CODEWITHSUNDEEP
rvectorization
vonjd
vonjd

Reputation: 4380

How to vectorize a function not 'inside' but 'outside' in R?

I am trying to program tic-tac-toe in R - here are my two functions for making a move and evaluating all valid moves (the ones are X's, zeros are O's and NA is not taken yet):

move <- function(board,square,mark)
{
  if (square < 1 || square > 9 || !is.na(board[square]))
  {
    return(NA)
  }
  else
    board[square] <- mark
    return(board)
}

valid.moves <- function(board)
{
  return(which(is.na(board)))
}

Now setting up a test position, evaluating all valid moves and then make those moves...

test.last <- matrix(c(1,1,NA,0,NA,0,NA,0,1),nrow=3)
moves <- valid.moves(test.last)
move(test.last,moves,1)

...gives a result which I didn't intend:

     [,1] [,2] [,3]
[1,]    1    0    1
[2,]    1    1    0
[3,]    1    0    1

I wanted to have three different boards with the respective valid moves (which will then be evaluated with another function whether it is a winning position) and not one board with all valid moves made at once.

I don't want to do this with a loop but the vectorization should not take place all at once 'inside' the move function but 'outside' of it - so basically I want to do the following without a loop (the eval.pos function to evaluate the position is of the form eval.pos <- function(board){}):

for (i in 1:length(moves))
{
  after.moves <- move(test.last,moves[i],1)
  print(after.moves)
  print(eval.pos(after.moves))
}

How can I accomplish this without a loop?

Upvotes: 1

Views: 101

Answers (2)

IRTFM
IRTFM

Reputation: 263352

Expanding my suggestion in the comment. How to use matrix indices to generate a list of move options:

 valid.moves <- function(board)
 {
     return(which(is.na(board), arr.ind=TRUE))
 }

> moves <- valid.moves(test.last)
> moves
     row col
[1,]   3   1
[2,]   2   2
[3,]   1   3

> lapply(1:3, function( mv) {start <- test.last 
                             start[matrix(moves[mv,],ncol=2)] <- 1
                             start})
[[1]]
     [,1] [,2] [,3]
[1,]    1    0   NA
[2,]    1   NA    0
[3,]    1    0    1

[[2]]
     [,1] [,2] [,3]
[1,]    1    0   NA
[2,]    1    1    0
[3,]   NA    0    1

[[3]]
     [,1] [,2] [,3]
[1,]    1    0    1
[2,]    1   NA    0
[3,]   NA    0    1

Upvotes: 1

Neal Fultz
Neal Fultz

Reputation: 9687

move2 <- function(board, square, mark) {
  lapply(square, function(x,i,value) `[<-`(x,i,value), x=board, value=mark) 
}

Note that the anonymous function() is needed because [<- is primitive.

Upvotes: 1

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