[R] Occurrence by season

[ani jaya]

Dear r community,

I have a data frame that shows the month of occurrence of the maximum
value on let say 10 stations in 30 years.  I want to make percentages
based on seasons (DJF, MAM, JJA, and SON). I can do that by the code
below but only when we put season by JFM, AMJ, ...(or 123; 456;
789;..).
I believe it is not that hard to put 12 in front of 1 but my mind stuck.

If someone can help me, I would be appreciated.

Thank you in advance

Ani


freqw<-matrix(NA, 4,10)
for(j in 1:4){
  for(i in 1:10){
      freqw[j,i]<-sum(table(when[i+1])[(j*3-2):(j*3)],na.rm=T)
  }
}
freqw<-freqw/30*100


> dput(when)
structure(list(Group.1 = structure(1:30, .Label = c("1985", "1986",
"1987", "1988", "1989", "1990", "1991", "1992", "1993", "1994",
"1995", "1996", "1997", "1998", "1999", "2000", "2001", "2002",
"2003", "2004", "2005", "2006", "2007", "2008", "2009", "2010",
"2011", "2012", "2013", "2014"), class = "factor"), V1 = c(8L,
1L, 1L, 12L, 12L, 1L, 4L, 1L, 2L, 3L, 7L, 4L, 1L, 6L, 2L, 2L,
11L, 4L, 2L, 2L, 12L, 6L, 2L, 2L, 6L, 3L, 6L, 4L, 7L, 1L), V2 = c(1L,
1L, 1L, 1L, 12L, 12L, 4L, 1L, 2L, 2L, 4L, 1L, 11L, 1L, 1L, 3L,
3L, 1L, 12L, 2L, 5L, 2L, 12L, 2L, 1L, 5L, 10L, 4L, 12L, 1L),
    V3 = c(4L, 1L, 12L, 12L, 2L, 4L, 1L, 5L, 1L, 1L, 2L, 2L,
    1L, 1L, 2L, 2L, 4L, 1L, 2L, 12L, 3L, 2L, 12L, 4L, 6L, 4L,
    2L, 3L, 5L, 1L), V4 = c(1L, 12L, 12L, 3L, 2L, 12L, 7L, 2L,
    12L, 1L, 12L, 2L, 1L, 12L, 1L, 10L, 7L, 1L, 1L, 3L, 10L,
    12L, 12L, 12L, 5L, 12L, 3L, 2L, 12L, 1L), V5 = c(1L, 12L,
    12L, 3L, 2L, 12L, 7L, 2L, 12L, 3L, 4L, 12L, 5L, 2L, 1L, 1L,
    2L, 5L, 2L, 2L, 3L, 2L, 10L, 3L, 1L, 1L, 4L, 1L, 4L, 6L),
    V6 = c(12L, 1L, 12L, 2L, 1L, 12L, 3L, 1L, 4L, 12L, 6L, 12L,
    1L, 7L, 2L, 2L, 1L, 1L, 12L, 12L, 12L, 3L, 12L, 2L, 1L, 9L,
    12L, 1L, 4L, 12L), V7 = c(5L, 2L, 6L, 12L, 3L, 4L, 12L, 12L,
    6L, 2L, 11L, 12L, 2L, 5L, 11L, 4L, 3L, 4L, 1L, 12L, 10L,
    3L, 10L, 3L, 3L, 9L, 1L, 3L, 12L, 11L), V8 = c(4L, 5L, 5L,
    8L, 7L, 12L, 1L, 3L, 2L, 5L, 2L, 4L, 1L, 4L, 1L, 6L, 6L,
    2L, 3L, 5L, 4L, 1L, 3L, 12L, 4L, 12L, 12L, 3L, 6L, 7L), V9 = c(3L,
    2L, 3L, 11L, 7L, 2L, 4L, 4L, 5L, 6L, 6L, 3L, 4L, 6L, 1L,
    9L, 9L, 12L, 3L, 12L, 5L, 6L, 3L, 7L, 4L, 8L, 2L, 2L, 6L,
    6L), V10 = c(5L, 10L, 3L, 6L, 6L, 5L, 5L, 6L, 5L, 6L, 12L,
    6L, 7L, 2L, 1L, 6L, 4L, 5L, 4L, 2L, 7L, 6L, 7L, 7L, 2L, 7L,
    3L, 6L, 7L, 6L)), row.names = c(NA, -30L), class = "data.frame")