如何遍历数据集减去下面一行中的值-使用R

Question

I need help with the below question in R language.

Lets say I have a data set:

X   Y
1   1
2   2
3   3
4   4

How would I go about looping through the dataset subtracting the X value in the current row from the X value in the row below, then going to the second row etc?

Currently I have the below:

df <- (df[row(df)-1,1] - df[row(df)+1,1])

I would like to get the following:

X
-1
-1
-1
N/a

However it seems to being doing the calculations twice and I am getting?

 X
-1
-1
-1
N/a
-1
-1
-1
N/a

I cant figure out why, any help would be appreciated?

Answer 1

As @Sotos pointed out, you can solve this with diff .

But the reason this isn't working is because row() returns row numbers for both columns

> row(df)
       [,1] [,2]
 [1,]    1    1
 [2,]    2    2
 [3,]    3    3
 [4,]    4    4

If you select either column alone your code works:

df <- (df[row(df)[,1]-1,1] - df[row(df)[,1]+1,1])

Answer 2

You could use diff() . Also you could do it using a matrix multiplication approach.

Example

set.seed(42)
x <- sample(10, 10, replace=TRUE)
> x
[1] 10 10  3  9  7  6  8  2  7  8

> diff(x)
[1]  0 -7  6 -2 -1  2 -6  5  1

# difference matrix approach
lbd <- matrix(0, nrow=length(x) - 1, ncol=length(x))  # setup lambda
diag(lbd) <- -1
diag(lbd[, -1]) <- 1

> lbd
      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
 [1,]   -1    1    0    0    0    0    0    0    0     0
 [2,]    0   -1    1    0    0    0    0    0    0     0
 [3,]    0    0   -1    1    0    0    0    0    0     0
 [4,]    0    0    0   -1    1    0    0    0    0     0
 [5,]    0    0    0    0   -1    1    0    0    0     0
 [6,]    0    0    0    0    0   -1    1    0    0     0
 [7,]    0    0    0    0    0    0   -1    1    0     0
 [8,]    0    0    0    0    0    0    0   -1    1     0
 [9,]    0    0    0    0    0    0    0    0   -1     1

> lbd %*% x  # matrix multiplication, same result as in `diff(x)` above
      [,1]
 [1,]    0
 [2,]   -7
 [3,]    6
 [4,]   -2
 [5,]   -1
 [6,]    2
 [7,]   -6
 [8,]    5
 [9,]    1  

Using your data:

x1 <- 1:4
lbd1 <- matrix(0, nrow=length(x1) - 1, ncol=length(x1))
diag(lbd1) <- -1
diag(lbd1[, -1]) <- 1

> lbd1 %*% x1
     [,1]
[1,]    1
[2,]    1
[3,]    1

> diff(x1)    # same
[1] 1 1 1