如何将for循环输出保存为R中的data.frame？

Question

I'd like to know how I can save the output of a for loop as data.frame ? let's say using the mtcars dataset I have the following for loop script:

for (i in seq_len(nrow(mtcars))) { 
  if (i  <= 30) {

    next
  }

  print(mtcars[i,])  
}

              mpg cyl disp  hp drat   wt qsec vs am gear carb
Maserati Bora  15   8  301 335 3.54 3.57 14.6  0  1    5    8
            mpg cyl disp  hp drat   wt qsec vs am gear carb
Volvo 142E 21.4   4  121 109 4.11 2.78 18.6  1  1    4    2

but if I am going to save it in output file as dataframe I will get the following:

   output <- as.data.frame(c())
    for (i in seq_len(nrow(mtcars))) { 
      if (i  <= 30) {

        next
      }

      output<- c(output,mtcars[i,])  
    }


   ### then the output is 

   > output
    $mpg
    [1] 15

    $cyl
    [1] 8

    $disp
    [1] 301

    $hp
    [1] 335

    $drat
    [1] 3.54

    $wt
    [1] 3.57

    $qsec
    [1] 14.6

    $vs
    [1] 0

    $am
    [1] 1

    $gear
    [1] 5

    $carb
    [1] 8

    $mpg
    [1] 21.4

    $cyl
    [1] 4

    $disp
    [1] 121

    $hp
    [1] 109

    $drat
    [1] 4.11

    $wt
    [1] 2.78

    $qsec
    [1] 18.6

    $vs
    [1] 1

    $am
    [1] 1

    $gear
    [1] 4

    $carb
    [1] 2

I know the simple indexing output <- mtcars[-(1:30), ] , but this is not a solution for my real life situation (which is more complex). I really need the loop to do what I want.

Answer 1

Here is a solution that is most similar to your code.
The points are using the initialisation (indexing ...[NULL, ] ) and the function rbind()

output <- mtcars[NULL,]
for (i in seq_len(nrow(mtcars))) { 
  if (i  <= 30) {
        next
  }
  # ...
  output <- rbind(output, mtcars[i, ]) 
}