[r] Non-numeric Argument to Binary Operator Error in R

The issue I believe is how CurrentDay is entered. It was previously created as:

Transaction <- function(PnL, Day)
  results <- list(a = PnL, b = Day)
  return(results)

Both PnL and Day are numeric values.

Day <- Transaction(PnL, Day)["b"]

Where Transaction returned a list and b is an integer.

moving_avg <- function(StockData, MA, CurrentDay){
  #MA = Days long the MA is
  #CurrentDay = What day we are currently on
  MAValue <- NULL
  total <- 0
  start <- CurrentDay - MA
  for(i in 1:length(MA)) {
    total <- total + StockData[[start, 4]]
    start <- start + 1
  }
  MAValue <- total/MA
  return(MAValue)
}

Anyone know why I am receiving this error?

Error in CurrentDay - MA : non-numeric argument to binary operator

Because your question is phrased regarding your error message and not whatever your function is trying to accomplish, I will address the error.

- is the 'binary operator' your error is referencing, and either CurrentDay or MA (or both) are non-numeric.

A binary operation is a calculation that takes two values (operands) and produces another value (see wikipedia for more). + is one such operator: "1 + 1" takes two operands (1 and 1) and produces another value (2). Note that the produced value isn't necessarily different from the operands (e.g., 1 + 0 = 1).

R only knows how to apply + (and other binary operators, such as -) to numeric arguments:

> 1 + 1
[1] 2
> 1 + 'one'
Error in 1 + "one" : non-numeric argument to binary operator

When you see that error message, it means that you are (or the function you're calling is) trying to perform a binary operation with something that isn't a number.

EDIT:

Your error lies in the use of [ instead of [[. Because Day is a list, subsetting with [ will return a list, not a numeric vector. [[, however, returns an object of the class of the item contained in the list:

> Day <- Transaction(1, 2)["b"]
> class(Day)
[1] "list"
> Day + 1
Error in Day + 1 : non-numeric argument to binary operator

> Day2 <- Transaction(1, 2)[["b"]]
> class(Day2)
[1] "numeric"
> Day2 + 1
[1] 3

Transaction, as you've defined it, returns a list of two vectors. Above, Day is a list contain one vector. Day2, however, is simply a vector.