Updating a subset of a dataframe

Question

I have a dataframe as such

col.1 <- c(\"a\", \"b\", \"c\", \"d\", \"e\", \"b\", \"c\")
col.2 <- c(22, 33, 55, 66, 66, 22, 54)
df <- data.frame(col.1         


        

Answer 1

We could also use data.table. We convert the 'data.frame' to 'data.table' (setDT(df)), set the 'key' column as 'col.1', subset the rows with 'search.df', and assign (:=) the corresponding values of 'col.2' to 100. It should be fast for big datasets as we are replacing in place.

library(data.table)#v1.9.6+
setDT(df, key='col.1')[search.df, col.2 := 100]
df
#    col.1 col.2
#1:     a    22
#2:     b   100
#3:     b   100
#4:     c    55
#5:     c    54
#6:     d   100
#7:     e    66

Answer 2

df[df[,1] %in% search.df, 2] <- 100

or if you want to use column elements of the data frame directly

df$col.2[df$col.1 %in% search.df] <- 100

For simplicity, the same broken down:

# get index of rows to be updated by checking each value 
# in col1 against search.df => e.g. FALSE, TRUE, FALSE, ...
index <- df[,1] %in% search.df

# update col2 where index is TRUE to a new value
df[index, 2] <- 100