Question

How do you perform such summation in R?

sum_{i=1}^3 (x^2)

i=1 is lower bound
i=3 is upper bound
x^2 is the operation

So we will perform

1^2 + 2^2 + 3^2

Using standard loop:

tot <-0
for (x in 1:3) {
  tot <- tot + x^2
}

Answer 1

First, I’ll point out that to generate a vector containing the elements 1,2,3 you can do:

x <- 1:3

Secondly, R is a vectorised language – meaning if x is a vector and I do x + 5 it’ll add 5 to each element of x for me without needing a for loop.

# Recalling that "x <- x + 5" is the same as
for ( i in 1:length(x) ) {
    x[i] <- x[i] + 5
}
# try to do something that makes  x squared, i.e. x == c(1,4,9).

Thirdly, look at ?sum, whereby sum(x) adds up all the elements in x.