R - 根据条件组合行以获得平均值/平均值

Question

Hei, I have a question regarding merging rows to get the average of a column, based on conditions of column value in R.

I would like to merge 2 rows in a data frame to get the average value of a column based on conditions on other column: For instance (see example of data set below) when the columns: depth == 20 & Species == "Diatoms" & locationID =="A", I would like to get the average value of the column quantity, add this value into 1 of the 2 row and delete the other.

 structure(list(depth = c(20, 20, 2, 4, 10), Species = c("Diatoms", 
"Diatoms", "Dinoflagellates", "Dinoflagellates", "Ciliates"), 
    locationID = c("A", "A", "B", "C", "A"), quantity = c(2, 
    4, 1, 2, 5)), row.names = c(NA, -5L), class = c("tbl_df", 
"tbl", "data.frame"))```

Answer 1

Since it looks like your data frame is set up as a tibble, I'm guessing you are using dplyr. In that case, you should be able to use a combination of group_by() and summarize() to do what you are trying to do. Are you looking for average quantities for each species, location and pair, or just one?

example using mtcars data set:

library(dplyr)

mtcars %>%
      group_by(gear,  cyl,carb)%>%
      summarize(hp.mean = mean(hp))

# # A tibble: 12 x 4
# # Groups:   gear, cyl [8]
# gear   cyl  carb hp.mean
# <dbl> <dbl> <dbl>   <dbl>
# 1     3     4     1    97  
# 2     3     6     1   108. 
# 3     3     8     2   162. 
# 4     3     8     3   180  
# 5     3     8     4   228  
# 6     4     4     1    72.5
# 7     4     4     2    79.5
# 8     4     6     4   116. 

This approach merges all rows with matching gear, cyl and carb and averages (mean in this case) the hp for all of the matching rows.

If on the other hand you just want the mean of one case, you can subset or filter then take the mean.

mtcars %>%
      filter(
            gear == 3,
            cyl == 8, 
            carb == 3
      ) %>%
      pull(hp) %>%
      mean()
# [1] 180

# base approach to return single answer
mean(mtcars$hp[mtcars$gear == 3 &
               mtcars$cyl == 8 &
               mtcars$carb == 3
               ])
# [1] 180