R：在一个列中添加来自两个数据帧的匹配向量值

Question

I have a data frame which is configured roughly like this:

df <- cbind(c('hello', 'yes', 'example'),c(7,8,5),c(0,0,0))

words	frequency	count
hello	7	0
yes	8	0
example	5	0

What I'm trying to do is add values to the third column from a different data frame, which is similiar but looks like this:

df2 <- cbind(c('example','hello') ,c(5,6))

words	frequency
example	5
hello	6

My goal is to find matching values for the first column in both data frames (they have the same column name) and add matching values from the second data frame to the third column of the first data frame.

The result should look like this:

df <- cbind(c('hello', 'yes', 'example'),c(7,8,5),c(6,0,5))

words	frequency	count
hello	7	6
yes	8	0
example	5	5

What I've tried so far is:

df <- merge(df,df2, by = "words", all.x=TRUE) 

However, it doesn't work.

I could use some help understanding how could it be done. Any help will be welcome.

Answer 1

This is an "update join". My favorite way to do it is in dplyr :

library(dplyr)
df %>% rows_update(rename(df2, count = frequency), by = "words")

In base R you could do the same thing like this:

names(df2)[2] = "count2"
df = merge(df, df2, by = "words", all.x=TRUE)
df$count = ifelse(is.na(df$coutn2), df$count, df$count2)
df$count2 = NULL

Answer 2

Here is an option with data.table :

library(data.table)

setDT(df)[setDT(df2), on = "words", count := i.frequency]

Output

     words frequency count
    <char>     <num> <num>
1:   hello         7     6
2:     yes         8     0
3: example         5     5

Or using match in base R:

df$count[match(df2$words, df$words)] <- df2$frequency

Or another option with tidyverse using left_join and coalesce :

library(tidyverse)

left_join(df, df2 %>% rename(count.y = frequency), by = "words") %>%
  mutate(count = pmax(count.y, count, na.rm = T)) %>%
  select(-count.y)

Data

df <- structure(list(words = c("hello", "yes", "example"), frequency = c(7, 
8, 5), count = c(0, 0, 0)), class = "data.frame", row.names = c(NA, 
-3L))

df2 <- structure(list(words = c("example", "hello"), frequency = c(5, 6)), class = "data.frame", row.names = c(NA, 
-2L))