Apply function to columns in a list of data frames and append results

Question

I want to apply a function to a list of data frames. the function takes elements from two columns in each data frame adds them and then adds the output to each data frame in a new column.

Create dummy data:

df.1 <- data.frame(data=cbind(rnorm(5, 0), rnorm(5, 2), rnorm(5, 5)))
df.2 <- data.frame(data=cbind(rnorm(5, 0), rnorm(5, 2), rnorm(5, 5)))

names(df.1) <- c("a", "b", "c")
names(df.2) <- c("a", "b", "c")

ls.1<- list(df.1,df.2)
names(ls.1) <- c("cat", "dog")
ls.1

have a look at the data:

> ls.1
$cat
          a        b        c
1 0.7031868 1.730499 4.286386
2 0.1527551 2.794084 4.348707
3 1.1151157 0.154562 4.647605
4 0.5786497 1.407386 4.118078
5 0.9223104 2.995469 5.065981

$dog
            a         b        c
1  0.04024872 1.6760609 5.013490
2  0.18095899 2.1015250 3.452313
3 -0.86588484 2.1371948 6.389203
4 -0.39499567 0.5996709 5.399724
5 -1.31850123 3.0058084 5.530989

Pseudo code of what I want to do:

 my.fun <- function(b, c) {
  out.put <- b + c
  ls.1[i]$d <- out.put
}

What I want the output to look like:

> ls.1
$cat
          a        b        c        d
1 0.7031868 1.730499 4.286386 6.689551
2 0.1527551 2.794084 4.348707 5.553838
3 1.1151157 0.154562 4.647605 8.526398
4 0.5786497 1.407386 4.118078 5.999395
5 0.9223104 2.995469 5.065981 8.536797

$dog
            a         b        c        d
1  0.04024872 1.6760609 5.013490 6.689551
2  0.18095899 2.1015250 3.452313 5.553838
3 -0.86588484 2.1371948 6.389203 8.526398
4 -0.39499567 0.5996709 5.399724 5.999395
5 -1.31850123 3.0058084 5.530989 8.536797

I think it should be easily achievable with mapply or something similar but I I cannot figure it out....

Answer 1

lapply works fine here. Note that a return(x) is needed here, otherwise we would just return the new vector.

res <- lapply(ls.1, function(x){
 x$d <- x$b + x$c
 return(x)
})