using if_else, mutate, map and reduce to create a data frame of columns meeting conditions

gistfile1.txt

``` r
library(dplyr, warn.conflicts = FALSE)
library(purrr)
library(rlang, warn.conflicts = FALSE)
library(stringr)

filenames <- c("coronavirus_cases_202007061134.csv", "coronavirus_cases_202007071134.csv", "coronavirus_cases_202007081134.csv")
cases <- c(1000, 1500, 2000)

# couple of functions
pull_date <- function(x) {
  stringr::str_extract(x, "[:digit:]+") %>%           # assumption that all digits are in a single string
    stringr::str_trunc(width = 8, ellipsis = "") %>%  # assumption that all dates are full 8 digits
    as.numeric()
}

add_column <- function(df, filename, check_var, fill_var) {
  df %>% 
    # '{{ }}' doesn't work here on LHS: '!!' required. See https://rlang.r-lib.org/reference/nse-force.html
    # mutate({{ filename }} := if_else(pull_date(filename) >= {{ check_var }}, {{ fill_var }}, na_dbl))
    mutate(!!filename := if_else(pull_date(filename) >= {{ check_var }}, {{ fill_var }}, na_dbl))
}


# this gives you a second column of just the dates in numeric
df_base <- dplyr::tibble(filenames, cases) %>% 
  dplyr::mutate(file_date = pull_date(filenames))

# call add_column() for each filename in turn
# then combine all the results back into a single df
purrr::map(filenames, ~ add_column(df_base, ., file_date, cases)) %>% 
  purrr::reduce(left_join)

#> Joining, by = c("filenames", "cases", "file_date")
#> Joining, by = c("filenames", "cases", "file_date")
#> # A tibble: 3 x 6
#>   filenames  cases file_date coronavirus_case~ coronavirus_cas~ coronavirus_cas~
#>   <chr>      <dbl>     <dbl>             <dbl>            <dbl>            <dbl>
#> 1 coronavir~  1000  20200706              1000             1000             1000
#> 2 coronavir~  1500  20200707                NA             1500             1500
#> 3 coronavir~  2000  20200708                NA               NA             2000
```

<sup>Created on 2020-07-13 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0)</sup>