Jim Slonder - 2 months ago 14

R Question

In R, I'm trying to make a simple function like the one below, just summing the elements in the row of a data frame which are k positions away the (i,j) element. If the element is on the edge (e.g. j=1 or j=n) I'd like for the element to the left or right which doesn't exist to be treated as 0. But with my current function I end up with an error if the element to the right doesn't exist, or a vector if the one on the left doesn't exist due to R's behavior with negative indices. Is there a nicer way to write this function without just using if statements to deal with the three cases (element is in the middle, too far left, or too far right)?

`sum_nearby <- function(dat, i, j, k) {`

dat[i, j - k] + dat[i, j + k]

}

Edit: I figured out a solution. Just pad the dataframe with zeros.

`sum_nearby <- function(dat, i, j, k) {`

for(i in 1:k) dat <- cbind(0,dat,0)

dat[i, j] + dat[i, j + 2*k]

}

Answer

You can do

```
sum_nearby <- function(dat, i, j, k) {
left <- max(1, j - k)
right <- min(j + k, ncol(dat))
dat[i, left] + dat[i, right]
}
```

This means that close to the boundary, the k-neighbourhood is not symmetric.

Let's consider a simplified case / example with a vector:

```
f <- function (x, j, k) {
left <- max(1, j - k)
right <- min(j + k, length(x))
x[left] + x[right]
}
```

Say

```
x <- seq(2, 10, by = 2)
# [1] 2 4 6 8 10
```

Let's test the summation effect for all elements with `k = 2`

:

```
sapply(1:5, f, k = 2, x = x)
# [1] 8 10 12 14 16
```

- The first
`8`

is actually`x[1] + x[3]`

, instead of`x[-1] + x[3]`

. - The second
`10`

is`x[1] + x[4]`

, rather than`x[0] + x[4]`

.

If you simply want to ignore those "out-of-bound" values, use an `if`

:

```
sum_nearby <- function(dat, i, j, k) {
if (j - k < 0) dat[i, j + k]
else if (j + k > ncol(dat)) dat[i, j - k]
else dat[i, j + k] + dat[i, j - k]
}
```