Loop through dataframe rows and add value in a new column (R) - r

Question

I have a dataframe (df) with a column of Latitude (Lat), and I need to match up the corresponding Longitude value (based off relationships in another dataset). New column name is 'Long_matched'.
Here, I am trying to write a new value in the column 'Long_matched' at the corresponding row to latitudes between -33.9238 and -33.9236. The data in 'Lat' has many more decimal places (e.g: -33.9238026666667, -33.9236026666667, etc.). As I will be applying this code to multiple datasets over the same geographical location (hence the long decimals will vary slightly), I want to write Longitude values which fall within a a 0.0002 degree range.
Some attempts of code I have tried include:
df$Long_matched <- ifelse(df$Lat< -33.9236 & df$Lat> -33.9238, 151.2279 , "N/A")
or
df$Long_matched[df$Lat< -33.9236 & df$Lat> -33.9238] <- 151.2279
I think I need to use a for loop to loop through the rows and an if statement, but struggling to figure this out - any help would be appreciated!
Resulting output should look something like this:
Lat Long_matched
-33.9238026666667 151.2279
-33.9236026666667 (new long value will go here)

Answer 1

Everything said in the comments applies, but here is a trick you can try:
In the following code, you will need to replace text with numbers.
Latitude_breaks <- seq(min_latitude, max_latitude, 0.0002) # you need to replace `min_latitude`, `max_latitude`, and `increment` with numbers
Longitude_values <- seq(first, last, increment) # you need to replace `first`, `last` and `increment` with numbers
df <- within(df, {
# make a categorical version of `Lat`
Lat_cat <- cut(Lat, Latitude_breaks)
Long_matched <- Longitude_values[Lat_cat]
})
A few notes:
the values between min_latitude and min_latitude + 1 will be assigned to the values of Longitude marked first.
The length of Latitude_beaks should be one more than the length of Longitude_values.
Values of Lat outside of Latitude_breaks will becomes NAs.
This works by exploiting a nice feature of factors - they are stored as integers. So we can use them to index another vector - in this case, Longitude_values.