[R] add specific fields in for loop

@vi@e@gross m@iii@g oii gm@ii@com
2022-11-15T17:22:22Z

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 at 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 at 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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