Reverse the order of lines in a text file while preserving the contents of each line
- Task
-
- Read 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.
Reference: Bash tac command
11l
:start:
V fileData = File(:argv[1]).read().split("\n")
L(line) reversed(fileData)
print(line)
Action!
In the following solution the input file rodgers.txt is loaded from H6 drive. Altirra emulator automatically converts CR/LF character from ASCII into 155 character in ATASCII charset used by Atari 8-bit computer when one from H6-H10 hard drive under DOS 2.5 is used.
DEFINE PTR="CARD"
DEFINE BUFFERSIZE="1000"
BYTE ARRAY buffer(BUFFERSIZE)
PTR ARRAY lines(100)
BYTE count=[0]
PROC AddLine(CHAR ARRAY line)
CHAR ARRAY dst
IF count=0 THEN
dst=buffer
ELSE
dst=lines(count-1)
dst==+dst(0)+1
FI
IF dst+line(0)+1>=buffer+BUFFERSIZE THEN
Print("End of memory!")
Break()
FI
SCopy(dst,line)
lines(count)=dst
count==+1
RETURN
PROC ReadFile(CHAR ARRAY fname)
CHAR ARRAY line(255)
BYTE dev=[1]
Close(dev)
Open(dev,fname,4)
WHILE Eof(dev)=0
DO
InputSD(dev,line)
AddLine(line)
OD
Close(dev)
RETURN
PROC Main()
CHAR ARRAY s
BYTE i,LMARGIN=$52,oldLMARGIN
oldLMARGIN=LMARGIN
LMARGIN=0 ;remove left margin on the screen
Put(125) PutE() ;clear the screen
ReadFile("H6:RODGERS.TXT")
FOR i=0 TO count-1
DO
s=lines(count-1-i)
PrintE(s)
OD
LMARGIN=oldLMARGIN ;restore left margin on the screen
RETURN
- Output:
Screenshot from Atari 8-bit computer
--- Will Rodger until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Ada
with Ada.Text_Io;
with Ada.Containers.Indefinite_Vectors;
with Ada.Command_Line;
procedure Reverse_Lines_In_File is
subtype Line_Number is Natural;
package Line_Vectors
is new Ada.Containers.Indefinite_Vectors
(Index_Type => Line_Number,
Element_Type => String);
use Line_Vectors,
Ada.Text_Io,
Ada.Command_Line;
File : File_Type;
Buffer : Vector;
begin
if Argument_Count = 1 then
Open (File, In_File, Argument (1));
Set_Input (File);
end if;
while not End_Of_File loop
Buffer.Prepend (Get_Line);
end loop;
if Is_Open (File) then
Close (File);
end if;
for Line of Buffer loop
Put_Line (Line);
end loop;
end Reverse_Lines_In_File;
ALGOL W
begin % reverse the order of the lines read from standard input %
% record to hold a line and link to the next %
record LinkedLine ( string(256) text; reference(LinkedLine) next );
string(256) line;
reference(LinkedLine) lines;
% allow the program to continue after reaching end-of-file %
ENDFILE := EXCEPTION( false, 1, 0, false, "EOF" );
% handle the input %
lines := null;
readcard( line );
while not XCPNOTED(ENDFILE) do begin
lines := LinkedLine( line, lines );
readcard( line )
end while_not_eof ;
% show the lines in reverse order %
while lines not = null do begin
integer len;
% find the length of the line with trailing spaces removed %
len := 255;
line := text(lines);
while len > 0 and line( len // 1 ) = " " do len := len - 1;
% print the line, note Algol W does not allow variable length substrings %
write( s_w := 0, line( 0 // 1 ) );
for cPos := 1 until len do writeon( s_w := 0, line( cPos // 1 ) );
lines := next(lines)
end while_lines_ne_null
end.
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
AWK
# syntax: GAWK -f REVERSE_THE_ORDER_OF_LINES_IN_A_TEXT_FILE_WHILE_PRESERVING_THE_CONTENTS_OF_EACH_LINE.AWK filename
{ arr[NR] = $0 }
END {
for (i=NR; i>=1; i--) {
printf("%s\n",arr[i])
}
exit(0)
}
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
BASIC256
source = freefile
open (source, "text.txt")
textEnt$ = ""
dim textSal$(size(source)*8)
linea = 0
while not eof(source)
textEnt$ = readline(source)
linea += 1
textSal$[linea] = textEnt$
end while
for n = size(source) to 1 step -1
print textSal$[n];
next n
close source
end
- Output:
Igual que la entrada de FreeBASIC.
