Creating a function by taking few arguments and calculating - r

Question

I'm still working on a question from couple of days ago and would like to receive feedback/support on how I could create a function. Your expertise is highly appreciated.
I have created the following:
##### 1)
> raceIDs
[1] "GER" "SUI" "NZ2" "US1" "US2" "POR" "FRA" "AUS" "NZ1" "SWE"
##### 2)
#For each "raceIDs", there is a csv file which I have made a loop to read and created a list of data frames (assigned to the symbol "boatList")
#For example, if I select "NZ1" the output is:
> head(boatList[[9]]) #Only selected the first six lines as there is more than 30000 rows
Boat Date Secs LocalTime SOG
1 NZ1 01:09:2013 38150.0 10:35:49.997 22.17
2 NZ1 01:09:2013 38150.2 10:35:50.197 22.19
3 NZ1 01:09:2013 38150.4 10:35:50.397 22.02
4 NZ1 01:09:2013 38150.6 10:35:50.597 21.90
5 NZ1 01:09:2013 38150.8 10:35:50.797 21.84
6 NZ1 01:09:2013 38151.0 10:35:50.997 21.95
##### 3)
# A matrix showing the race times for each raceIDs
> raceTimes
start finish
GER "11:10:02" "11:35:05"
SUI "11:10:02" "11:35:22"
NZ2 "11:10:02" "11:34:12"
US1 "11:10:01" "11:33:29"
US2 "11:10:01" "11:36:05"
POR "11:10:02" "11:34:31"
FRA "11:10:02" "11:34:45"
AUS "11:10:03" "11:36:48"
NZ1 "11:10:01" "11:35:16"
SWE "11:10:03" "11:35:08"
What I need to do is I need to calculate the average speed (SOG) of a boat "while it was racing" (between start and finish times) by creating a function called meanRaceSpeed and having three arguments:
What I have tried so far is to create a function with 3 arguments (with a bit of help from experts here):
meanRaceSpeed <- function(raceIDs, boatList, raceTimes)
{
#Probably need to compare times, and thought it might be useful to convert character values into `DateTime` values but not to sure how to use it
#DateTime <- as.POSIXct(paste(boatList$Date, boatList$Time), format="%Y%m%d %H%M%S")
#To get the times for each boat
start_time <- raceTimes$start[rownames(raceTimes) = raceIDs]
finish_time <- raceTimes$finish[rownames(raceTimes) = raceIDs]
start_LocalTime <- min(grep(start_time, boatList$LocalTime))
finish_LocalTime <- max(grep(finish_time, boatList$LocalTime))
#which `SOG`s contain all the `LocalTimes` between start and finish
#take their `mean`
mean(boatList$SOG[start_LocalTime : finish_LocalTime])
}
### Obviously, my code does not work :( and I don't know where.
So basically, I need to create a function with three arguments and the expected result is:
#e.g For NZ1
> meanRaceSpeed("NZ1", boatList, raceTimes)
[1] 18.32 #Mean speed for NZ1 between 11:10:01 - 11:35:16
#e.g for US1
> meanRaceSpeed("US1", boatList, raceTimes)
[1] 17.23 #Mean speed for US1 between 11:10:01 - 11:33:29
Any helps where I could have gone wrong? Highly appreciate your help please.

Answer 1

I'm going to give some general advice for R, but I will also help you with your specific question. Whenever I have a problem in R, I usually find that it helps to make things more explicit.
If the function isn't working with these methods (is that a data frame or a matrix in your function?) then you should try another method. If those table manipulation methods aren't working, try a different one. How?
Here's a few different things you can do to test your function, and a few suggestions that may move you along a bit. (I don't want to fix the whole thing for you, since it's your homework, but rather get you on your way.)
1) Why not try using a loop instead of brackets?
start_time <- raceTimes$start[rownames(raceTimes) = raceIDs]
Make that into a for loop. It's not too hard to do.
2) Debug your functions. There are a lot of tools to do this built into R, and in packages you can add. Since you, likely, don't have time for that with your homework. I'd suggest doing this. Take apart the function and apply each part of it with a variable you want. Are they of the right length? Are they the right data type? Are they getting the right answer before you put them all together? Make sure of that.
3) If all else fails, don't be afraid if the function and code is not elegant. R is not always an elegant language. (Actually, it's rarely an elegant language.) Especially when you're a beginner, your code will likely be ugly. Just make sure it works.

Answer 2

Since I, already, had experience with your data, I sat to make a complete example.
First, data that look like yours:
raceIDs <- c("GER", "SUI", "NZ2", "US1", "US2", "POR", "FRA", "AUS", "NZ1", "SWE")
raceTimes <- as.matrix(read.table(text = ' start finish
GER "11:10:02" "11:35:05"
SUI "11:10:02" "11:35:22"
NZ2 "11:10:02" "11:34:12"
US1 "11:10:01" "11:33:29"
US2 "11:10:01" "11:36:05"
POR "11:10:02" "11:34:31"
FRA "11:10:02" "11:34:45"
AUS "11:10:03" "11:36:48"
NZ1 "11:10:01" "11:35:16"
SWE "11:10:03" "11:35:08"', header = T))
#turn matrix to data.frame or, else, `$` won't work
raceTimes <- as.data.frame(raceTimes, stringsAsFactors = F)
blDF <- data.frame(Boat = rep(raceIDs, 3),
LocalTime = c(raceTimes$start, rep("11:20:25", length(raceIDs)), raceTimes$finish),
SOG = runif(3 * length(raceIDs), 15, 25), stringsAsFactors = F)
boatList <- split(blDF, blDF$Boat)
#remove `names` to create them from scratch
names(boatList) <- NULL
Then:
#create `names` by searching each element of
#`boatList` of what `boat` it contains
names(boatList) <- unlist(lapply(boatList, function(x) unique(x$Boat)))
#the function
meanRaceSpeed <- function(ID, boatList, raceTimes)
{ #named the first argument `ID` instead of `raceIDs`
start_time <- raceTimes$start[rownames(raceTimes) == ID]
finish_time <- raceTimes$finish[rownames(raceTimes) == ID]
start_LocalTime <- min(grep(start_time, boatList[[ID]]$LocalTime))
finish_LocalTime <- max(grep(finish_time, boatList[[ID]]$LocalTime))
mean(boatList[[ID]]$SOG[start_LocalTime : finish_LocalTime])
}
Test:
meanRaceSpeed("US1", boatList, raceTimes)
#[1] 19.7063
meanRaceSpeed("NZ1", boatList, raceTimes)
#[1] 21.74729
mean(boatList$NZ1$SOG) #to test function
#[1] 21.74729
mean(boatList$US1$SOG) #to test function
#[1] 19.7063