R multiply columns by values in second dataframe

2
votes

I have two dataframes: df1

plot   grass moss   rock    other     stuff  
a      4      0     0       text      3
b      2      2     3       text      4
c      10     0     0       text      0
d      3      1     0       text      9

and df2

Cover  value
grass   5
moss    2
rock    3

I would like to multiply the values in df1 by the corresponding value in df2. The solution should be applicable for a large dataset.

Result df3

plot  grass moss    rock    
a     20      0     0      
b     10      4     9      
c     50      0     0       
d     15      2     0
2
rdataframe
My impression is that you want to multiply the grass column in df1 by the factor in df2, etc for the other variables in df2. If this is the case, this is not reflected in your df3. - Paul Hiemstra

2 Answers

5
votes

Your data:

df1 <- read.table(
  text = 
    "plot   grass moss   rock    other     stuff  
    a      4      0     0       text      3
    b      2      2     0       text      4
    c      10     0     0       text      0
    d      3      1     0       text      9",
  header = TRUE
)

df2 <- read.table(
  text = 
    "Cover  value
    grass   5
    moss    2
    rock    3",
  header           = TRUE,
  stringsAsFactors = FALSE
)

(Make sure that Cover is a character vector, not a factor, for this solution.)

A simple for loop does the trick:

df3 <- df1
for(i in seq_len(nrow(df2)))
{
  df3[, df2$Cover[i]] <- df2$value[i] * df1[, df2$Cover[i]]
}
0
votes

For those looking for a tidyverse approach the following also works using the new across() and cur_column() functions:

df1 %>% 
  mutate(across(.cols = c(grass, moss, rock),
                .fns = function(x){
                  x * (df2 %>% 
                         filter(Cover == cur_column()) %>% 
                         pull(value))
                }))