检测递归函数中的无限循环（R）

Question

I have a test df:

testdf<-data.frame(x = seq(1,10), y= c(1, 1, 4, 3, 2, 6, 7, 4, 9, 10))

testdf

    x  y
1   1  1
2   2  1
3   3  4
4   4  3
5   5  2
6   6  6
7   7  7
8   8  4
9   9  9
10 10 10

I want to write a function that inputs a row number and "follows" the y value until it finds a row for which column x = column y.

get_acc_x<-function(rownum){
  if(testdf[rownum, 'x'] == testdf[rownum, 'y']){
    return(rownum)
  }else{
    get_acc_x(testdf[rownum, 'y'])
  }
} 

So, running get_acc_x(1) returns 1, get_acc_x(9) returns 9, get_acc_x(2) returns 1, get_acc_x(5) also returns 1, etc.

But, if I were to run this function on the number 8, it would get into an infinite loop, going back and forth between 3 and 4. What is the easiest way to detect an infinite loop in this situation? I want to keep track of past inputs so I can stop the function if the same input is used more than once, but I don't know how best to go about keeping track of the inputs.

Answer 1

You can pass in a parameter marking visited rows:

get_acc_x<-function(rownum, seen){
  if (seen[rownum]) {
    # Whatever you want to do, cycle detected
  }
  seen[rownum] <- T
  if(testdf[rownum, 'x'] == testdf[rownum, 'y']){
    return(rownum)
  }else{
    get_acc_x(testdf[rownum, 'y'], seen)
  }
} 

When calling, use get_acc_x(rownum, rep(F, nrow(df)) to pass in an all False param.

Answer 2

If you don't want to pass along visited nodes explicitly, you could read them off the call stack using sys.frames . If you think the recursion will be reasonably shallow, there shouldn't be too much of a hit to performance, and since it doesn't change the signature, you won't have to modify any of the calling code.

get_acc_x2<-function(rownum){
  if(testdf[rownum, 'x'] == testdf[rownum, 'y']){
    return(rownum)
  }else{
    rownum %in% sapply(head(sys.frames(), -1), `[[`, "rownum") &&
        stop('infinite recursion detected')
    get_acc_x2(testdf[rownum, 'y'])
  }
} 

Example:

> get_acc_x2(8)
Error in get_acc_x2(8) : infinite recursion detected

Answer 3

You need to pass the previously seen values as an argument. I've added a wrapper function which handles passing an initial empty vector.

x <- c(1,2,3,4,5,6,7,8,9,10)
y <- c(1,1,4,3,2,6,7,4,9,10)
df <- data.frame(x,y)


get_acc_x <- function(rownum,df) get_acc_x_rec(rownum,df,numeric())
get_acc_x_rec<-function(rownum,df,prev){
  if(df[rownum, 'x'] == df[rownum, 'y']){
return(rownum)
 }else{
if(is.element(df[rownum, 'y'],prev)) get_acc_x(df[rownum, 'y'],df,c(prev,rownum))
else stop("Damnit!")
 }
}