I am struggling with using map from the library purrr correctly. I would like to calculate the weighted mean of my sample by nesting a common observations in a list and then using map(). (I know this would also work with group_by)
MWE: Suppose I have observed 3 different subjects (indicated by ‘id’), I have their sample weights (‘weights’) and corresponding observations (‘obs’).
df <- tibble(id = c(1, 1, 2, 2, 3,3), weights = c(0.3,0.7,0.25,0.75,0.14,0.86), obs = 6:1)
df
# A tibble: 6 x 3
id weights obs
<dbl> <dbl> <int>
1 1 0.3 6
2 1 0.7 5
3 2 0.25 4
4 2 0.75 3
5 3 0.14 2
6 3 0.86 1
I would like to calculate the weighted average in each subject.Therefore, I nest the weights and observations.
df %>% nest(data = c(weights, obs))
# A tibble: 3 x 2
id data
<dbl> <list>
1 1 <tibble [2 x 2]>
2 2 <tibble [2 x 2]>
3 3 <tibble [2 x 2]>
Now I would like to use map to apply a function to each element of data. More precisely, I try to solve it as following
df %>% nest(data = c(weights, obs)) %>% map(data, ~ (.x$weights*.x$obs)/sum(.x$weights))
Warning in .f(.x[[i]], ...) : data set ‘.x[[i]]’ not found
Warning in .f(.x[[i]], ...) :
data set ‘~(.x$weights * .x$obs)/sum(.x$weights)’ not found
Warning in .f(.x[[i]], ...) : data set ‘.x[[i]]’ not found
Warning in .f(.x[[i]], ...) :
data set ‘~(.x$weights * .x$obs)/sum(.x$weights)’ not found
As you can see this results in a lot of error messages. In order to better understand map I tried to multiply the weights vector of each ID with 2.
df %>% nest(data = c(weights, obs)) %>% map(data, ~ .x$weights*2)
$id
[1] ".x[[i]]" "~.x$weights * 2"
$data
[1] ".x[[i]]" "~.x$weights * 2"
Warning messages:
1: In .f(.x[[i]], ...) : data set ‘.x[[i]]’ not found
2: In .f(.x[[i]], ...) : data set ‘~.x$weights * 2’ not found
3: In .f(.x[[i]], ...) : data set ‘.x[[i]]’ not found
4: In .f(.x[[i]], ...) : data set ‘~.x$weights * 2’ not found
and
df %>% nest(data = c(weights, obs)) %>% map(data, function(x) x$weights*2)
Warning in .f(.x[[i]], ...) : data set ‘.x[[i]]’ not found
Warning in .f(.x[[i]], ...) :
data set ‘function(x) x$weights * 2’ not found
Warning in .f(.x[[i]], ...) : data set ‘.x[[i]]’ not found
Warning in .f(.x[[i]], ...) :
data set ‘function(x) x$weights * 2’ not found
$id
[1] ".x[[i]]" "function(x) x$weights * 2"
$data
[1] ".x[[i]]" "function(x) x$weights * 2"
So I also get error messages here. I am quite lost even after reading the documentation of map. I do not see my error. I am happy about any insights!
Thanks a lot!
>Solution :
We may pass the map within mutate because the data column is not accessible outside the data, unless we use .$data
library(dplyr)
library(purrr)
df %>%
nest(data = c(weights, obs)) %>%
mutate(wtd_mean = map_dbl(data, ~ sum(.x$weights*.x$obs)/sum(.x$weights)))
-output
# A tibble: 3 × 3
id data wtd_mean
<dbl> <list> <dbl>
1 1 <tibble [2 × 2]> 5.3
2 2 <tibble [2 × 2]> 3.25
3 3 <tibble [2 × 2]> 1.14
There is also weighted.mean function from stats (base R)
df %>%
nest(data = c(weights, obs)) %>%
mutate(wtd_mean = map_dbl(data, ~ weighted.mean(.x$obs, .x$weights)))
# A tibble: 3 × 3
id data wtd_mean
<dbl> <list> <dbl>
1 1 <tibble [2 × 2]> 5.3
2 2 <tibble [2 × 2]> 3.25
3 3 <tibble [2 × 2]> 1.14