F#
// Reverse the order of lines in a text file while preserving the contents of each line. Nigel Galloway: August 9th., 2022
seq{use n=System.IO.File.OpenText("wr.txt") in while not n.EndOfStream do yield n.ReadLine()}|>Seq.rev|>Seq.iter(printfn "%s")
- Output:
-- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Factor
USING: io io.encodings.utf8 io.files sequences ;
"rodgers.txt" utf8 file-lines <reversed> [ print ] each
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Delphi
See maybe Free Pascal
FreeBASIC
open "text.txt" for input as #1
dim as string textEnt, textSal()
dim as integer n, linea = 0
do while not eof(1)
line input #1, textEnt
linea += 1
redim preserve textSal(linea)
textSal(linea) = textEnt
loop
for n = ubound(textSal) to 1 step -1
print textSal(n)
next n
close #1
sleep
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Free Pascal
program TAC;
{$IFDEF FPC}
{$MODE DELPHI}
{$ELSE}
{$APPTYPE CONSOLE}
{$ENDIF}
uses
sysutils, classes;
var
Sl:TStringList;
i,j : nativeInt;
begin
Sl := TStringList.Create;
Sl.Loadfromfile('Rodgers.txt');
i := 0;
j := Sl.Count-1;
While i<j do
Begin
Sl.Exchange(i,j);
inc(i);
dec(j);
end;
writeln(Sl.text);
end.
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
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))
}
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Haskell
import qualified Data.Text as T
import qualified Data.Text.IO as TIO
main :: IO ()
main = TIO.interact $ T.unlines . reverse . T.lines
- Output:
$ tac < tac.in --- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
J
;|.<;.2 text
--- Will Rodgers
until you can find a rock."
saying 'Nice Doggy'
"Diplomacy is the art of
where
text=: {{)n
"Diplomacy is the art of
saying 'Nice Doggy'
until you can find a rock."
--- Will Rodgers
}}
or
text=: fread 'filename'
if the text were stored in a file named filename
.
jq
Works with gojq, the Go implementation of jq
jq -nRr '[inputs] | reverse[]' input.txt
- 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 '\n' char or '\r\n' digraph 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.
readlines("diplomacyrodgers.txt", keep=true) |> reverse .|> print
- 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.
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 ">>>>>"
- 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 >>>>>
OCaml
let rec read_lines_reverse lst =
match read_line () with
| line -> read_lines_reverse (line :: lst)
| exception End_of_file -> lst
let () = read_lines_reverse [] |> List.iter print_endline
- Output:
$ ocaml tac.ml <file.txt --- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Pascal
See also Delphi and Free Pascal
The following is an ISO-compliant program.
Some compilers, however, such as the FPC (Free Pascal Compiler) or Delphi, cannot handle files without file names.
program tac(input, output);
procedure reverse;
var
line: text;
begin
{ Open for (over-)writing. }
rewrite(line);
{ `EOLn` is shorthand for `EOLn(input)`. }
while not EOLn do
begin
{ `line^` and `input^` refer to buffer variables [their values]. }
line^ := input^;
{ Write buffer and advance writing position. }
put(line);
{ Advance reading cursor and obtain next value [if such exists]. }
get(input)
end;
{ Consume “newline” character in `input` }
readLn;
{ Likewise, `EOF` is shorthand for `EOF(input)`. }
if not EOF then
begin
reverse
end;
{ (Re‑)open for reading. }
reset(line);
while not EOLn(line) do
begin
output^ := line^;
put(output);
get(line)
end;
{ `writeLn` is shorthand for `writeLn(output)`. }
writeLn
end;
begin
reverse
end.
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Note, this program will append a “new line” character to the final line if it did not exist.
Perl
as one-liner ..
// 20210803 Perl programming solution
< input.txt perl -e 'print reverse <>'
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Phix
with javascript_semantics string text = """ "Diplomacy is the art of saying 'Nice Doggy' until you can find a rock." --- Will Rodgers""" if platform()!=JS then integer fn = open("rogers.txt","r") if fn=-1 then fn = open("rogers.txt","w") puts(fn,text) close(fn) fn = open("rogers.txt","r") end if text = substitute(get_text(fn),"\r\n","\n") close(fn) end if sequence lines = split(text,"\n",false) printf(1,"%s\n",{join(reverse(lines),"\n")})
Obviously you can test the file handling by running the above, then changing eg Diplomacy to Diplomaxy and re-running it, and checking it outputs the previously saved c rather than the replacement x.
