functions functioning learning

[MarianoDem]

Hello, how are you doing, I am taking an R course and I have reached a point where it asks me to put together some functions which I am not being able to make work, I estimate that it is due to my brief introduction to the subject and I still do not have enough understanding to carry out that activity.

I would like to know if someone can tell me or help me how to perform a function that fulfills 3 activities, one is to choose the destination folder where I want the files to be searched, the second is to allow me to pre-select between one or more files that only have consecutive numbers as names. Starting from 1 to 300 and third point, within those database files, choose which columns I want to see.
it have to look samoethign like this

pollutantmean <- function(directory, pollutant, id = 1:332){

}
Could someone give me some kind of guide?

[FJCC]

Can you write the code to search in a directory named May2023 for a file named 42 and read the column named NO2? Please show how you would do that or, if you can't do all of it, show the parts you can do.

[MarianoDem]

something like
if the file is csv
read.csv(.../May2023/42.csv, header = TRUE, sep = " , ", ...)
...thats all i can think right now

[FJCC]

Your call to csv contains the directory name May2023 and the file name 42.csv. I'll note that you did not put quotes around ".../May2023/42.csv" which would have caused an error and I thing the directory name should not start with ".../". If you were building the file path up from its parts, you could use the paste0() function.

paste0("May2023/", "42", ".csv")
[1] "May2023/42.csv"

You could put that work into a function, like this

MakePath <- function(directory, file) {
   FullName = paste0(directory, file, ".csv")
   return(FullName)
 }
 MakePath(directory = "May2023/", file = "42")
[1] "May2023/42.csv"

You don't want to just make the file name, you want to use it in read.csv(). Can you figure out how to do that part within the MakePath function?

[FJCC]

Also, if you pass in a range of numbers like 40:42, the function will make a vector of file names, one for each number.

MakePath <- function(directory, file) {
   FullName = paste0(directory, file, ".csv")
   return(FullName)
 }
MakePath(directory = "May2023/", file = 40:42)
[1] "May2023/40.csv" "May2023/41.csv" "May2023/42.csv"

Do you know how to loop through that vector and pass each individual value to read.csv()?