Sum all elements in a vector except the ith element without for-loop

Question

As the title says. I have a vector and I want to sum all elements in the vector, except the ith element. I can achieve this with a for-loop e.g.

# vector to iterate over
v1 <- 1:5

# somewhere to store results
v2 <- c()

for (i in seq_along(v1)) {
v2[i] <-  sum(v1[-i])  
}

# desired output
v2
[1] 14 13 12 11 10

I am looking for performance gains, Is it possible to achieve v2 without a for-loop via vectorisation?

user_6851825 · Answer

This can be done without a loop using:

sum(v1) - v1

This will, for all elements of v1, provide the sum of all of v1 excluding that element. Since it's a vectorized approach, it will be faster than a loop. https://www.noamross.net/archives/2014-04-16-vectorization-in-r-why/

Sum all elements in a vector except the ith element without for-loop

1 Answers