Replace empty values with value from other column in a dataframe

Try ifelse:

df$baglocatie <- ifelse(df$baglocatie == "", df$knmilocatie, df$baglocatie)

You need to index also the replacing column:

df[ df$baglocatie == "", "baglocatie"  ]  <- df[ df$baglocatie == "", "knmilocatie" ]

Or you can use a support vector:

empty <- df$baglocatie == ""
df$baglocatie[empty] <- df$knmilocatie[empty]