Input loop
You are encouraged to solve this task according to the task description, using any language you may know.
Read from a text stream either word-by-word or line-by-line until the stream runs out of data. The stream will have an unknown amount of data on it.
Ada
This example reads in a text stream from standard input line by line and writes the output to standard output. <lang ada>with Ada.Text_Io; use Ada.Text_Io;
procedure Read_Stream is
Line : String(1..10); Length : Natural;
begin
while not End_Of_File loop
Get_Line(Line, Length); -- read up to 10 characters at a time
Put(Line(1..Length));
-- The current line of input data may be longer than the string receiving the data.
-- If so, the current input file column number will be greater than 0
-- and the extra data will be unread until the next iteration.
-- If not, we have read past an end of line marker and col will be 1
if Col(Current_Input) = 1 then
New_Line;
end if;
end loop;
end Read_Stream;</lang>
ALGOL 68
For file consisting of just one page - a typical linux/unix file: <lang algol68>main:(
PROC raise logical file end = (REF FILE f) BOOL: ( except logical file end ); on logical file end(stand in, raise logical file end); DO print(read string); read(new line); print(new line) OD; except logical file end: SKIP
)</lang> For multi page files, each page is seekable with PROC set = (REF FILE file, INT page, line, char)VOID: ~. This allows rudimentary random access where each new page is effectively a new record. <lang algol68>main:(
PROC raise logical file end = (REF FILE f) BOOL: ( except logical file end );
on logical file end(stand in, raise logical file end);
DO
PROC raise page end = (REF FILE f) BOOL: ( except page end );
on page end(stand in, raise page end);
DO
print(read string);
read(new line);
print(new line)
OD;
except page end:
read(new page);
print(new page)
OD;
except logical file end:
SKIP
)</lang> The boolean functions physical file ended(f), logical file ended(f), page ended(f) and line ended(f) are also available to indicate the end of a file, page and line.
AmigaE
<lang amigae>CONST BUFLEN=1024, EOF=-1
PROC consume_input(fh)
DEF buf[BUFLEN] : STRING, r
REPEAT
/* even if the line si longer than BUFLEN,
ReadStr won't overflow; rather the line is
"splitted" and the remaining part is read in
the next ReadStr */
r := ReadStr(fh, buf)
IF buf[] OR (r <> EOF)
-> do something
WriteF('\s\n',buf)
ENDIF
UNTIL r=EOF
ENDPROC
PROC main()
DEF fh
fh := Open('basicinputloop.e', OLDFILE)
IF fh
consume_input(fh)
Close(fh)
ENDIF
ENDPROC</lang>
AWK
This just reads lines from stdin and prints them until EOF is read.
<lang awk>{ print $0 }</lang>
or, more idiomatic:
<lang awk>1</lang>
AutoHotkey
This example reads the text of a source file line by line and writes the output to a destination file. <lang AutoHotkey>Loop, Read, Input.txt, Output.txt {
FileAppend, %A_LoopReadLine%`n
}</lang>
C
<lang c>#include <stdio.h>
- define MAX_LEN 20
/* line by line: */ /* This may not read the whole line; just up to MAX_LEN characters at a time. */ void process_lines(FILE *stream) {
char line[MAX_LEN + 1];
while (fgets(line, MAX_LEN + 1, stream) != NULL) {
/* process the string here */
/* the string includes the line return, if it reached the end of line */
}
}
/* word by word */ /* This may not read the whole word; just up to MAX_LEN characters at a time. */
- define Str(x) #x
- define Xstr(x) Str(x)
void process_words(FILE *stream) {
char word[MAX_LEN + 1];
while (fscanf(stream, "%" Xstr(MAX_LEN) "s", word) == 1) {
/* process the string here */
}
}</lang>
C++
The following functions send the words resp. lines to a generic output iterator <lang cpp>
- include <istream>
- include <string>
- include <vector>
- include <algorithm>
- include <iostream>
- include <iterator>
// word by word template<class OutIt> void read_words(std::istream& is, OutIt dest) {
std::string word;
while (is >> word)
{
// send the word to the output iterator
*dest = word;
}
}
// line by line: template<class OutIt> void read_lines(std::istream& is, OutIt dest) {
std::string line;
while (std::getline(is, line))
{
// store the line to the output iterator
*dest = line;
}
}
int main() {
// 1) sending words from std. in std. out (end with Return)
read_words(std::cin,
std::ostream_iterator<std::string>(std::cout, " "));
