Find all m-tuples that sum to n - r

Question

I want to find ALL the non-negative integer solutions to the equation i+j+k+l+m=n where n is a non-negative integer. That is, I want to find all possible 5-tuples (i,j,k,l,m) with respect to a certain n, in R.
I wrote a code which is not working. I am suspicious there is something wrong in the looping.
For your convenience, I have taken n=3, so I am basically trying to compute all vectors (i,j,k,l,m) which are 35 in number, and the matrix a(35 by 5) is the matrix that is supposed to display those vectors. The whole thing is in the function "sample(n)", where if I put n=3 i.e. sample(3) when called will give me the matrix a. Please note that a (35 by 5) is defined beforehand with all entries 0.
sample=function(n){
i=0
j=0
k=0
l=0
m=0
for(p in 1:35){
while(i<=3){
while(j<=3){
while(k<=3){
while(l<=3){
m=n-(i+j+k+l)
if(m>-1){
a[p,]=c(i,j,k,l,m)
}
l=l+1}
k=k+1}
j=j+1}
i=i+1}
}
return(a)
}
When I call sample(3), I get my original a i.e. the matrix with all elements 0. What is wrong with this code? Please rectify it.

Answer 1

I don't think a brute-force approach will bring you much joy for this task. Instead you should look for existing functions that can be used and are efficient (i.e. implemented in C/C++).
n <- 3
library(partitions)
blockparts(rep(n, 5), n)
#[1,] 3 2 1 0 2 1 0 1 0 0 2 1 0 1 0 0 1 0 0 0 2 1 0 1 0 0 1 0 0 0 1 0 0 0 0
#[2,] 0 1 2 3 0 1 2 0 1 0 0 1 2 0 1 0 0 1 0 0 0 1 2 0 1 0 0 1 0 0 0 1 0 0 0
#[3,] 0 0 0 0 1 1 1 2 2 3 0 0 0 1 1 2 0 0 1 0 0 0 0 1 1 2 0 0 1 0 0 0 1 0 0
#[4,] 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 2 2 2 3 0 0 0 0 0 0 1 1 1 2 0 0 0 1 0
#[5,] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 3

Answer 2

I believe that your code isn't answering your stated problem (as I understand it), on top of possible errors in your code.
One way to think of the problem is that, given the quadruple (i,j,k,l), the value of m = n - (i + j + k + l), while noting that the quadruple (i,j,k,l) is constrained so that n >= i+j+k+l AND i,j,k,l >= 0. For example, consider the following algorithm:
Let i freely take any value between 0 and n.
Given i, j can take values between 0 and n-i.
Given (i,j), k takes values between 0 and n-i-j.
Given (i,j,k), l takes values between 0 and n-i-j-k.
Given (i,j,k,l), m is defined as m = n - i - j - k -l.
The following code ought to answer your question. Please comment if this is not what you were looking for.
sample.example = function(n){
a=array(0,c(0,5))
for(i in 0:n){
for(j in seq(from=0,to=n-i,by=1)){
for(k in seq(from=0,to=n-i-j,by=1)){
for(l in seq(from=0,to=n-i-j-k,by=1)){
m = n - i -j - k - l
a = rbind(a,c(i,j,k,l,m))
}}}}
return(a)
}