I have a data frame with the following data:
z = data.frame(date = strptime(c(20110101,20110102,20110103,20110104,20110105,20110106),
format = '%Y%m%d'),
rate1=c(1,2,3,4,5,6),
rate2=c(2,1,3,6,8,4),
rate3=c(4,1,3,6,8,3),
rate4=c(7,8,9,2,1,8))
Use pmax to get the maximum value of the 'rate columns' for each row:
z$max = pmax(rate1,rate2,rate3,rate4)
# date rate1 rate2 rate3 rate4 max
# 1 2011-01-01 1 2 4 7 7
# 2 2011-01-02 2 1 1 8 8
# 3 2011-01-03 3 3 3 9 9
# 4 2011-01-04 4 6 6 2 6
# 5 2011-01-05 5 8 8 1 8
# 6 2011-01-06 6 4 3 8 8
The pmax function allows me to get the maximum value for each row, but I was wondering how I can get the index of the maximum value for that record.
Where z$max equal the maximum values c(7, 8, 9, 6, 8, 8), I would like to get the corresponding column indices c(5, 5, 5, 3, 3, 5)
Is this possible? I know this seems like something simple but I cannot find the answer anywhere.
Very simple in base R:
z$wmax <- apply(z[, -c(1,6)],1, which.max)
Actually that gives you 1 less than what you were asking for since I excluded the first column but that can easily be remedied by adding one.
z$max_col_n <- apply(z[, -c(1,6)],1, which.max) +1