// 2) appending lines from std. to vector (end with Ctrl+Z) std::vector<std::string> v; read_lines(std::cin, std::back_inserter(v)); return 0;
}
</lang>
An alternate way to read words or lines is to use istream iterators:
<lang cpp> template<class OutIt> void read_words(std::istream& is, OutIt dest) {
typedef std::istream_iterator<std::string> InIt;
std::copy(InIt(is), InIt(),
dest);
}
namespace detail {
struct ReadableLine : public std::string
{
friend std::istream & operator>>(std::istream & is, ReadableLine & line)
{
return std::getline(is, line);
}
};
}
template<class OutIt> void read_lines(std::istream& is, OutIt dest) {
typedef std::istream_iterator<detail::ReadableLine> InIt;
std::copy(InIt(is), InIt(),
dest);
} </lang>
C#
<lang csharp>using System; using System.IO;
class Program {
static void Main(string[] args)
{
StreamReader b = new StreamReader("file.txt"); //or any other Stream
string line = b.ReadLine();
while (line != null)
{
Console.WriteLine(line);
line = b.ReadLine();
}
}
}</lang>
Clojure
<lang lisp>(defn basic-input [fname]
(line-seq (java.io.BufferedReader. (java.io.FileReader. fname))))</lang>
Common Lisp
<lang lisp>(defun basic-input (filename)
(with-open-file (stream (make-pathname :name filename) :direction :input)
(loop for line = (read-line stream nil nil)
while line
do (format t "~a~%" line))))</lang>
D
<lang d>import tango.io.Console; import tango.text.stream.LineIterator;
void main (char[][] args) {
foreach (line; new LineIterator!(char)(Cin.input)) {
// do something with each line
}
}</lang>
<lang d>import tango.io.Console; import tango.text.stream.SimpleIterator;
void main (char[][] args) {
foreach (word; new SimpleIterator!(char)(" ", Cin.input)) {
// do something with each word
}
}</lang>
Note that foreach variables 'line' and 'word' are transient slices. If you need to retain them for later use, you should .dup them.
F#
Using a sequence expression: <lang fsharp> let lines_of_file file =
seq { use stream = System.IO.File.OpenRead file
use reader = new System.IO.StreamReader(stream)
while not reader.EndOfStream do
yield reader.ReadLine() }
</lang> The file is reopened every time the sequence is traversed and lines are read on-demand so this can handle arbitrarily-large files.
Factor
<lang factor>"file.txt" utf8 [ [ process-line ] each-line ] with-file-reader</lang>
Forth
<lang forth>4096 constant max-line
- read-lines
begin stdin pad max-line read-line throw
while pad swap \ addr len is the line of data, excluding newline
2drop
repeat ;</lang>
Fortran
The code read line-by-line, but the maximum length of the line is limited (by a parameter)
<lang fortran>program BasicInputLoop
implicit none
integer, parameter :: in = 50, &
linelen = 1000
integer :: ecode
character(len=linelen) :: l
open(in, file="afile.txt", action="read", status="old", iostat=ecode)
if ( ecode == 0 ) then
do
read(in, fmt="(A)", iostat=ecode) l
if ( ecode /= 0 ) exit
write(*,*) trim(l)
end do
close(in)
end if
end program BasicInputLoop</lang>
gnuplot
The following gnuplot script echoes standard input to standard output line-by-line until the end of the stream. <lang gnuplot>!cat</lang> It makes use of the ability of gnuplot to spawn shell commands. In that sense it might be considered cheating. Nevertheless, this is a valid gnuplot script that does meet the requirements of the task description.
It seems impossible to complete this task with just standard gnuplot commands.
Haskell
The whole contents of a file can be read lazily. The standard functions lines and words convert that lazily into the lists of lines resp. words. Usually, one wouldn't use extra routines for that, but just use readFile and then put 'lines' or words somewhere in the next processing step.
<lang haskell>import System.IO
readLines :: Handle -> IO [String] readLines h = do
s <- hGetContents h return $ lines s
readWords :: Handle -> IO [String] readWords h = do
s <- hGetContents h return $ words s</lang>
Icon
<lang Icon>link str2toks
- call either words or lines depending on what you want to do.
procedure main()
words()
end
procedure lines()
while write(read())
end
procedure words()
local line while line := read() do line ? every write(str2toks())
end</lang> Note: str2toks is from the Icon Programming Library. To see the Icon language features available for the str2toks implementation please follow the external links.
