Calculate the days between dates by hand - r

Question

I suspect R has a built-in method to do this, but I'm new to programming and was trying to figure out how to calculate the days between dates by hand as practice. Here is my code:
isLeapYear <- function(year) {
if (year %% 400 == 0) {
return (TRUE)
}
if (year %% 100 == 0) {
return (FALSE)
}
if (year %% 4 == 0) {
return (TRUE)
}
else{ return (FALSE)}
}
daysInMonth <- function(year,month){
if (month == 1 | month == 3| month == 5 | month == 7 | month == 8 | month == 10 | month == 12) {
return (31)
}
else if (month == 2){
if (isLeapYear(year)){
return (29)
}
else {return (28)}
}
else {return (30)}
}
nextDay <- function(year, month, day){
if (day < daysInMonth(year,month)){
return (list(year, month, day + 1))
}
else{
if (month == 12){
return (list(year + 1, 1, 1))
}
else{return (list(year, month + 1, 1))}
}
}
dateIsBefore <- function(year1, month1, day1, year2, month2, day2){
if (year1 < year2){
return(TRUE)
}
if (year1 == year2){
if (month1 < month2){
return(TRUE)
}
if (month1 == month2){
return (day1 < day2)
}
}
return (FALSE)
}
daysBetweenDates <- function(year1, month1, day1, year2, month2, day2){
days <- 0
while (dateIsBefore(year1, month1, day1, year2, month2, day2)){
result = nextDay(year1,month1,day1)
year1 = result[1]
month1 = result[2]
day1 = result[3]
days = days+1
}
return (days)
}
I wrote a code to determine the number of day between two dates using python. I'm now trying to convert it to R for another assignment I'm doing. When I run each indiviual function, they seem to work fine. When I call daysBetweenDates(2012,1,1,2012,2,28) I get the following error:
Error in day + 1 : non-numeric argument to binary operator

Answer 1

> as.Date("2012/02/28") - as.Date("2012/01/01")
# Time difference of 58 days
> as.Date("2012/01/01") - as.Date("2012/02/28")
# Time difference of -58 days
Improvement suggested by Dirk in comments on just to get the number as a difference using as.integer(), here it is
> as.integer(as.Date("2012/02/28") - as.Date("2012/01/01"))
# [1] 58
> as.integer(as.Date("2012/01/01") - as.Date("2012/02/28"))
# [1] -58

Answer 2

The reason your code isn't working is because of the way you are assigning the three elements of result to year1, month1, and day1 in the while loop in your daysBetweenDates function. If you change it like this it should work:
daysBetweenDates <- function(year1, month1, day1, year2, month2, day2){
days <- 0
while (dateIsBefore(year1, month1, day1, year2, month2, day2)){
result = nextDay(year1,month1,day1)
year1 = result[[1]]
month1 = result[[2]]
day1 = result[[3]]
days = days + 1
}
return (days)
}
If you do return <- list(2012, 1, 1) and then return[1] and compare that with return[[1]] you should see the difference. This is common mistake when using lists. You might want to take a look at this for an excellent discussion of subsetting: http://adv-r.had.co.nz/Subsetting.html.