[R] to create a new variable based on values in other variables

[Bert Gunter]

You can use subscripting to generalize and avoid multiply nested
ifelse's which, I agree, can be a nightmare. However, you have to be
very careful about the logic of the conditions you create and the
order in which you apply them. It is very easy to wipe out an earlier
relationship with a later one (I speak from sad experience here). Note
that because the following uses subscripting, it's vectorized. But you
of course have to set up all your conditions manually. Note that your
last example condition is redundant, btw. As Thierry indicated,
depending on what you do, there can be shortcuts. To keep the solution
generalizable, I have not used any.

> tem2 <-data.frame(m1=c(12, 23, 22, 23), m2=c(23, 23, 3, 5), m3 =c(12, 23, 12.5, 5))
> tem2$ravg <- rowMeans(tem2[,c("m1","m2")])
## or use with() or within() for more complex functions that  you have
to code yourself.
> cond1 <- with(tem2,m1!= 23 & m2 == 23)
> cond2  <- with(tem2, m1 == 23 & m2 != 23)
## etc.
> tem2 <- within(tem2,{
+       ravg[cond1] <- m1[cond1]
+       ravg[cond2] <- m2[cond2]
+    })
>
> tem2
  m1 m2   m3 ravg
1 12 23 12.0 12.0
2 23 23 23.0 23.0
3 22  3 12.5 12.5
4 23  5  5.0  5.0


Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )

On Tue, Apr 7, 2020 at 11:34 AM Yuan Chun Ding <ycding using coh.org> wrote:
>
> Hi R users,
>
>
> I want to create a new variable, Ravg, in data frame tem2 based on values of two other variables m1 and m2.
>
> the condition:
>
> if m1 = 23 and m2 =23 then Ravg =23;
> else if m1 != 23 and m2=23 then Ravg =m1;
> else if m1 =23 and m2 !=23 then Ravg=m2;
> else Ravg=average of m1 and m2;
>
> the Ravg variable should be  same as  m3 variable in the following small example.
>
> my R code did not generate errors but not generate a new variable.
>
> Ravg <- "rare_allele"
>   tem2 <-data.frame(m1=c(12, 23, 22, 23), m2=c(23, 23, 3, 5), m3 =c(12, 23, 12.5, 5))
>   for (r in 1:nrow(tem2)) {
>   if (tem2$m1[r] ==23 & tem2$m2[r] ==23) {
>     tem2[[Ravg]][r] ==23} else if(tem2$m1[r] ==23 & tem2$m2[r] !=23){
>     tem2[[Ravg]][r] ==tem2$m2[r]} else if (tem2$m1[r] !=23 & tem2$m2[r] ==23) {
>     tem2[[Ravg]][r] ==tem2$m1[r]} else {
>     tem2[[Ravg]][r] == mean(tem2$m1[r] + tem2$m2[r])}
>                           }
>
> Thank you,
>
> Ding
>
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