KnapSack dynamic programming in R with recursive function - r

Question

I created this simple code in R to solve the Knapsack program with a recursive funtion
n <- c(0,1,2,3,4)
v <- c(10,40,30,50)
w <- c(5,4,6,3)
k <- 10
myfunction <- function(n,k){
if (n==0 | k==0){
output <- 0
} else if (w[i] > k) {
output <- myfunction[i-1,w]
} else {
output <- max(v[i]+ myfunction(i-1, k-w[i]),myfunction(i-1,k))
}
return(myfunction)
}
However, I don't get a value as an output, but the whole function. For example if I put in:
myfunction(4,10)
I don't get an value of 90, but the whole funtion typed out.
these are the values

Answer 1

There were several errors beyond the ones pointed out by #etienne. Here's an annotated debugging session. First we fix the returned object:
> myfunction <- function(n,k){
+ if (n==0 | k==0){
+ output <- 0
+ } else if (w[i] > k) {
+ output <- myfunction[i-1,w]
+ } else {
+ output <- max(v[i]+ myfunction(i-1, k-w[i]),myfunction(i-1,k))
+ }
+ return(output)
+ }
> myfunction(4,10)
Error in if (w[i] > k) { : argument is of length zero
Obviously neither w nor k are of length zero which suggests it must be i. (As also pointed out by etienne). Looking at your code it appears you actually intended i to be the index that decreased until the terminating condition was met. So replace n by i in the few instances where it appeared:
> myfunction <- function(i,k){
+ if (i==0 | k==0){
+ output <- 0
+ } else if (w[i] > k) {
+ output <- myfunction[i-1,w]
+ } else {
+ output <- max(v[i]+ myfunction(i-1, k-w[i]),myfunction(i-1,k))
+ }
+ return(output)
+ }
> myfunction(4,10)
Error in myfunction[i - 1, w] :
object of type 'closure' is not subsettable
So you also made the mistake of using square-brackets where parentheses (aka bracket in the non-US sections of the world) were needed:
> myfunction <- function(i,k){
+ if (i==0 | k==0){
+ output <- 0
+ } else if (w[i] > k) {
+ output <- myfunction(i-1,w)
+ } else {
+ output <- max(v[i]+ myfunction(i-1, k-w[i]),myfunction(i-1,k))
+ }
+ return(output)
+ }
> myfunction(4,10)
[1] 90
Success, well, almost. Most of the warnings are because you used | instead of || in one of the conditionals:
Warning messages:
1: In if (i == 0 | k == 0) { :
the condition has length > 1 and only the first element will be used
2: In if (w[i] > k) { :
the condition has length > 1 and only the first element will be used
3: In if (i == 0 | k == 0) { :
the condition has length > 1 and only the first element will be used
4: In if (i == 0 | k == 0) { :
the condition has length > 1 and only the first element will be used
5: In if (i == 0 | k == 0) { :
the condition has length > 1 and only the first element will be used
6: In if (i == 0 | k == 0) { :
the condition has length > 1 and only the first element will be used
So replace that instance with a logical ||. To deal with the other warning that didn't seem to sabotage your logic, realize that w[i] is length-0 when i == 0, so add a logical clause in the conditional that first tests for that possibility and use the correct "double-AND-sign" ( && ):
myfunction <- function(i,k){
if (i==0 || k==0){
output <- 0
} else if (length( w[i]) && w[i] > k) {
output <- myfunction(i-1,w)
} else {
output <- max(v[i]+ myfunction(i-1, k-w[i]), myfunction(i-1,k))
}
return(output)
}
Now you get:
> myfunction(4,10)
[1] 90