R/days_since_first_assessment.R

In CANS.works: Import, manipulate and analyze data from the Massachusetts CANS tool

days_since_first_assessment <-
function (cans) {

# Make sure MHID is factor:
cans[,2]<-as.factor(cans[,2])

# Make sure Date of Assessment is of class Date:
cans[,32] <- as.Date(cans[,32],format="%m/%d/%Y")

# Note: in next step the single bracket [] is used to identify a list of columns
# in data frame, as required in aggregate(). This step creates a dataframe with
# nrow equal to number of initial dates of assessment. This dataframe allows us to
# translate from Date of Assessment to days elapsed since first.
date.key<- aggregate(cans[32], by=cans[2], min)

# Create the vector of elapsed days (integers) that can replace the date of assessment.
# (you might also do this with merge() which uses match() internally;
# the following method is straightforward and fast enough for our purposes, 
# but see Adler for faster methods using environments)
init.date <- date.key[match(cans[, 2], date.key[, 1]), 2]
# calculate the diffence in dates
days <- as.numeric(cans[, 32]-init.date)
}