vanao veneri - 4 months ago 20

R Question

I would like to use apply with a data.frame that contains lists in one column. That is, one column contains vectors and I wand to perform various operations on these vectors. For example, I would like to find out how many elements there are with a certain value.

What's my mistake here?

`#reproducible code`

set.seed(1)

some_list <- replicate(40, sample(c(1:8), size=sample(3:6, 1), replace=TRUE))

exdf <- expand.grid(id=c(1:10), content=c(1:4))

exdf$dv <- some_list

exdf$edge <- replicate(40, sample(3:5, size=1))

#Prolem: get elements with lesser value than "edge" in "dv"

result <- sapply(dv~., data=exdf, function(x) which(x[, "dv"] < x[, "edge"]))

Answer

We can use `Map`

to do the comparison between corresponding elements in the columns

```
Map(function(x,y) which(x <y), exdf$dv, exdf$edge)
```

If we are only interested in the number of elements, use the `sum`

and with `mapply`

it returns a `vector`

```
mapply(function(x,y) sum(x <y), exdf$dv, exdf$edge)
```

NOTE: `Map`

is just a wrapper for `mapply`

which always returns a `list`

output.

Or another option is `unnest`

(from `tidyr`

) with `dplyr`

functions

```
library(dplyr)
library(tidyr)
unnest(exdf, .id = "grp") %>%
group_by(grp) %>%
summarise(n = sum(dv < edge))
```