[R] Problem with diff(strptime(...

From: Jim Lemon <jim_at_bitwrit.com.au>
Date: Thu, 20 Mar 2008 21:59:06 +1100


Hi all,

I have been chipping away at a problem I encountered in calculating rates per year from a moderately large data file (46412 rows). When I ran the following command, I got obviously wrong output:

interval<-
  c(NA,as.numeric(diff(
  strptime(mkdf$MEAS_DATE,"%d/%m/%Y")))/365.25)

The values in MEAS_DATE looked like this:

mkdf$MEAS_DATE[1:10]
[1] 1/5/1962 1/5/1963 1/5/1964 1/3/1965 1/4/1966 1/4/1967
  1/6/1968
[8] 25/3/1969 1/4/1971 1/2/1974

146 Levels: 10/10/1967 1/10/1947 1/10/1965 1/10/1967 1/10/1983 ... 9/1/1992

To abbreviate three evenings of work, I finally found that values 17170 and 17171 were the same. If I ran the entire set, or anything over 1:17170, I would get output like this:

interval[1:10]
[1] NA 86340.86 86577.41 71911.29 93673.92 86340.86
  101006.98
[8] 70255.44 174337.58 245292.81

If I ran any set of values up to 17170, I would get the correct output:

interval[1:10]
[1] NA 0.9993155 1.0020534 0.8323066 1.0841889 0.9993155
  1.1690623
[8] 0.8131417 2.0177960 2.8390372

If I changed value 17171 by one day (and added that level), the command worked correctly:

interval[1:10]
[1] NA 0.9993155 1.0020534 0.8323066 1.0841889 0.9993155
  1.1690623
[8] 0.8131417 2.0177960 2.8390372

There have been a few messages about this problem, but apparently no solution. The problem can be seen with these examples (I haven't included the real data as it is not mine):

foodate<-c("1/7/1991","1/8/1991","1/8/1991","3/8/1991") as.numeric(diff(strptime(foodate,"%d/%m/%Y"))/365.25) [1] 7333.0595 0.0000 473.1006

foodate<-factor(c("1/7/1991","1/8/1991","1/8/1991","3/8/1991")) as.numeric(diff(strptime(foodate,"%d/%m/%Y"))/365.25) [1] 7333.0595 0.0000 473.1006

foodate<-factor(c("1/7/1991","1/8/1991","2/8/1991","3/8/1991"))  > as.numeric(diff(strptime(foodate,"%d/%m/%Y"))/365.25) [1] 0.084873374 0.002737851 0.002737851

Beats me.

Jim



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