Java
Some people prefer Scanner or BufferedReader, so a way with each is presented. <lang java>import java.util.Scanner; ... Scanner in = new Scanner(System.in);//stdin //new Scanner(new FileInputStream(filename)) for a file //new Scanner(socket.getInputStream()) for a network stream while(in.hasNext()){ String input = in.next(); //in.nextLine() for line-by-line //process the input here }</lang> Or <lang java>import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; ... try{ BufferedReader inp = new BufferedReader(new InputStreamReader(System.in));//stdin //new BufferedReader(new FileReader(filename)) for a file //new BufferedReader(new InputStreamReader(socket.getInputStream())) for a network stream while(inp.ready()){ String input = inp.readLine();//line-by-line only //in.read() for character-by-character //process the input here } } catch (IOException e) { //There was an input error }</lang>
JavaScript
These implementations of JavaScript define a readline() function, so:
$ js -e 'while (line = readline()) { do_something_with(line); }' < inputfile
As above, this operates on standard input <lang javascript>var text_stream = WScript.StdIn; var i = 0;
while ( ! text_stream.AtEndOfStream ) {
var line = text_stream.ReadLine(); // do something with line WScript.echo(++i + ": " + line);
}</lang>
Logo
There are several words which will return a line of input.
- readline - returns a line as a list of words
- readword - returns a line as a single word, or an empty list if it reached the end of file
- readrawline - returns a line as a single word, with no characters escaped
<lang logo>while [not eof?] [print readline]</lang>
Lua
<lang lua>lines = {} str = io.read() while str do
table.insert(lines,str) str = io.read()
end</lang>
MAXScript
this function will read a file line by line. <lang MAXScript>fn ReadAFile FileName = ( local in_file = openfile FileName while not eof in_file do ( --Do stuff in here-- print (readline in_file) ) close in_file )</lang>
Modula-3
<lang modula3>MODULE Output EXPORTS Main;
IMPORT Rd, Wr, Stdio;
VAR buf: TEXT;
<*FATAL ANY*>
BEGIN
WHILE NOT Rd.EOF(Stdio.stdin) DO buf := Rd.GetLine(Stdio.stdin); Wr.PutText(Stdio.stdout, buf); END;
END Output.</lang>
OCaml
<lang ocaml>let rec read_lines ic =
try let line = input_line ic in line :: read_lines ic with End_of_file -> []</lang>
The version above will work for small files, but it is not tail-recursive.
Below will be more scalable:
<lang ocaml>let read_line ic =
try Some (input_line ic) with End_of_file -> None
let read_lines ic =
let rec loop acc = match read_line ic with | Some line -> loop (line :: acc) | None -> List.rev acc in loop []
- </lang>
Or with a higher order function:
<lang ocaml>let read_lines f ic =
let rec loop () = try f(input_line ic); loop() with End_of_file -> () in loop()
read_lines print_endline (open_in Sys.argv.(1))</lang>
Oz
<lang oz>%% Returns a list of lines. %% Text: an instance of Open.text (a mixin class) fun {ReadAll Text}
case {Text getS($)} of false then nil
[] Line then Line|{ReadAll Text}
end
end</lang>
Pascal
<lang pascal>{ for stdio }
var
s : string ;
begin
repeat
readln(s); until s = "" ;
{ for a file }
var
f : text ; s : string ;
begin
assignfile(f,'foo'); reset(f); while not eof(f) do readln(f,s);
closefile(f);
end;</lang>
Perl
The angle brackets operator ( <...> ) reads one line at a time from a filehandle in scalar context: <lang perl>open FH, "< $filename" or die "can't open file: $!"; while (my $line = <FH>) {
chomp $line; # removes trailing newline # process $line
} close FH or die "can't close file: $!";</lang>
Or you can get a list of all lines when you use it in list context: <lang perl>@lines = <FH>;</lang>
Or a simpler program for lines of files entered as command line arguments or standard input: <lang perl>while (<>) {
# $_ contains a line
}</lang>
PHP
<lang php>$fh = fopen($filename, 'r'); if ($fh) {
while (!feof($fh)) {
$line = rtrim(fgets($fh)); # removes trailing newline
# process $line
}
fclose($fh);
}</lang>
Or you can get an array of all the lines in the file: <lang php>$lines = file($filename);</lang>
Or you can get the entire file as a string: <lang php>$contents = file_get_contents($filename);</lang>
PicoLisp
This reads all lines in a file, and returns them as a list of lists <lang PicoLisp>(in "file.txt"
(make
(until (eof)
(link (line)) ) ) )</lang>
PL/I
<lang PL/I> declare line character (200) varying;
open file (in) title ('TEXT.DAT,type(text),recsize(200)' ); on endfile (in) stop;
do forever;
get edit (line) (L); put skip list (line);
end; </lang>
Python
Python file objects can be iterated like lists:
<lang python>my_file = open(filename, 'r') try:
for line in my_file:
pass # process line, includes newline
finally:
my_file.close()</lang>
One can open a new stream for read and have it automatically close when done, with a new "with" statement: <lang python>from __future__ import with_statement
with open(filename, 'r') as f:
for line in f:
pass # process line, includes newline</lang>
You can also get lines manually from a file: <lang python>line = my_file.readline() # returns a line from the file lines = my_file.readlines() # returns a list of the rest of the lines from the file</lang> This does not mix well with the iteration, however.
