How to find the difference in value in every two consecutive rows in R?

Question

I have a big table of total 969 rows and I need to find the difference between every two rows, e.g. row1 and row2, row2 and row3, row3 and row4 etc. How can I do that? I was told to do it by the command diff() but I have no idea where to start.

Answer 1

Here's an example of how to use diff() on the built-in mtcars data.frame. You have to select a column to perform the diff over:

mtcars
                     mpg cyl  disp  hp drat    wt  qsec vs am gear carb
Mazda RX4           21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4
Mazda RX4 Wag       21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4
Datsun 710          22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1
Hornet 4 Drive      21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1
[..snip..]
Ford Pantera L      15.8   8 351.0 264 4.22 3.170 14.50  0  1    5    4
Ferrari Dino        19.7   6 145.0 175 3.62 2.770 15.50  0  1    5    6
Maserati Bora       15.0   8 301.0 335 3.54 3.570 14.60  0  1    5    8
Volvo 142E          21.4   4 121.0 109 4.11 2.780 18.60  1  1    4    2

Calculate the successive differences of e.g. the column "qsec":

diff(mtcars$qsec)
 [1]  0.56  1.59  0.83 -2.42  3.20 -4.38  4.16  2.90 -4.60  0.60 -1.50  0.20
[13]  0.40 -0.02 -0.16 -0.40  2.05 -0.95  1.38  0.11 -3.14  0.43 -1.89  1.64
[25]  1.85 -2.20  0.20 -2.40  1.00 -0.90  4.00

Answer 2

You could simply subtract a data.frame consisting of rows 1:(n-1) of the original data.frame from a second one consisting of rows 2:n. (Here n is the number of rows in the original data.frame):

# Example data
df <- data.frame(a=1:4, b=4:1, c=11:14, d=c(2,4,10,0))
#   a b  c  d
# 1 1 4 11  2
# 2 2 3 12  4
# 3 3 2 13 10
# 4 4 1 14  0

# Calculate the differences
diff_df <- df[-1,] - df[-nrow(df),]
diff_df
#   a  b c   d
# 2 1 -1 1   2
# 3 1 -1 1   6
# 4 1 -1 1 -10

You can then rename the rows as you see fit using something like:

row.names(diff_df) <- paste("d", seq_len(nrow(diff_df)), sep="")
diff_df
#    a  b c   d
# d1 1 -1 1   2
# d2 1 -1 1   6
# d3 1 -1 1 -10