generate list of dates based on one date in r - r

Question

I am new to R and am finding it difficult to generate a series of rows where each generated row has a calculated date.
For example, going from a dataset like this:
Name date_birth
Greg 01/02/2015
Fred 02/02/2015
...to generate the following:
Name date_birth age date_atage<br/>
Greg 01/02/2015 0 01/02/2015
Greg 01/02/2015 1 02/02/2015
Greg 01/02/2015 2 03/02/2015
Fred 02/02/2015 0 02/02/2015
Fred 02/02/2015 1 03/02/2015
Fred 02/02/2015 2 04/02/2015
I have been studying sites like R-blogger, general instructional blogs and this site and I have been trying to figure out a loop statement involving the Seq statement, so that for each individual (e.g. Greg, Fred, etc) the process can be repeated where dates are calculated and placed in their own rows. Your first thought may be that this is simpler to do in Excel, but it isn't, as I need to repeat this for over 800 individuals (i.e. not just Greg and Fred), and for up to 300 days of age.

Answer 1

We can use data.table
library(data.table)
setDT(df1)[, .(date_birth, date_at_age = format(seq(as.Date(date_birth,
"%d/%m/%Y"), length.out=3, by = "1 day"), "%d/%m/%Y")) ,
by = Name][,age := seq_len(.N)-1 , by = Name][]
# Name date_birth date_at_age age
#1: Greg 01/02/2015 01/02/2015 0
#2: Greg 01/02/2015 02/02/2015 1
#3: Greg 01/02/2015 03/02/2015 2
#4: Fred 02/02/2015 02/02/2015 0
#5: Fred 02/02/2015 03/02/2015 1
#6: Fred 02/02/2015 04/02/2015 2

Answer 2

This is a long form way of getting the same place that data.table will take you.
Have a look at how you use dates in R. I've taken your original format and converted it to a date (code line 2). See http://strftime.org/ for more codes.
Set some dummy data:
df = data.frame(name=c("Gregg", "Joan"), DOB=c("01/02/2015", "02/02/2015"), stringsAsFactors=F)
Make date format:
df$DOB = as.Date(df$DOB, format="%d/%m/%Y")
Loop over each name, making 301 instances and adding day to DoB
df = lapply(1:nrow(df), function(i){
x = data.frame(name=rep(df[i, 1], times=301),
DoB=rep(df[i, 2], times=301),
age=0:300)
x$newDate = x$DoB + x$age
x
})
Convert list to a data frame:
df = do.call("rbind.data.frame", df)
Check output:
head(df)

Answer 3

Setup
df <- cbind(c("Greg","Fred"),c("01/02/2015","02/02/2015"))
max_age <- 2
start_at <- 0
Script
new_df <- data.frame(rep(NA,(max_age+1)*dim(df)[1]))
new_df[,1] <- rep(df[,1],each=max_age-start_at+1) #Names
new_df[,2] <- rep(df[,2],each=max_age-start_at+1) #Birth date
new_df[,3] <- rep(seq(from=start_at,to=max_age),dim(df)[1]) #Age
library(lubridate)
new_df[,4] <- dmy(new_df[,2]) + days(new_df[,3]) #Date at age
colnames(new_df) <- c("names","date_birth","age","date_at_age")