R for loop not working

Question

I\'m trying to use R to find the max value of each day for 1 to n days. My issue is there are multiple values in each day. Heres my code. After I run it incorrect number of

Answer 1

Here is the solution using plyr package

mydata<-structure(list(Day = structure(c(2L, 2L, 2L, 2L, 3L, 3L, 3L, 
3L), .Label = c("", "x", "y"), class = "factor"), Value = c(0L, 
1L, 2L, 3L, 12L, 0L, 1L, 5L), Time = c(5L, 6L, 7L, 8L, 1L, 2L, 
3L, 4L)), .Names = c("Day", "Value", "Time"), row.names = c(NA, 
8L), class = "data.frame")
library(plyr)
ddply(mydata,.(Day),summarize,max.value=max(Value))

  Day max.value
1   x         3
2   y        12

Updated1: If your day is say 10/02/2012 12:00:00 AM, then you need to use:

mydata$Day<-with(mydata,as.Date(Day, format = "%m/%d/%Y"))
ddply(mydata,.(Day),summarize,max.value=max(Value))

Please see here for the example.

Updated2: as per new data: If your day is like the one you updated, you don't need to do anything. You can just use the code as following:

    mydata1<-structure(list(Day = c(20130310L, 20130310L, 20130310L, 20130310L, 
    20130311L, 20130311L, 20130311L, 20130311L), Time = structure(c(1L, 
    2L, 3L, 4L, 1L, 2L, 3L, 4L), .Label = c("9:30:00", "9:31:00", 
    "9:32:00", "9:33:00"), class = "factor"), Value = c(5L, 1L, 2L, 
    3L, 12L, 0L, 1L, 5L)), .Names = c("Day", "Time", "Value"), class = "data.frame", row.names = c(NA, 
    -8L))



ddply(mydata,.(Day),summarize,Time=Time[which.max(Value)],max.value=max(Value))
       Day    Time max.value
1 20130310 9:30:00         5
2 20130311 9:30:00        12

If you want the time to appear in the output, then just use Time=Time[which.max(Value)] which gives the time at the maximum value.