[R] add specific fields in for loop

[Kai Yang]

 Hello Bert and Avi,Sorry, it is typo. it should be:
for (i in colnames(df)){
  ......
}
below is the code I'm currently using
try2.un$ab2 <-
  ifelse(grepl("ab2",try2.un$data1), try2.un$data1,
         ifelse(grepl("ab2",try2.un$data2), try2.un$data2,
                ifelse(grepl("ab2",try2.un$data3), try2.un$data3,
                       ifelse(grepl("ab2",try2.un$data4), try2.un$data4,
                              ifelse(grepl("ab2",try2.un$data5), try2.un$data5,NA
                              ) ) ) ) )


As you can see, it uses 5 fields (data1 -- 5 ) in ifelse function. I want to turn it to for loop, because the number of data(s) fields is dynamic. In this sample is 5, But it maybe more than 15 in some of situation. So, I want use loop to solve it and avoid to write those many ifelse statement. Also, in try2.un data frame, there are many other fields that I don't need to use in the loop. 
I'm not sure if the loop is a correct solution. But I'm willing to learn any more suggestion from you.
Thanks,
Kai
    On Tuesday, November 15, 2022 at 09:23:03 AM PST, avi.e.gross using gmail.com <avi.e.gross using gmail.com> wrote:  
 
 Kai,

As Bert pointed out, it may not be clear what you want.

As a GUESS, you have some arbitrary data.frame object with multiple columns and you want to do something on selected columns. Consider changing your idea to be in several stages for simplicity and then optionally later rewriting it.

So step 1 is to get a vector of column names. The normal way to do this in base R is not with a function called columns(df) but colnames(df) ...

Step 2 is to use one of many techniques that take that vector of names and select the ones you want to keep. In base R there are many ways to do that including using regular expressions as in the "grep" family of functions. You may end up with a new vector of names perhaps shorter or in a different order.

Step 3 is to use those names in your loop. If you want say to convert a column from character to numeric, and your loop index is "current" you might write something like:
    df[current] <- as.numeric(df[current])

There are many ways and it depends on what exactly you want to do. There are packages designed to make some of these things fairly simple, such as dplyr where you can ask to match names that start or end a certain way or that are of certain types.

Avi

-----Original Message-----
From: R-help <r-help-bounces using r-project.org> On Behalf Of Kai Yang via R-help
Sent: Tuesday, November 15, 2022 11:18 AM
To: R-help Mailing List <r-help using r-project.org>
Subject: [R] add specific fields in for loop

Hi Team,
I can write a for loop like this:
for (i in columns(df)){
  ......
}

But it will working on all column in dataframe df. If I want to work on some of specific fields (say: the fields' name content 'date'), how should I modify the for loop? I changed the code below, but it doesn't work.
for (i in columns(df) %in% 'date' ){
  .....
}


Thank you,
Kai

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