https://stackoverflow.com/questions/72324198/how-to-use-a-for-loop-to-extract-columns-from-a-data-frame/72324469#72324469

data-frame-from-loop.R

# original ----------------------------------------------------------------

new <- list()      # construct as list -- data.frames are fancy lists
cols <- c(1, 5, 3) # use a vector of column indices
for (i in seq_along(cols)) {
  # append the list at each column
  new[[i]] <- mtcars[, cols[i], drop = FALSE]
}

new <- as.data.frame(new)      # make list into data.frame
identical(new, mtcars[, cols]) # check that this produces the same thing
#> [1] TRUE
head(new)
#>                    mpg drat disp
#> Mazda RX4         21.0 3.90  160
#> Mazda RX4 Wag     21.0 3.90  160
#> Datsun 710        22.8 3.85  108
#> Hornet 4 Drive    21.4 3.08  258
#> Hornet Sportabout 18.7 3.15  360
#> Valiant           18.1 2.76  225
str(new)
#> 'data.frame':    32 obs. of  3 variables:
#>  $ mpg : num  21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...
#>  $ drat: num  3.9 3.9 3.85 3.08 3.15 2.76 3.21 3.69 3.92 3.92 ...
#>  $ disp: num  160 160 108 258 360 ...


# updates -----------------------------------------------------------------

data <- Reduce(
  cbind,
  lapply(
    1:20,
    function(i) {
      out <- data.frame(
        id = order(runif(5)),
        event = runif(5) < .5,
        other_col = runif(5)
      )
      colnames(out) <- paste0(colnames(out), i)
      out
    }
  )
)

# just a quick peak
str(data[, c(1:3, 9:12, 21:24)])
#> 'data.frame':    5 obs. of  11 variables:
#>  $ id1       : int  3 2 1 4 5
#>  $ event1    : logi  FALSE FALSE TRUE TRUE FALSE
#>  $ other_col1: num  0.617 0.951 0.511 0.185 0.667
#>  $ other_col3: num  0.6856 0.0524 0.5786 0.9265 0.2291
#>  $ id4       : int  4 2 1 5 3
#>  $ event4    : logi  TRUE TRUE FALSE FALSE FALSE
#>  $ other_col4: num  0.0849 0.8345 0.8465 0.1958 0.2534
#>  $ other_col7: num  0.656 0.353 0.604 0.973 0.381
#>  $ id8       : int  2 3 5 4 1
#>  $ event8    : logi  TRUE FALSE FALSE TRUE TRUE
#>  $ other_col8: num  0.646 0.693 0.534 0.624 0.625

result <- lapply(1:20, function(i) {
  # make pattern (must have letters before number)
  pattern <- paste0("[a-z]", i, "$") 
  
  # find the column indeces that match the pattern
  ind <- grep(pattern, colnames(data))
  
  # extract those indices
  res <- data[, ind, ]
  
  # optional: rename columns
  colnames(res) <- sub(paste0(i, "$"), "", colnames(res))
  res
})

head(result)
#> [[1]]
#>   id event other_col
#> 1  3 FALSE 0.6174577
#> 2  2 FALSE 0.9509916
#> 3  1  TRUE 0.5107370
#> 4  4  TRUE 0.1851543
#> 5  5 FALSE 0.6670226
#> 
#> [[2]]
#>   id event other_col
#> 1  3  TRUE 0.8261719
#> 2  4 FALSE 0.4171351
#> 3  1  TRUE 0.5640345
#> 4  5  TRUE 0.6825371
#> 5  2 FALSE 0.4381013
#> 
#> [[3]]
#>   id event  other_col
#> 1  4 FALSE 0.68559712
#> 2  3 FALSE 0.05241906
#> 3  2 FALSE 0.57857342
#> 4  1  TRUE 0.92649458
#> 5  5  TRUE 0.22908630
#> 
#> [[4]]
#>   id event  other_col
#> 1  4  TRUE 0.08491369
#> 2  2  TRUE 0.83452439
#> 3  1 FALSE 0.84650621
#> 4  5 FALSE 0.19578470
#> 5  3 FALSE 0.25342999
#> 
#> [[5]]
#>   id event other_col
#> 1  4 FALSE 0.8912857
#> 2  1 FALSE 0.1261470
#> 3  3 FALSE 0.7962369
#> 4  5  TRUE 0.3911494
#> 5  2 FALSE 0.6041862
#> 
#> [[6]]
#>   id event other_col
#> 1  4  TRUE 0.8987728
#> 2  2  TRUE 0.2830371
#> 3  5 FALSE 0.6696249
#> 4  3 FALSE 0.6249742
#> 5  1 FALSE 0.4754757