Alternate way for R loop

Question

This loop is workable for small amount of data but when it comes to huge volume of data, it took quite long for looping. So I want to find out is there any alternate way to do it so it can help to speed up the process time by using R programming:

#set correction to the transaction
mins<-45
for (i in 1:nrow(tnx)) {
 if(tnx$id[i] == tnx$id[i+1]){
    #check trip within 45 mins
    if(tnx$diff[i]>=mins){
        tnx$FIRST[i+1] <- TRUE
        tnx$LAST[i] <- TRUE
    }
 }
 else{
        tnx$LAST[i]<-TRUE
     }
 }

Thanks in advance.

EDIT

What I am trying to do is set the true false value in first and last column by checking the diff column.

Data like:

tnx <- data.frame(
  id=rep(c("A","C","D","E"),4:1),
  FIRST=c(T,T,F,F,T,F,F,T,F,T),
  LAST=c(T,F,F,T,F,F,T,F,T,T),
  diff=c(270,15,20,-1,5,20,-1,15,-1,-1)
)

EDIT PORTION FOR @thelatemail

#   id diff FIRST  LAST
#1   A  270 TRUE  TRUE
#2   A   15  TRUE FALSE
#3   A   20 FALSE FALSE
#4   A   -1 FALSE TRUE
#5   C    5 TRUE  FALSE
#6   C   20 FALSE FALSE
#7   C   -1 FALSE TRUE
#8   D   15 TRUE  FALSE
#9   D   -1 FALSE TRUE
#10  E   -1 TRUE  TRUE

Answer 1

Something like this should work: I reset the FIRST and LAST values to make it obvious in this example:

tnx$FIRST <- FALSE
tnx$LAST <- FALSE

The next two parts use ?ave to respectively set tnx$FIRST to TRUE for the first row in each id group, and tnx$LAST to TRUE for the last row in each id group.

tnx$FIRST <- as.logical(
              with(tnx, ave(diff,id,FUN=function(x) seq_along(x)==1) ))
tnx$LAST <- as.logical(
              with(tnx, ave(diff,id,FUN=function(x) seq_along(x)==length(x))))

The final two parts then:
- set tnx$LAST to TRUE when tnx$diff is >=45.
- set tnx$FIRST to TRUE when the previous value for tnx$diff is >=45

tnx$LAST[tnx$diff >= 45] <- TRUE
tnx$FIRST[c(NA,head(tnx$diff,-1)) >= 45] <- TRUE


#   id diff FIRST  LAST
#1   A  270  TRUE  TRUE
#2   A   15  TRUE FALSE
#3   A   20 FALSE FALSE
#4   A   -1 FALSE  TRUE
#5   C    5  TRUE FALSE
#6   C   20 FALSE FALSE
#7   C   -1 FALSE  TRUE
#8   D   15  TRUE FALSE
#9   D   -1 FALSE  TRUE
#10  E   -1  TRUE  TRUE