Re: How to assign value to one row of database based on the value on another column?

Às 10:52 de 20/01/2026, Eric Berger escreveu:
> Here is one approach with base R, starting with your first 2 lines,
> then a 1-liner
> 
> df1 = data.frame(x = 1:20,
>                  y = rep(letters[1:5],4),
>                  w = c(rep(LETTERS[1],10), rep(LETTERS[2],10)),
>                  z = rnorm(20),
>                  stringsAsFactors = FALSE)
> df2 = data.frame(x = sample(df1$x, 5))
> 
> 
> df2 <- merge(df2, df1, by="x", sort=FALSE)[,c("x","z")]
> 
> HTH,
> Eric
> 
> 
> On Tue, Jan 20, 2026 at 12:25 PM Luigi Marongiu
> <[email protected]> wrote:
>>
>> Dear all,
>> this is a basic question but I could not find a specific answer online.
>> I have a dataframe (df1) with, in particular, an identification value
>> `x` and an output value `z`.
>> I also have another dataframe (df2) that shares `x`.
>> I need to assign `z` to df2 where df1$x is equal to df2$x.
>> What would be a straightforward way of doing this?
>> I can do it, but selecting one element at a time, checking the
>> identity of the x elements, and then assigning z. There should be a
>> more R way...
>> Thank you
>>
>>
>> ```
>> df1 = data.frame(x = 1:20,
>>                  y = rep(letters[1:5],4),
>>                  w = c(rep(LETTERS[1],10), rep(LETTERS[2],10)),
>>                  z = rnorm(20),
>>                  stringsAsFactors = FALSE)
>> df2 = data.frame(x = sample(df1$x, 5))
>> df2$z = NA
>> for (i in df1$x) {
>>    df2$z[df2$x==i] = df1[df1$x==i,]$z
>> }
>> ```
>>
>> ______________________________________________
>> [email protected] mailing list -- To UNSUBSCRIBE and more, see
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>> PLEASE do read the posting guide https://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
> 
> ______________________________________________
> [email protected] mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide https://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
Hello,

Another option is


i <- match(df2$x, df1$x)
df2$z <- df1$z[i]


or simply


df2$z <- df1$z[match(df2$x, df1$x)]


Hope this helps,

Rui Barradas