How to make add a values in column with values based on a list

Question

Im working on a dataset which I simplify here.

Suppose I have one data below:

col1 <- c("AAA", "BBB", "CCC")
col2 <- c("auto", "bike", "car")

data1 <- data.frame(col1, col2)

And another data below:

colA <- c("AAA", "BBB", "CCC", "DDD")
colB <- c("1", "2", "3", "4")

data2 <- data.frame(colA, colB)

I would like to add col2 from data1 as another column in data2 that matches colA. I'd like to use ifelse in a loop but im really confused how to do this. Below is my poor attempt.

for(i in data1$col1){
  data2$col2 <- ifelse(data1$col1 == i, data1$col2, NA)
}

but im getting this error

Error in $<-.data.frame(*tmp*, "col2", value = c("auto", NA, NA)) : replacement has 3 rows, data has 4

Im new in loop. Please guide. Thank you.

You may need match or named vector i.e. data2$col2 <- setNames(data1$col2, data1$col1)[data2$colA] — akrun
– akrun, Commented Aug 16, 2022 at 21:50

dcsuka · Accepted Answer · 2022-08-16 21:48:50Z

2

Try a left_join like this:

library(dplyr)

left_join(data1, data2, by = c("col1" = "colA"))

#   col1 col2 colB
# 1  AAA auto    1
# 2  BBB bike    2
# 3  CCC  car    3

Or alternatively:

left_join(data2, data1, by = c("colA" = "col1"))

#   colA colB col2
# 1  AAA    1 auto
# 2  BBB    2 bike
# 3  CCC    3  car
# 4  DDD    4 <NA>

answered Aug 16, 2022 at 21:48

dcsuka

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2 Comments

Kelbino Over a year ago

Thanks dcsuka, this works perfectly and thus helps for my particular dataset. But I do also want to learn how to use loop to achieve this. Do you think its possible?

dcsuka Over a year ago

Yes, but it is much slower, less efficient, and less concise. There is a de facto taboo against for loops in R. If this solution helped you, consider leaving a green check mark so others know it is correct.

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