Input loop

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. <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; </ada>

AWK

This just reads lines from stdin and prints them until EOF is read.

//{ print $0 }

C

<c>#include <stdio.h>

1. 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. */

1. define Str(x) #x
2. 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 */
 }

} </c>

C++

The following functions store the words resp. lines in a vector. Of course, instead arbitrary processing of the words could be done. <cpp>#include <istream>

1. include <string>
2. include <vector>

// word by word void read_words(std::istream& is, std::vector<std::string>& words) {

 std::string word;
 while (is >> word)
 {
   // store the word in the vector
   words.push_back(word);
 }

}

// line by line: void read_lines(std::istream& is, std::vector<std::string>& lines) {

 std::string line;
 while (std::getline(is, line))
 {
   // store the line in the vector
   lines.push_back(line);
 }</cpp>

An alternate way to read all words into a vector is to use iterators:

<cpp>#include <istream>

1. include <string>
2. include <iterator>
3. include <algorithm>
4. include <vector>

void read_words(std::istream& is, std::vector<std::string>& words) {

 std::copy(std::istream_iterator<std::string>(is), std::istream_iterator<std::string>(),
           std::back_inserter(words));

}</cpp>

For arbitrary processing, replace std::copy with std::for_each or std::transform calling an appropriate function (or function object).

D

Library: Tango

<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
   }

}</d>

Library: Tango

<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
   }

}</d>

Note that foreach variables 'line' and 'word' are transient slices. If you need to retain them for later use, you should .dup them.

Forth

Works with: GNU 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 ;

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.

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

Java

Some people prefer Scanner or BufferedReader, so a way with each is presented. <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 }</java> Or <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 netowrk stream while(inp.ready()){ String input = inp.readLine();//line-by-line only //process the input here } } catch (IOException e) { //There was an input error }</java>

OCaml

<ocaml>let rec read_lines ic =

 try let line = input_line ic in
   line :: read_lines ic
 with End_of_file ->
   []</ocaml>

The version above will work for small files, but it is not tail-recursive.
Below will be more scalable:

<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 []

</ocaml>

Or with a higher order function:

<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)) </ocaml>

Ruby

Ruby input streams are Enumerable objects like Arrays, so one can use the standard #each:

stream.each do |line|

 # process line

end

One can open a new stream for read and have it automatically close when done:

File.open(filename, "r") do |stream|

 stream.each do |line|
   # process line
 end

end

UnixPipes

the pipe 'yes XXX' produces a sequence

read by lines

yes 'A B C D ' | while read x ; do echo -$x- ; done

read by words

yes 'A B C D ' | while read -d\  a ; do echo -$a- ; done