -4
5   1    -4
6   1    -4
7   1    -4
8   1    -4
9   1    -4
10  1    -4

It seems unlikely that you want to calculate the difference in sequential IDs, so you could choose to apply it on all columns except the first like so:

apply( df[-1] , 2 , diff )

Or you could use data.table (not that it adds anything here I just really want to start using it!), and I am again assuming that you do not want to apply diff to the ID column:

DT <- data.table(df)
DT[ , list(ID,Score,Diff=diff(Score))  ]
    ID Score Diff
 1:  1    40   -4
 2:  2    36   -4
 3:  3    32   -4
 4:  4    28   -4
 5:  5    24   -4
 6:  6    20   -4
 7:  7    16   -4
 8:  8    12   -4
 9:  9     8   -4
10: 10     4   -4

And thanks to @AnandaMahto an alternative syntax that gives more flexibility to choose which columns to run it on could be:

DT[, lapply(.SD, diff), .SDcols = 1:2]

Here .SDcols = 1:2 means you want to apply the diff function to columns 1 and 2. If you have 20 columns and didn't want to apply it to ID you could use .SDcols=2:20 as an example.