[R] Loop too slow for Bid calc - BUT cannot figure out how to do with matrix

[Duncan Murdoch]

On 04/10/2010 5:29 PM, rivercode wrote:
> Hi,
> 
> I am trying to create Bid/Ask for each second from a high volume stock and
> the only way I have been able to solve this is using loops to create the
> target matrix from the source tick data matrix.  Looping  is too slow and
> not practical to use on multiple stocks. For example:

I would use loops, but something like this:

lastrow <- Bids[1,]
time <- [ some starting time ]
for (i in 2:400000) {
   thisrow <- Bids[i,]
   while (thisrow[,2] > time) {
     output lastrow for time
     time <- time + 1
   }
   lastrow <- thisrow
}

I don't see how that would be too slow, but if it was, just rewrite it in C.

Duncan Murdoch

> 
> Bids Matrix (a real one is 400,000++ length):
> 
> Bid       Time
> 10.03    11:05:03.124
> 10.04    11:05:03.348
> 10.05    11:05:04.010
> 
> One Second Bid Matrix (Bid price for every second of the day):
> 
> Bid       Second
> 10.02   11:05:03
> ??        11:05:04    <----Last bid price before 11:05:04.xxx, which is
> 11.04 at 11:05:03.348 
> 
> The challenge is how to create the one second bid matrix, without looping
> through the Bids Matrix to find the first timestamp that is greater than the
> OneSecond  timestamp then getting the previous row price from
> BidsMatrix...which would have been the bid at the beginning of that second.  
> 
> I am new to R, so need some help to do this “properly”. 
> 
> #  OneSecond.  Matrix above called “One Second Bid Matrix”
> #  BidsMatrix.  Matrix above called “Bids Matrix”
> 
> bidrow = 1
> 
> # looping through each second
> for (sec in 1:length(OneSecond$Second) )  
>     {
> 	t = as.POSIXlt(onesec$Second[sec],origin = "1970-01-01")
> 	sec.onesec = as.numeric(format(t, "%H%M%S")) # convert date/time to format
> HHMMSS as a number
> 			
> 	  # Find bid for second, which is the last bid before a change in the
> second
> 	  for (r in bidrow:length(BidsMatrix$Price))
> 		{
>                  # convert the BidsMatrix timestamp to number of format
> %H%M%S 
>   		 bidTS = unlist(strsplit(as.character(BidsMatrix$Time[r]),
> split="\\."))[1] # remove milliseconds
> 		 bidTS = gsub(":", "", bidTS) # remove ":" from time
> 		 bidTS = as.numeric(bidTS) # convert to number
> 	  
> 				
> 	    if (bidTS > sec.onesec)  
> 		  {
> 		    onesec$Bid[sec] = bids$Price[r -1] # Price of previous bid
>     		    bidrow = r  # save bidrow as starting point to find next bid.
> 		    break
> 		  } #if
> 		
> 		}# for
> 	
> }# for
> 
> Hope this is clear and  thanks for your help.
> 
> Chris
>