Time data

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Hi
Date sent:      	Tue, 20 Dec 2005 11:12:28 +0100 (CET)
From:           	Marc Bernard <bernarduse1 at yahoo.fr>
To:             	r-help at stat.math.ethz.ch
Subject:        	[R] Time data
Dear All, I wonder how to compute the age from the date of birth and
the date of examination. Suppose that have the following data:

  df <- as.data.frame(rbind(c(1,"10/08/1950","15/03/1998"),
  c(1,"10/08/1950","20/07/1998"), c(1,"10/08/1950","23/10/1998")))

  names(df) <- c("ID", "date_birth", "date_exam")

  where:
  date_birth: is the date of birth
  date_exam: date of examination

  I used the following program to compute the value of the age  :

  difftime(strptime(as.character(df$date_exam), '%d/%m/%Y'),
  strptime(as.character(df$date_birth), '%d/%m/%Y'))/365.25

  which gives me as an output:

Time differences of 47.59491, 47.94251, 48.20260 days
  theses values are actually the 3 ages (but 

  My questions are:

  1- Why in the output it says "days" instead of "years")
this is in attributes of an output and is not changed by calculations
  2- How can I obtain the output as a numeric vector, without the
  statement "Time difference of ....". This is in order to use it in
  my calculations.
as.numeric(as.Date(df$date_exam)-as.Date(df$date_birth))/365

However you can use it in calculations without problem as I suppose 
that "Time difference of" results from print method of a difftime 
object.
  3- Is there a way quicker and less redondant  to compute the age
  form the date_birth and date_exam?
as.Date(df$date_exam)-as.Date(df$date_birth)

HTH
Petr
  Thanks a lot,

  Bernard,

---------------------------------

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Petr Pikal
petr.pikal at precheza.cz
Dear All, I wonder how to compute the age from the date of birth and the date of examination. Suppose that have the following data:

 df <- as.data.frame(rbind(c(1,"10/08/1950","15/03/1998"), c(1,"10/08/1950","20/07/1998"), c(1,"10/08/1950","23/10/1998")))

 names(df) <- c("ID", "date_birth", "date_exam")

 where:
 date_birth: is the date of birth
 date_exam: date of examination

 I used the following program to compute the value of the age  :

First ensure that df stores its dates using "Date" class in the first
place.  If your data is stored in the correct representation then
everything becomes easier subsequently:

fmt <- "%d/%m/%Y"
df[,2] <- as.Date(df[,2], fmt)
df[,3] <- as.Date(df[,3], fmt)

# converting them to numeric gives the number of days since
# the Epoch and one can just subtact those:

(as.numeric(df$date_exam) - as..numeric(df$date_birth)) / 365

R News 4/1 Help Desk article has more info on dates.
 difftime(strptime(as.character(df$date_exam), '%d/%m/%Y'), strptime(as.character(df$date_birth), '%d/%m/%Y'))/365.25

 which gives me as an output:

Time differences of 47.59491, 47.94251, 48.20260 days
 theses values are actually the 3 ages (but

 My questions are:

 1- Why in the output it says "days" instead of "years")

 2- How can I obtain the output as a numeric vector, without the statement "Time difference of ....". This is in order to use it in my calculations.

 3- Is there a way quicker and less redondant  to compute the age form the date_birth and date_exam?

 Thanks a lot,

 Bernard,

---------------------------------

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