Selecting row by min value and merging - r

Question

This is row selection problem in R.
I want to get only one row in each dataset based on the minimum value of the variable fifteen. I have tried two approaches below, none of them returning the desired output.
For list 1> SPX[[1]] the date.frame is set up like this:
SPX1[[1]]
+----------------------------------+---------+------------------------+----------------+
| | stkPx | expirDate | fifteen |
+----------------------------------+---------+------------------------+----------------+
| 1 | 1461.62 | 2013-01-19 | 2 |
| 2 | 1461.25 | 2013-01-25 | 8 |
| 3 | 1461.35 | 2013-02-01 | 3 |
| . | . | . | . |
| . | . | . | . |
+----------------------------------+---------+------------------------+----------------+
The first approach is aggregating and the merge. As this has to be done for a list the code is in a loop:
df.agg<- list() # creates a list
for (l in 1:length(SPX1)){
df.agg[[l]]<- SPX1[[l]] %>%
aggregate(fifteen ~ ticker, data=SPX1[[l]], min) #Finding minimum value of fifteen for ticker
df.minSPX1 <- merge(df.agg[[l]], SPX1[[l]]) #Merge dataset so we only get row with min fifteen value
}
I get :
Error in get(as.character(FUN), mode = "function", envir = envir) :
object 'SPX1' of mode 'function' was not found
Another approach, however it just changes all the values of the first column to one, not deleting any rows when merging:
TESTER<- which.min(SPX1[[1]]$fifteen) # Finds which row has minimum value of fifteen
df.minSPX1 <- merge(TESTER, SPX1[[1]],by.x=by) #Try to merge so I only get the row with min. fifteen
I have tried reading other answers on SO but maybe because of the way the lists are set up this won't work?
I hope you can tell me where I get this wrong.

Answer 1

Try This
df<- lapply(SPX, function(x) x[x$fifteen==min(x$fifteen),])
df<- as.data.frame(df)
Edit:
As suggested by #Gregor-Thomas this will work when there is a tie.
df<- lapply(SPX, function(x) x[which.min(x$fifteen), ])
df<- as.data.frame(df)