trying to add a column to a dataframe from an existing dataframe in r

Question

I have created a new dataframe (called "averages") in r based on an already existing dataframe (named "old). "averages" has a column with all the unique values from a column in "old". Now I would like to add another column from "old" to "averages", with values from "old" that match the values associated with the unique values from the other column in "old".

What I used to make the new dataframe:

> averages <- data.frame(unique(old$Tree), avg) ## creating new dataframe

This was the code I tried to add the new column:

> averages <- data.frame(unique(old$Tree), avg, old$Site) ## creating new dataframe

And I get this error:

    Error in data.frame(unique(old$Tree), avg, old$Site) : arguments imply differing number of rows: 50, 1110

I tried fixing the issue with this but thought it wouldn't work (it didn't):

> averages <- data.frame(unique(old$Tree), avg, unique(old$Site)) ## creating new dataframe

    Error in data.frame(unique(old$Tree), avg, unique(old$Site)) : arguments imply differing number of rows: 50, 3 

Any help on how to add this new column would be much appreciated!

Answer 1

It looks like you could do it using dplyr

install.packages("tidyverse")
library(tidyverse)

df <- left_join(averages, old, by = c("ID_col_in_averages" = "ID_col_in_old"))

where if I understand correctly both ID colums to use in the join are Tree