Reply to: https://twitter.com/emjonaitis/status/509400828437098497

indexing.R

# Tweet from Erin Jonaitis:
# Hey nerds. y[x==1] is shorthand for "elements of y where the corresponding x
# is 1." What if I want y[i] such that x[i+e]==1?

set.seed(42)

# random data for example
y <- round(rnorm(30), 2)
x <- sample(1:3, size = 30, replace = TRUE)

# view the two vectors together
dat <- rbind(y, x)
dat

# simple index of y
y[x == 1]

# find the index of the x vector which equals 1
idx <- which(x == 1)
dat[, idx]

# solve for i, assuming that e is a vector of lenght 1
# idx = i + e
# i = idx - e
e <- 3
i <- idx - e 

dat[, i]

rbind(dat[, i], 
      dat["x", i + e])