When (multiple) filenames are given on the command line:
<lang python>import fileinput
for line in fileinput.input():
pass # process line, includes newline</lang>
The fileinput module can also do inplace file editing, follow line counts, and the name of the current file being read etc.
R
Note that read.csv and read.table provide alternatives for files with 'dataset' style contents. <lang R>lines <- readLines("file.txt")</lang>
REBOL
<lang REBOL>REBOL [ Title: "Basic Input Loop" Author: oofoe Date: 2009-12-06 URL: http://rosettacode.org/wiki/Basic_input_loop ]
- Slurp the whole file in
x: read %file.txt
- Bring the file in by lines
x: read/lines %file.txt
- Read in first 10 lines
x: read/lines/part %file.txt 10
- Read data a line at a time
f: open/lines %file.txt while [not tail? f][ print f/1 f: next f ; Advance to next line. ] close f</lang>
REXX
Reading line by line from the standard input using linein and lines did not work. <lang rexx>do while stream(stdin, "State") <> "NOTREADY"
call charout ,charin(stdin)
end</lang>
<lang rexx>/* -- AREXX -- */ do until eof(stdin)
l = readln(stdin) say l
end</lang>
Ruby
Ruby input streams are Enumerable objects like Arrays, so one can use the standard #each:
<lang ruby>stream.each do |line|
# process line
end</lang>
One can open a new stream for read and have it automatically close when done:
<lang ruby>File.open(filename, "r") do |stream|
stream.each do |line| # process line end
end</lang>
Or, <lang ruby>File.foreach(filename) do |line|
# process line
end</lang>
Scala
<lang scala>scala.io.Source.fromFile(filename).getLines.foreach {
line => // do something
}</lang>
<lang scala>scala.io.Source.fromPath(filename).getLines().foreach {
line => // do something
}</lang>
Slate
<lang slate>(File newNamed: 'README') reader sessionDo: [| :input | input lines do: [| :line | inform: line]].</lang>
Smalltalk
<lang smalltalk>|f| f := FileStream open: 'afile.txt' mode: FileStream read. [ f atEnd ] whileFalse: [ (f nextLine) displayNl ] .</lang>
Tcl
<lang tcl>set fh [open $filename] while {[gets $fh line] != -1} {
# process $line
} close $fh</lang> For “small” files, it is often more common to do this: <lang tcl>set fh [open $filename] set data [read $fh] close $fh foreach line [split $data \n] {
# process line
}</lang>
Unicon
See Icon.
UnixPipes
the pipe 'yes XXX' produces a sequence
read by lines <lang bash>yes 'A B C D ' | while read x ; do echo -$x- ; done</lang> read by words <lang bash>yes 'A B C D ' | while read -d\ a ; do echo -$a- ; done</lang>
UNIX Shell
The following echoes standard input to standard output line-by-line until the end of the stream.
<lang bash>cat < /dev/stdin > /dev/stdout</lang>
Since cat defaults to reading from standard input and writing to standard output, this can be further simplified to the following.
<lang bash>cat</lang>
Visual Basic .NET
This reads a stream line by line, outputing each line to the screen.
<lang vbnet>Sub Consume(ByVal stream As IO.StreamReader)
Dim line = stream.ReadLine
Do Until line Is Nothing
Console.WriteLine(line)
line = stream.ReadLine
Loop
End Sub</lang>
- Programming Tasks
- Text processing
- Selection/Short Circuit/Console Program Basics
- Basic language learning
- Ada
- ALGOL 68
- AmigaE
- AWK
- AutoHotkey
- C
- C++
- C sharp
- Clojure
- Common Lisp
- D
- Tango
- F Sharp
- Factor
- Forth
- Fortran
- Gnuplot
- Haskell
- Icon
- Java
- JavaScript
- Logo
- Lua
- MAXScript
- Modula-3
- OCaml
- Oz
- Pascal
- Perl
- PHP
- PicoLisp
- PL/I
- Python
- R
- REBOL
- REXX
- Ruby
- Scala
- Slate
- Smalltalk
- Tcl
- Unicon
- UnixPipes
- UNIX Shell
- Visual Basic .NET
- TI-89 BASIC/Omit