New Column Based on Conditions - r

Question

To set the scene, I have a set of data where two columns of the data have been mixed up. To give a simple example:
df1 <- data.frame(Name = c("Bob", "John", "Mark", "Will"), City=c("Apple", "Paris", "Orange", "Berlin"), Fruit=c("London", "Pear", "Madrid", "Orange"))
df2 <- data.frame(Cities = c("Paris", "London", "Berlin", "Madrid", "Moscow", "Warsaw"))
As a result, we have two small data sets:
> df1
Name City Fruit
1 Bob Apple London
2 John Paris Pear
3 Mark Orange Madrid
4 Will Berlin Orange
> df2
Cities
1 Paris
2 London
3 Berlin
4 Madrid
5 Moscow
6 Warsaw
My aim is to create a new column where the cities are in the correct place using df2. I am a bit new to R so I don't know how this would work.
I don't really know where to even start with this sort of a problem. My full dataset is much larger and it would be good to have an efficient method of unpicking this issue!

Answer 1

If the 'City' values are only different. We may loop over the rows, create a logical vector based on the matching values with 'Cities' from 'df2', and concatenate with the rest of the values by getting the matched values second in the order
df1[] <- t(apply(df1, 1, function(x)
{
i1 <- x %in% df2$Cities
i2 <- !i1
x1 <- x[i2]
c(x1[1], x[i1], x1[2])}))
-output
> df1
Name City Fruit
1 Bob London Apple
2 John Paris Pear
3 Mark Madrid Orange
4 Will Berlin Orange

Answer 2

using dplyr package this is a solution, where it looks up the two City and Fruit values in df1, and takes the one that exists in the df2 cities list.
if none of the two are a city name, an empty string is returned, you can replace that with anything you prefer.
library(dplyr)
df1$corrected_City <- case_when(df1$City %in% df2$Cities ~ df1$City,
df1$Fruit%in% df2$Cities ~ df1$Fruit,
TRUE ~ "")
output, a new column created as you wanted with the city name on that row.
> df1
Name City Fruit corrected_City
1 Bob Apple London London
2 John Paris Pear Paris
3 Mark Orange Madrid Madrid
4 Will Berlin Orange Berlin

Answer 3

Another way is:
library(dplyr)
library(tidyr)
df1 %>%
mutate(across(1:3, ~case_when(. %in% df2$Cities ~ .), .names = 'new_{col}')) %>%
unite(New_Col, starts_with('new'), na.rm = TRUE, sep = ' ')
Name City Fruit New_Col
1 Bob Apple London London
2 John Paris Pear Paris
3 Mark Orange Madrid Madrid
4 Will Berlin Orange Berlin