R function for finding a specific repeat count for data.frame

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I want to create a function that will give a new dataframe only with rows in which for the selected column the value is counted exactly 2 times in the original data.frame

I try this:

duplicates <- function(df$x, as.bool = TRUE) {
  is.dup <- (duplicated(x) & rev(duplicated(rev(x))))
  if (as.bool) { is.dup } else { x[is.dup] }
}
filter r function
2 Answers
oneDuplicate <- function(df, vec){

    ndf <- df[df[[vec]] %in% (which(table(df[[vec]]) == 2) |> names()),]

  return(ndf)

}

oneDuplicate(attitude, "advance")

    63         64         51       54     63       73      47
5      81         78         56       66     71       83      47
6      43         55         49       44     54       49      34
11     64         53         53       58     58       67      34
15     77         77         54       72     79       77      46
19     65         70         46       57     75       85      46
21     50         40         33       34     43       64      33
24     40         37         42       58     50       57      49
25     63         54         42       48     66       75      33
27     78         75         58       74     80       78      49
Admin1

In lack of data, I'm using the mtcars data. You can do:

duplicates <- function(data, var)
{
  library(tidyverse)
  data |> 
    add_count(!!sym(var)) |> 
    filter(n == 2) |> 
    select(-n)
}

duplicates(mtcars, "mpg")

    mpg cyl  disp  hp drat    wt  qsec vs am gear carb
1  21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4
2  21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4
3  22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1
4  21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1
5  22.8   4 140.8  95 3.92 3.150 22.90  1  0    4    2
6  19.2   6 167.6 123 3.92 3.440 18.30  1  0    4    4
7  15.2   8 275.8 180 3.07 3.780 18.00  0  0    3    3
8  10.4   8 472.0 205 2.93 5.250 17.98  0  0    3    4
9  10.4   8 460.0 215 3.00 5.424 17.82  0  0    3    4
10 30.4   4  75.7  52 4.93 1.615 18.52  1  1    4    2
11 15.2   8 304.0 150 3.15 3.435 17.30  0  0    3    2
12 19.2   8 400.0 175 3.08 3.845 17.05  0  0    3    2
13 30.4   4  95.1 113 3.77 1.513 16.90  1  1    5    2
14 21.4   4 121.0 109 4.11 2.780 18.60  1  1    4    2

In this case, each of the "mpg" values appears exactly two times in the data.

If you want to do these checks for several vars, simply wrap this into an lapply command. The output will be a list of data frames.

lapply(as.list(c('mpg', 'gear')), function(x) duplicates(mtcars, x))
user_27257730
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