Looping through dates in R - examples of issues with indexing with dates.

dateloopexamples.R

# examples of issues with indexing with dates for will turn to number
# Solution 
# - DONT use dates as elements of your index 
# - DO index alonmg length of your date list
library(data.table)

date.list <- c(as.Date("2015/3/1"), as.Date("2015/3/2"))

# 1 loop through dates?
for (i.date  in unique(date.list)){
 str(i.date) 
}
# Fail. You will see index i.date is number :(

for (i.date  in date.list){
 # str(i.date) 
  print(paste0(i.date, ":", is.numeric(i.date)))
}


# 2 try as factors
for (i.date  in as.factor(date.list)){
  str(i.date) 
  print(paste0(i.date, ":", is.numeric(i.date)))
}


# https://stat.ethz.ch/pipermail/r-help/2008-December/182516.html
# To loop through dates need to turn into sequence.

# SOLUTION
# do own date list
#  watch out in for loop  - use 1:length not just length
for (i in 1:length(date.list)){
  str(i)
  str(date.list[i])
  print(paste0(date.list[i], ":", is.numeric(date.list[i])))
}


dt <- data.table(x = c("p1","p2"),
                 max.date = date.list)

for (i  in 1:length(unique(dt$max.date))) {
  str(i)
  str(dt[i, max.date]) 
#  print(paste0(dt[i, max.date], ":", is.numeric(dt[i, max.date]) ))
}