Adding vector elements with condition in R

Question

Suppose I have a vector of elements that I want to add:

a <- c(1,2,-7,5)

Here are some additional test cases:

a <- c(1,2,-3,5)
a <- c(1,2,-7,-3,5)

I know I can use sum(a) to get the result, but what if I had a condition to watch out for:

current_sum = 0
for(i in 1:length(a)){    
 last_sum = current_sum
 current_sum = current_sum + a[i]
 if(current_sum < 0)
 {
  current_sum = last_sum
  current_sum = current_sum + (a[i]*-1)
 }
}

Here, each time the sum is negative, we go back to the previous sum and add the opposite of that number that made the sum go negative. Outputting 15 as a result of the first example

Obviously, the vector of elements is not known before hand and performance is of issue. Is there any fully vectorized method or a more efficient way to do this in general (avoiding loops) ?

Answer 1

I find the R/C interface very useful for the tasks in which performance is important, there isn't an obvious vectorized R solution and the code in C is very easy to write. Try this:

require(inline)
.internalSumPositive<-cfunction(sig=c(v="SEXP"), language="C", body="
   double sum=0.0;
   int i,n = length(v);
   double *values = REAL(v);
   SEXP ret = PROTECT(allocVector(REALSXP,1));
   for (i=0;i<n;i++) {
      sum += values[i];
      if (sum<0) sum = sum - 2*values[i];
   }
   REAL(ret)[0] = sum;
   UNPROTECT(1);
   return ret;")

sumPositive<-function(v) {
   if (!is.numeric(v)) stop("Argument must be numeric")
   if (length(v)==0) return(numeric(0))
   .internalSumPositive(as.numeric(v))
}

Then you can try:

sumPositive(c(1,2,-7,5))
#[1] 15
sumPositive(c(1,2,-3,5))
#[1] 5
sumPositive(c(1,2,-7,-3,5))
#[1] 12

I won't report benchmark since it's not even a contest with the other proposed R solutions (this can be tens of thousands times faster).

Answer 2

Try this

f1 <- function(x) repeat{pos<-min(which(cumsum(x)<0))
                    x[pos]<-abs(x[pos])
                    if(all(cumsum(x)>=0)){return(sum(x));break}}
a <- c(1,2,-7,-3,5)
f1(a)
#[1] 12