Reverse the order of lines in a text file while preserving the contents of each line
- Task
-
- Read an an entire (input) file (into memory or buffers).
- Display the lines/records of the entire file in reverse order.
- Show the results here, on this page.
For the input file, use the following five lines (records):
"Diplomacy is the art of ◄■■■■■■ starts in column 3.
saying 'Nice Doggy' ◄■■■■■■ starts in column 5,
until you can find a rock." ◄■■■■■■ starts in column 2,
◄■■■■■■ (a blank line),
--- Will Rodgers ◄■■■■■■ starts in column 30.
You can (or may) assume there are no superfluous trailing blanks, and that line four has one blank.
Also, don't include the rightmost informative comments (◄■■■■■■), as they are not meant to be part of the file.
Go
<lang go>package main
import (
"bytes" "fmt" "io/ioutil" "log" "runtime"
)
func main() {
fileName1 := "rodgers.txt"
fileName2 := "rodgers_reversed.txt"
lineBreak := "\n"
if runtime.GOOS == "windows" {
lineBreak = "\r\n"
}
// read lines from input file
b, err := ioutil.ReadFile(fileName1)
if err != nil {
log.Fatal(err)
}
lines := bytes.Split(b, []byte(lineBreak))
// remove final blank line, if any, added by some editors
if len(lines[len(lines)-1]) == 0 {
lines = lines[:len(lines)-1]
}
// write lines in reverse order to output file
for i, j := 0, len(lines)-1; i < j; i, j = i+1, j-1 {
lines[i], lines[j] = lines[j], lines[i]
}
b = bytes.Join(lines, []byte(lineBreak))
if err = ioutil.WriteFile(fileName2, b, 0o666); err != nil {
log.Fatal(err)
}
// print contents of output file to terminal
b, err = ioutil.ReadFile(fileName2)
if err != nil {
log.Fatal(err)
}
fmt.Println(string(b))
}</lang>
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Julia
The optional
keep
argument to
readlines
means to keep the newline char at the end of each line. The
|>
symbolism is the pipe operator. and the
.|>
symbolism means to pipe each line in the read array to print separately. <lang julia>readlines("diplomacyrodgers.txt", keep=true) |> reverse .|> print</lang>
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Nim
We provide a procedure which takes input and output files as parameters. Assumptions are the following:
– a line is a sequence of bytes terminated by CR, LF or CR-LF;
– there is enough memory to process the file in memory i.e to store the input file as a string, the sequence of lines, the reverse sequence of lines and the output file as a string. <lang Nim>import algorithm, strutils
proc reverseLines(infile, outfile: File) =
let lines = infile.readAll().splitLines(keepEol = true)
outfile.write reversed(lines).join("")
when isMainModule:
let infile = open("reverse_file_lines.txt")
echo ">>>>> Input file:"
stdout.write infile.readAll()
infile.setFilePos(0)
echo ">>>>>"
echo '\n'
echo ">>>>> Output file:"
reverseLines(infile, stdout)
echo ">>>>>"</lang>
- Output:
>>>>> Input file:
"Diplomacy is the art of
saying 'Nice Doggy'
until you can find a rock."
--- Will Rodgers
>>>>>
>>>>> Output file:
--- Will Rodgers
until you can find a rock."
saying 'Nice Doggy'
"Diplomacy is the art of
>>>>>
Perl
as one-liner .. <lang perl>// 20210803 Perl programming solution
< input.txt perl -e 'print reverse <>'</lang>
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Raku
Simplest thing that could possibly satisfy the extremely vague task description and completely glossing over all of the questions raised on the discussion page.
ASSUMPTIONS:
- File is redirected into STDIN from command line.
- Is a Unix or Windows format text file.
- Is in UTF8 encoding or some subset thereof.
- May hold entire file in memory.
<lang perl6>.put for reverse lines</lang>
REXX
No assumptions were made concerning line/record termination, as REXX takes care of that. <lang rexx>/*REXX pgm reads a file, and displays the lines (records) of the file in reverse order. */ parse arg iFID . /*obtain optional argument from the CL.*/ if iFID== | iFID=="," then iFID='REVERSEF.TXT' /*Not specified? Then use the default.*/ call lineout iFid /*close file, good programming practice*/
do j=1 while lines(iFID)>0 /*read the file, one record at a time. */
@.j= linein(iFID) /*assign contents of a record to array.*/
end /*j*/
recs= j - 1 /*J will be 1 more ('cause of DO loop)*/
do k=recs by -1 for recs /*process array (@.k) in reverse order.*/
say @.k /*display a record of the file ──► term*/
end /*k*/
call lineout iFid /*close file, good programming practice*/</lang>
- output when using the default input:
--- Will Rodgers
until you can find a rock."
saying 'Nice Doggy'
"Diplomacy is the art of
Ring
<lang ring> load "stdlib.ring" see "working..." + nl see "Input file lines:" + nl
fp = fopen("..\New\text.txt","r") r = "" txt = "" while isstring(r)
r = fgetc(fp)
if r = -1
loop
ok
if r = char(10)
txt += nl
else
txt += r
ok
end
see txt + nl see "Reversed file lines: " + nl txt = str2list(txt) txt = reverse(txt) txt = list2str(txt) see txt fclose(fp)
see nl + "done..." + nl </lang>
- Output:
working...
Input file lines:
"Diplomacy is the art of
saying 'Nice Doggy'
until you can find a rock."
--- Will Rodgers
Reversed file lines:
--- Will Rodgers
until you can find a rock."
saying 'Nice Doggy'
"Diplomacy is the art of
done...
Wren
<lang ecmascript>import "/ioutil" for File, FileUtil
var fileName1 = "rodgers.txt" var fileName2 = "rodgers_reversed.txt"
// read lines from input file var lines = FileUtil.readLines(fileName1) // remove final blank line, if any, added by some editors if (lines[-1] == "") lines.removeAt(-1)
// write lines in reverse order to output file File.create(fileName2) { |file|
for (i in lines.count-1..1) file.writeBytes(lines[i] + FileUtil.lineBreak) file.writeBytes(lines[0])
} // print contents of output file to terminal System.print(File.read(fileName2))</lang>
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of