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Python
Interactive program which takes input from a file :
#Aamrun, 4th October 2021
import sys
if len(sys.argv)!=2:
print("Usage : python " + sys.argv[0] + " <filename>")
exit()
dataFile = open(sys.argv[1],"r")
fileData = dataFile.read().split('\n')
dataFile.close()
[print(i) for i in fileData[::-1]]
Input file :
"Diplomacy is the art of saying 'Nice Doggy' until you can find a rock." --- Will Rodgers
Sample run and output:
- Output:
C:\My Projects\BGI>python rosetta7.py diplomaticQuote.txt --- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of C:\My Projects\BGI>
Quackery
[ sharefile drop
[] swap
[ carriage over find split
dup $ "" != while
behead drop
unrot nested swap join
swap again ]
drop nested swap join
witheach [ echo$ cr ] ] is task ( $ --> )
$ "rosetta/input.txt" task
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
R
text <- scan("Rodgers.txt", character(), sep = "\n")
print(text)
reversed <- rev(text)
print(reversed)
write(reversed, "SaveTheOutput.txt")
- Output:
Read 5 items [1] " \"Diplomacy is the art of" [2] " saying 'Nice Doggy'" [3] "until you can find a rock.\"" [4] " " [5] " --- Will Rodgers" [1] " --- Will Rodgers" [2] " " [3] "until you can find a rock.\"" [4] " saying 'Nice Doggy'" [5] " \"Diplomacy is the art of"
Raku
Not going to bother testing with the task recommended file. It demonstrates nothing to do with file handling, record separators, memory conservation or anything useful. May as well just be "Reverse this list" for all the good it does.
Lots of assumptions
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.
.put for reverse lines
Few assumptions
Processes a small (configurable) number of bytes at a time so file can be multi-terabyte and it will handle with ease. Does assume Latin 1 for reduced complexity.
No assumptions were made concerning line/record termination, full stop.
Run the following to generate nul.txt. (digits 1 through 6 repeated 8 times with double null as record separators):
raku -e'print join "\x00\x00", (1..6).map: * x 8' > nul.txt
my $input-record-separator = "\x00\x00";
my $fh = open("nul.txt".IO, :r, :bin);
$fh.seek(0, SeekFromEnd); # start at the end of the file
my $bytes = 5 min $fh.tell - 1; # read in file 5 bytes at a time (or whatever)
$fh.seek(-$bytes, SeekFromCurrent);
my $buffer = $fh.read($bytes).decode('Latin1'); # assume Latin1 for reduced complexity
loop {
my $seek = ($fh.tell < $bytes * 2) ?? -$fh.tell !! -$bytes * 2;
$fh.seek($seek, SeekFromCurrent);
$buffer = $buffer R~ $fh.read((-$seek - $bytes) max 0).decode('Latin1');
if $buffer.contains: $input-record-separator {
my @rest;
($buffer, @rest) = $buffer.split: $input-record-separator;
.say for reverse @rest; # emit any full records that have been processed
}
last if $fh.tell < $bytes;
}
say $buffer; # emit any remaining record
- Output:
66666666 55555555 44444444 33333333 22222222 11111111
REXX
version 1
This will work for all REXXes, but it reads all the file's lines/records into memory (storage or buffers).
No assumptions were made concerning line/record termination, as REXX takes care of that.
/*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 #=1 while lines(iFID)>0 /*read the file, one record at a time. */
@.#= linein(iFID) /*assign contents of a record to array.*/
end /*#*/
recs= # - 1 /*# 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*/
- output when using the default input:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
version 2
This will work for all the following REXXes (and perhaps other REXXes as well):
- Regina REXX
- R4 REXX
- ROO REXX
- CMS REXX compiler
- CMS OREXX
/*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*/
options nofast_lines_BIF_default /*an option just for Regina REXX. */
#= lines(iFID) /*#: the number of lines in the file. */
do j=# by -1 for # /*read file (backwards), from bot──►top*/
say linein(iFID, j) /*display record contents ──► terminal.*/
end /*j*/
call lineout iFID /*close file, good programming practice*/
- output is identical to the 1st REXX version.
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
- 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...
Ruby
puts File.readlines("diplomacy.txt").reverse
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
sed
1!G
h
$!d
- Output:
$ sed -f tac.sed file.txt --- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
UNIX Shell
tac rodgers.txt
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
Notice that tac is cat in reverse order.
Wren
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))
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of
XPL0
Usage: rev <will.txt
char Array(1000, 1000); \(tacky)
int Line, Char, I;
def LF=$0A, EOF=$1A;
[Line:= 0;
repeat I:= 0;
repeat Char:= ChIn(1);
Array(Line, I):= Char; I:= I+1;
until Char = LF or Char = EOF;
Line:= Line+1;
until Char = EOF;
for Line:= Line-2 downto 0 do
[I:= 0;
repeat Char:= Array(Line, I); I:= I+1;
ChOut(0, Char);
until Char = LF;
];
]
- Output:
--- Will Rodgers until you can find a rock." saying 'Nice Doggy' "Diplomacy is the art of