根据另一列中的值减去一列中的数据

Question

I want to subtract the values in the Temp column based on the values in the sensor column. Grouped by each date and location, I want to subtract Temp with the sensor value of 1 from Temp with a sensor value of 2. See data sample below.

date <- c("2016-03-21","2016-03-21","2016-03-21","2016-03-21","2016-03-21","2016-03-21")
location <- c(1,1,2,2,3,3)
sensor <- c(1,16,1,16,1,16)
Temp <- c(35,34,45,42,46,47)
df <- data.frame(date,location,sensor,Temp)

This is my attempt at trying to do so using dplyr ...

test <- df %>% group_by(date,location,sensor) %>% lfMaxTemp$Temp["sensor"==1]-lfMaxTemp$Temp["sensor"==16]

This is the result I would like:

        date location diff
1 2016-03-21        1    1
2 2016-03-21        2    3
3 2016-03-21        3   -1

Answer 1

library(dplyr)
df %>% group_by(date, location) %>% summarise(diff = Temp[sensor==1]- Temp[sensor==16])
#        date location  diff
#1 2016-03-21        1     1
#2 2016-03-21        2     3
#3 2016-03-21        3    -1

Answer 2

We can use data.table . Convert the 'data.frame' to 'data.table' ( setDT(df) ), grouped by 'date', 'location', order the rows based on 'sensor' in descending, and we summarise the output by the taking the difference between the 1st and 2nd observations with diff

library(data.table)
setDT(df)[order(-sensor), .(Diff  = diff(Temp)), .(date, location)]
#         date location Diff
#1: 2016-03-21        1    1
#2: 2016-03-21        2    3
#3: 2016-03-21        3   -1

NOTE: Here we assume that there are only 1 and 16 values in 'Temp' for each 'Temp'

In case, there are other values in 'Temp', just do a filter before the group by step

 setDT(df)[Temp %in% c(1, 16)][order(-sensor), .(Diff  = diff(Temp)), .(date, location)]

data

df <- data.frame(date,location,sensor,Temp)

Comments

It is not recommended to use as.data.frame(cbind(.. to construct the data.frame as it can result in all the columns to be in class factor/character .