R, data frame manipulation

Question

I have the following data frame in R.

X1 <- c(451,2,6,2,7)
X2 <- c(0,1,6,3,4)
df <- data.frame(X1, X2)

I would like to add two extra columns (X3 and X4), where the first row is mirrored (X3=X2 and X1=X4), and from the second row onwards, X3 equals X4 from the previous row plus one, and X4 equals X3 (same row) plus X1 (same row).

The final output should look like this

X1 <- c(451,2,6,2,7)
X2 <- c(0,1,6,3,4)
X3 <- c(0,452,455,462,465)
X4 <- c(451,454,461,464,472)
df <- data.frame(X1, X2, X3, X4)

Do you have any suggestions on how to do this in R?

Answer 1

You could do:

a <- df$X1
a[1] <- a[1] - 1
b <- cumsum(a+1)

transform(df, X3 = c(X2[1], head(b+1, -1)), X4 = b)
  X1 X2  X3  X4
1 451  0   0 451
2   2  1 452 454
3   6  6 455 461
4   2  3 462 464
5   7  4 465 472

Answer 2

Although there are much more efficient ways of going about this, writing a for loop is also a good idea:

X3 <- rep(df1$X2[1], nrow(df))
X4 <- c(df$X1[1], rep(0, nrow(df) - 1))

for(i in 2:nrow(df)) {
  X3[i] <- X4[i - 1] + 1
  X4[i] <- X3[i] + df$X1[i]
}

cbind(df, as.data.frame(cbind(X3, X4)))

   X1 X2  X3  X4
1 451  0   0 451
2   2  1 452 454
3   6  6 455 461
4   2  3 462 464
5   7  4 465 472