R: insert value in dataframe cell searching extracting it from another dataframe - r

Question

I have one dataframe looking as follows:
Date Element Problem Losses
1 2020-09-29 54 Energy loss NA
2 2020-09-30 54 Fault NA
3 2020-09-30 54 Energy loss NA
4 2020-09-29 40 Cooling NA
5 2020-09-29 50 Voltage NA
I would like to insert certain values in the Losses column whenever the problem column has the substring "Energy".
The values I need to insert are in another dataframe, looking like this:
Date Element Losses
1 2020-09-29 54 13.24
2 2020-09-30 54 12.16
This is just an example, as the actual dataframes I'm using are pretty big, so I'd like to do this with some type of merge by the Date and Element columns, instead with looping through both dataframes.
EDIT:
I've tried using a merge by the Element column, so first I get the Losses repeteadly for all the corresponding elements, and then putting those rows where I don't have my desired substring back as Nan.
My problem here is that merging by Element deletes all my other rows, getting only the following:
Date Element Problem Losses
1 2020-09-29 54 Energy loss 13.24
2 2020-09-30 54 Fault NA
3 2020-09-30 54 Energy loss 12.16

Answer 1

Base R solution:
transform(df, Losses = insert_df$Losses[match(paste0(Date, Element, grepl("Energy", Problem)),
paste0(insert_df$Date, insert_df$Element, "TRUE"))])
Data:
df <- structure(list(Date = structure(c(18534, 18535, 18535, 18534,
18534), class = "Date"), Element = c(54L, 54L, 54L, 40L, 50L),
Problem = c("Energy loss", "Fault", "Energy loss", "Cooling",
"Voltage"), Losses = c(NA, NA, NA, NA, NA)), row.names = c(NA,
-5L), class = "data.frame")
insert_df <- structure(list(Date = structure(18534:18535, class = c("IDate",
"Date")), Element = c(54L, 54L), Losses = c(13.24, 12.16)), class = "data.frame", row.names = c(NA,
-2L))