library(HMDHFDplus)
LT <- readHMDweb("USA","fltper_1x1",username = us, password = pw)
head(LT)

library(reshape2)
lx <-acast(LT, Age~Year, value.var = "lx")

dx <- lx * 0

years <- unique(LT$Year)
ages  <- unique(LT$Age)

NY <- length(years)
na <- length(ages)

# first do one year at a time
lx <- rbind(lx, 0)

yr <- 20
a <- 20

for (yr in 1:NY){ # columns
   # then do one age at a time
   for (a in 1:na){ # rows
	   dx[a, yr] <- lx[a, yr] - lx[a + 1, yr]
   }
}

dx[20,20]