extract variable from string variable using data.table and self-defined function

Question

# simulate data
id=c(1:30)
house_type=sample(c("3STR","2STR","1STR","DETC,1STR","2STR,DETC","OTHERS"),20, replace=TRUE)
house=data.table(id,house_type)

# I want to create a house_type2 variable in house data.table
# In the real data, there are many other house type, with 1/2/3STR embeded in these house_typ. 
# I want to extract the STR (how many story) information from the data.

house[,story:="others"][str_detect("3STR",house_type),story:="3STR"][
  str_detect("2STR",house_type),story:="2STR"][
    str_detect("1STR",house_type),story:="1STR"]

The simulated data looks really sample; in the real data, the house_type string has more variation. I managed to do it with data.table chaining syntax. But I am asking a more elegant way to do it: define a function to extract story and apply function on the house_type column.

Cole · Accepted Answer · 2020-07-25 11:41:46Z

You could use regular expressions to extract the number:

library(data.table)
library(stringr)
# simulate data
set.seed(123L)
id=c(1:30)
house_type=sample(c("3STR","2STR","1STR","DETC,1STR","2STR,DETC","OTHERS"),20, replace=TRUE)
house=data.table(id,house_type)
#> Warning in as.data.table.list(x, keep.rownames = keep.rownames, check.names
#> = check.names, : Item 2 has 20 rows but longest item has 30; recycled with
#> remainder.

#OP
house[,story:="others"][str_detect("3STR",house_type),story:="3STR"][
    str_detect("2STR",house_type),story:="2STR"][
        str_detect("1STR",house_type),story:="1STR"]

## Solutions
house[, story2 := as.integer(sub("\\D*(\\d+).*", "\\1", house_type))]
#> Warning in eval(jsub, SDenv, parent.frame()): NAs introduced by coercion
house[, story3 := fifelse(is.na(story2), "other", paste0(story2, "STR"))]

house
#>        id house_type  story story2 story3
#>     <int>     <char> <char>  <int> <char>
#>  1:     1       1STR   1STR      1   1STR
#>  2:     2     OTHERS others     NA  other
#>  3:     3       1STR   1STR      1   1STR
#>  4:     4       2STR   2STR      2   2STR
#>  5:     5       2STR   2STR      2   2STR
#>  6:     6     OTHERS others     NA  other
#>  7:     7       1STR   1STR      1   1STR
#>  8:     8  2STR,DETC others      2   2STR
#>  9:     9  DETC,1STR others      1   1STR
#> 10:    10     OTHERS others     NA  other
#> 11:    11     OTHERS others     NA  other
#> 12:    12       3STR   3STR      3   3STR
#> 13:    13       2STR   2STR      2   2STR
#> 14:    14       1STR   1STR      1   1STR
#> 15:    15  2STR,DETC others      2   2STR
#> 16:    16       1STR   1STR      1   1STR
#> 17:    17       1STR   1STR      1   1STR
#> 18:    18       3STR   3STR      3   3STR
#> 19:    19  DETC,1STR others      1   1STR
#> 20:    20       3STR   3STR      3   3STR
#> 21:    21       1STR   1STR      1   1STR
#> 22:    22     OTHERS others     NA  other
#> 23:    23       1STR   1STR      1   1STR
#> 24:    24       2STR   2STR      2   2STR
#> 25:    25       2STR   2STR      2   2STR
#> 26:    26     OTHERS others     NA  other
#> 27:    27       1STR   1STR      1   1STR
#> 28:    28  2STR,DETC others      2   2STR
#> 29:    29  DETC,1STR others      1   1STR
#> 30:    30     OTHERS others     NA  other
#>        id house_type  story story2 story3

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