indexing a matrix in R - r

Question

A novice R user here. So i have a data set formated like:
Date Temp Month
1-Jan-90 10.56 1
2-Jan-90 11.11 1
3-Jan-90 10.56 1
4-Jan-90 -1.67 1
5-Jan-90 0.56 1
6-Jan-90 10.56 1
7-Jan-90 12.78 1
8-Jan-90 -1.11 1
9-Jan-90 4.44 1
10-Jan-90 10.00 1
In R syntax:
datacl <- structure(list(Date = structure(1:10, .Label = c("1990/01/01",
"1990/01/02", "1990/01/03", "1990/01/04", "1990/01/05", "1990/01/06",
"1990/01/07", "1990/01/08", "1990/01/09", "1990/01/10"), class = "factor"),
Temp = c(10.56, 11.11, 10.56, -1.67, 0.56, 10.56, 12.78,
-1.11, 4.44, 10), Month = c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L)), .Names = c("Date", "Temp", "Month"), class = "data.frame", row.names = c(NA,
-10L))
i would like to subset the data for a particular month and apply a change factor to the temp then save the results. so i have something like
idx <- subset(datacl, Month == 1) # Index
results[idx[,2],1] = idx[,2]+change # change applied to only index values
but i keep getting an error like
Error in results[idx[, 2], 1] = idx[, 2] + change:
only 0's may be mixed with negative subscripts
Any help would be appreciated.

Answer 1

First, give the change factor a value:
change <- 1
Now, here is how to create an index:
# one approach to subsetting is to create a logical vector:
jan.idx <- datacl$Month == 1
# alternatively the which function returns numeric indices:
jan.idx2 <- which(datacl$Month == 1)
If you want just the subset of data from January,
jandata <- datacl[jan.idx,]
transformed.jandata <- transform(jandata, Temp = Temp + change)
To keep the entire data frame, but only add the change factor to Jan temps:
datacl$Temp[jan.idx] <- datacl$Temp[jan.idx] + change

Answer 2

First, note that subset does not produce an index, it produces a subset of your original dataframe containing all rows with Month == 1.
Then when you are doing idx[,2], you are selecting out the Temp column.
results[idx[,2],1] = idx[,2] + change
But then you are using these as an index into results, i.e. you're using them as row numbers. Row numbers can't be things like 10.56 or -1.11, hence your error. Also, you're selecting the first column of results which is Date and trying to add temperatures to it.
There are a few ways you can do this.
You can create a logical index that is TRUE for a row with Month == 1 and FALSE otherwise like so:
idx <- datac1$Month == 1
Then you can use that index to select the rows in datac1 you want to modify (this is what you were trying to do originally, I think):
datac1$Temp[idx] <- datac1$Temp[idx] + change # or 'results' instead of 'datac1'?
Note that datac1$Temp[idx] selects the Temp column of datac1 and the idx rows.
You could also do
datac1[idx,'Temp']
or
datac1[idx,2] # as Temp is the second column.
If you only want results to be the subset where Month == 1, try:
results <- subset(datac1, Month == 1)
results$Temp <- results$Temp + change
This is because results only contains the rows you are interested in, so there's no need to do subsetting.

Answer 3

Personally, I would use ifelse() and leverage the syntactic beauty that is within() for a nice one liner datacl <- within(datacl, Temp <- ifelse(Month == 1, Temp + change,Temp)). Well, I said one liner, but you'd need to define change somewhere else too.