If else statement to delete repeated values - r

Question

I'm a novice R user and have created a small script that is doing some trigonometry with movement data. I need to add a final column that deletes repeated values from the column before it.
I've tried adding an if else statement that seems to work when isolated, but keep having errors when it is put into the for loop. I'd appreciate any advice.
# trig loop
list.df <- vector("list", max(Sp_test$ID))
names1 <- c(1:max(Sp_test$ID))
for(i in 1:max(Sp_test$ID)) {
if(i %in% unique(Sp_test$ID)) {
idata <- subset(Sp_test, ID == i)
idata$originx <- idata[1,3]
idata$originy <- idata[1,4]
idata$deltax <- idata[,"UTME"]-idata[,"originx"]
idata$deltay <- idata[,"UTMN"]-idata[,"originy"]
idata$length <- sqrt((idata[,"deltax"])^2+(idata[,"deltay"]^2))
idata$arad <- atan2(idata[,"deltay"],idata[,"deltax"])
idata$xnorm <- idata[,"deltax"]/idata[,"length"]
idata$ynorm <- idata[,"deltay"]/idata[,"length"]
sumy <- sum(idata$ynorm, na.rm=TRUE)
sumx <- sum(idata$xnorm, na.rm=TRUE)
idata$vecsum <- atan2(sumy,sumx)
idata$width <- idata$length*sin(idata$arad-idata$vecsum)
# need if else statement excluding a repeat from the position just before it
list.df[[i]] <- idata
names1[i] <- i
} }
# this works alone, I think the problem is when it gets to the first of the dataset and there is not one before it
if (idata$width[j]==idata$width[j-1]) {
print("NA")
} else {
print(idata$width[j])
}

Answer 1

I think you want to use the function diff for this. diff(idata$width) will give the differences between successive values of idata$width. Then
idata$width[c(FALSE, diff(idata$width) == 0)] <- NA
I think does what you want. The initial FALSE is since there is no value corresponding to the first element (since as you rightly noted, the first element doesn't have an element before it).