Replacing values in a column where there is a match

Question

I\'m new to R programming and I\'m stuck on the example below.

Basically I have two data sets:

dataset1:

ID       Category        
1        CatZ         


        

Answer 1

Another option is to use data.table package.

Using the same setup as @tmfmnk in his answer:

Construct the sample data set:

df1 <- read.table(text = "ID       Category        
1        CatZZ         
2        CatVV         
3        CatAA  
4        CatQQ", header = TRUE, stringsAsFactors = FALSE)

df2 <- read.table(text = "ID  Category  
1   Cat600  
                  3   Cat611", header = TRUE, stringsAsFactors = FALSE)

Load the data.table package and convert dataframes to data tables:

library(data.table)
df1 <- data.table(df1)
df2 <- data.table(df2)

Perform a left join

(take all values from df1, where ID matches with df2, and add there the category from df2, then create a new column combining info from df1 and df2)

a <- df2[df1, on = "ID"][, a := ifelse(is.na(Category), i.Category, Category)]

There is a nice question and answer on data.table joins here: Left join using data.table

Also, to get exactly the result you asked for, you can do:

a <- df2[df1, on = "ID"][, list(ID, Category = ifelse(is.na(Category), i.Category, Category))]