Trouble with R and vector length NA values appended - r

Question

I've looked this over and I can't quite understand why this is giving me NA values appended onto the vector I want. Prompt below:
"The function should return a vector where the first element is the sum of the first n elements of the input vector, and the rest of the vector is a copy of the other elements of the input vector. For example, if the input vector is (2, 3, 6, 7, 8) and n = 2, then the output should be the vector (5, 6, 7, 8)"
testA<- c(1,2,3,4,5)
myFunction <- function(vector1, n)
{
sum1=0
for(i in 1:n)
{
sum1<-sum1+vector1[i]
newVector<-c(sum1,vector1[n+1:length(vector1)])
}
return(newVector)
}
print(newVector)
myFunction(testA, 3)
Output is: [1] 6 4 5 NA NA NA when it should just be 6 4 5

Answer 1

There is no need for a for loop here; you can do something like this
test <- c(2, 3, 6, 7, 8)
myfunction <- function(x, n) c(sum(x[1:n]), x[-(1:n)])
myfunction(test, 2)
#[1] 5 6 7 8
testA <- c(1,2,3,4,5)
myfunction(testA, 3)
#[1] 6 4 5
Explanation: sum(x[1:n]) calculates the sum of the first n elements of x and x[-(1:n)] returns x with the first n elements removed.

Answer 2

It can be done with head and tail
n <- 2
c(sum(head(test, 2)), tail(test, -2))
#[1] 5 6 7 8
data
test <- c(2, 3, 6, 7, 8)

Answer 3

Here I try to compare the efficiency of above two functions, which are answer post https://stackoverflow.com/a/52472214/3806250 with the question post.
> testA <- 1:5
> myFunction <- function(vector1, n) {
+ sum1 <- 0
+ for(i in 1:n) {
+ sum1 <- sum1 + vector1[i]
+ newVector <- c(sum1, vector1[n+1:length(vector1)])
+ }
+ newVector <- newVector[!is.na(newVector)]
+ return(newVector)
+ }
>
> microbenchmark::microbenchmark(myFunction(testA, 3))
Unit: microseconds
expr min lq mean median uq max neval
myFunction(testA, 3) 3.592 4.1055 77.37798 4.106 4.619 7292.85 100
>
> myfunction <- function(x, n) c(sum(x[1:n]), x[-(1:n)])
>
> microbenchmark::microbenchmark(myfunction(testA, 2))
Unit: microseconds
expr min lq mean median uq max neval
myfunction(testA, 2) 1.539 1.54 47.04373 2.053 2.053 4462.644 100

Answer 4

Thank you for everyone's answers! I was really tired last night and couldn't come up with this simple solution:
function(vector1, n)
{
sum1=0
for(i in 1:n) #scans input vector from first element to input 'n' element
{
sum1<-sum1+vector1[i]#Find sum of numbers scanned
newVector<-c(sum1,vector1[n+1:length(vector1)])#new output vector starting with the sum found then concatonates rest of the original vector after 'n' element
length(newVector)<-(length(newVector)-(n)) #NA values were returned, length needs to be changed with respect to 'n'
}
return(newVector)
print(newVector)
}

Answer 5

There are already great solutions, but here is another option which does minimum modifications on you original code:
testA<- c(1,2,3,4,5)
myFunction <- function(vector1, n)
{
sum1=0
for(i in 1:n)
{
sum1<-sum1+vector1[i]
}
newVector<-c(sum1,vector1[(n+1):length(vector1)]) # we take this line out of the for loop
# and put the n+1 in between parenthesis
return(newVector)
}
newVector <- myFunction(testA, 3)
print(newVector)
The problem on the original code/example was that n+1:length(vector1) was supossed to return [1] 4 5, in order to do the appropiate subsetting (obtaining the last elements in the vector which weren't included in the sum of the first n elements), but it is actually returning [1] 4 5 6 7 8. Since there are no elements in positions 6:8 in testA, this is the reason why there are appearing missing values/NAs.
What n+1:length(vector1) is actually doing is first obtaining the secuence 1:length(vector1) and then adding n to each element. Here is an example of this behaviour using values:
3+1:5
#> [1] 4 5 6 7 8
We can solve this by putting n+1 between parenthesis on the original code. In our example using values:
(3+1):5
#> [1] 4 5
Also, taking the assignment of newVector out of the loop improves performance, because the binding between sum1 and the subsetted vector only needs to be done once the sum of the first n elements is completed.