function applied to summarise + group_by doesn't work correctly

Question

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:

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:

Answer 1

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))