I am trying to change the sign of a number to negative if it is below a specific target and it is to remain positive if it is above. - r

Question

I put the section of code in bold that seems to be the problem. Here is the code:
## price impact analysis
rm(list=ls())
### import data from excel spreadsheets
chtr_trades <- read.csv("F:/FRE 6951 Mkt Micro Struc/CHTRTRADES.csv")
chtr_quotes <- read.csv("F:/FRE 6951 Mkt Micro Struc/CHTRQUOTES.csv")
## initialize bid ask
max_bid <- NULL
min_ask <- NULL
### cleans data
maxrm <- function(x) {
max(x, na.rm=TRUE)
}
minrm <- function(x) {
min(x, na.rm=TRUE)
}
## retrieve max bid and ask for each iteration
max_bid<- tapply(chtr_quotes[,4],chtr_quotes[,3], maxrm)
min_ask<- tapply(chtr_quotes[,5],chtr_quotes[,3], minrm)
time <- levels(chtr_quotes[,3])
## calculate previous second midpoint
midpoint <- (min_ask + max_bid)/2
askbidtime <- data.frame(midpoint,time,max_bid,min_ask)
row.names(askbidtime) <- seq(nrow(askbidtime))
askbidtime[,2] <- as.POSIXct(askbidtime[,2], format="%H:%M:%S")
ordered.askbidtime <- askbidtime[order(askbidtime$time),]
row.names(ordered.askbidtime) <- seq(nrow(ordered.askbidtime))
chtr_trades_revised <-chtr_trades[which(as.POSIXct(chtr_trades[,3],format="%H:%M:%S") %in% ordered.askbidtime[,2]),]
midpoint<-NULL
midpoint[1:5] <- NA
for(i in 6:3917) {
midpoint[i] <- as.numeric(ordered.askbidtime[which(ordered.askbidtime[,2]==as.POSIXct(chtr_trades_revised[i,3],format="%H:%M:%S"))-1,1])
}
***## sign trades
chtr_trades_revised$midpoint
chtr_trades_revised$midpoint <- midpoint
for(i in 6:3917) {
if((!is.na(chtr_trades_revised$midpoint[i])) & (chtr_trades_revised$midpoint[i] > chtr_trades_revised$PRICE[i])) {
chtr_trades_revised$signed_volume <- -chtr_trades_revised$SIZE
}
if((!is.na(chtr_trades_revised$midpoint[i])) & (chtr_trades_revised$midpoint[i] < chtr_trades_revised$PRICE[i])) {
chtr_trades_revised$signed_volume <- chtr_trades_revised$SIZE
}
}***
Here are the results. In the last column rows 4062 and 4054 should be positive but it makes the entire column negative:
SYMBOL DATE TIME PRICE SIZE midpoint signed_volume
4060 CHTR 20130718 2014-08-26 15:59:44 124.46 100 124.485 -100
4061 CHTR 20130718 2014-08-26 15:59:52 124.46 100 124.495 -100
4062 CHTR 20130718 2014-08-26 15:59:55 124.52 100 124.490 -100
4063 CHTR 20130718 2014-08-26 15:59:58 124.53 100 124.410 -100
4064 CHTR 20130718 2014-08-26 16:00:00 124.57 7951 124.550 -7951
4065 CHTR 20130718 2014-08-26 16:00:00 124.53 100 124.550 -100

Answer 1

Here's a cute way:
foo<- 1:10
threshold <- 5
foo<- foo*(-1)^(foo < threshold)
foo
[1] -1 -2 -3 -4 5 6 7 8 9 10

Answer 2

Another method:
foo = 1:10 ; threshold = 5
foo
[1] 1 2 3 4 5 6 7 8 9 10
foo = ifelse(foo>=threshold, foo, -foo)
foo
[1] -1 -2 -3 -4 5 6 7 8 9 10