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I extract my data 

fluo <- read.csv("data/ctd_SOMLIT.csv", sep=";", stringsAsFactors=FALSE)

I display in three columns : the day, the month and the year based on the original date : Y - m - d

fluo$day <- day(as.POSIXlt(fluo$DATE, format = "%Y-%m-%d"))
fluo$month <- month(as.POSIXlt(fluo$DATE, format = "%Y-%m-%d"))
fluo$year <- year(as.POSIXlt(fluo$DATE, format = "%Y-%m-%d"))

This is a part of my data_frame:

data.frame

Then, I do summarise and group_by in order to apply the function : 

prof_DCM = fluo[max(fluo$FLUORESCENCE..Fluorescence.),2]

=> I want the depth of the max of FLUORESCENCE measured for each month, for each year.

mean_fluo <- summarise(group_by(fluo, month, year), 
                       prof_DCM = fluo[max(fluo$FLUORESCENCE..Fluorescence.),2])
mean_fluo <- arrange(mean_fluo, year, month)
View(mean_fluo)

But it's not working ... The values of prof_DCM still the same all along the column 3 of the data_frame:

same value for column 3

zx8754
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Loulou
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  • Do not post your data as an image, please learn how to give a [reproducible example](http://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example/5963610). – Jaap Oct 13 '16 at 09:52

1 Answers1

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Maybe try the following code. 

library(dplyr)
mean_fluo <- fluo %>%
group_by(month,year) %>%
filter(FLUORESCENCE..Fluorescence. == max(FLUORESCENCE..Fluorescence.)) %>%
arrange(year,month)

View(mean_fluo)

You can select the variables you want to keep with 'select'

mean_fluo <- fluo %>%
group_by(month,year) %>%
filter(FLUORESCENCE..Fluorescence. == max(FLUORESCENCE..Fluorescence.)) %>%
arrange(year,month)%>%
select(c(month,year,PROFONDEUR))
Micky
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