View Category
Files
Reading

Read the contents of a file into a string

python
contents = open('myFile.txt', 'rt').read()
erlang
Text = readfile("Solution607.erl"),
Text = readfile("Solution608.erl"),
clojure
(slurp "/tmp/foobar")
cpp
IO::FileStream^ file; String^ buffer;

try
{
file = gcnew IO::FileStream("test.txt", IO::FileMode::Open);
buffer = gcnew String((gcnew IO::BinaryReader(file))->ReadChars(file->Length));
}
IO::StreamReader^ stream; String^ buffer;

try
{
stream = gcnew IO::StreamReader("test.txt");
buffer = stream->ReadToEnd();
}
String^ buffer = IO::File::ReadAllText("test.txt");
fsharp
let file = new FileStream("test.txt", FileMode.Open)
let buffer = new String((new BinaryReader(file)).ReadChars(Convert.ToInt32(file.Length)))
let stream = new StreamReader("test.txt")
let buffer = stream.ReadToEnd()
let buffer = File.ReadAllText("test.txt")
fantom
contents := File(`file.text`).readAllStr

Process a file one line at a time

Open the source file to your solution and print each line in the file, prefixed by the line number, like:
1> First line of file
2> Second line of file
3> Third line of file
python
for no, line in enumerate(open(__file__)):
print "{0}> {1}".format(no+1, line.rstrip())
erlang
Reader = fun (IODevice) -> io:get_line(IODevice, "") end,
Worker = fun (Line, N) -> io:format("~B> ~s", [N, Line]), N + 1 end,

while_not_eof("Solution609.erl", Reader, Worker, 1).
Reader = fun (Filename) -> {ok, Contents} = file:read_file(Filename), Contents end,
Transformer = fun (Line, N) -> string:concat(string:concat(integer_to_list(N), "> "), Line) end,
Printer = fun (Line) -> io:format("~s~n", [Line]) end,

Lines = string:tokens(binary_to_list(Reader("Solution610.erl")), "\n"),
NewLines = lists:zipwith(Transformer, Lines, lists:seq(1, length(Lines))),
lists:foreach(Printer, NewLines).
clojure
(defn read-line-by-line [fn]
(reduce str (map (partial format "%d> %s\n")
(iterate inc 1)
(read-lines fn))))
cpp
IO::StreamReader^ stream; String^ ln; int i = 0;

try
{
stream = gcnew IO::StreamReader("test.txt");
while ((ln = stream->ReadLine())) Console::WriteLine("{0}> {1}", ++i, ln);
}
int i = 0;
for each(String^ line in IO::File::ReadAllLines("test.txt")) Console::WriteLine("{0}> {1}", ++i, line);
fsharp
let stream = new StreamReader("test.txt")
let mutable i = 1
let mutable line = stream.ReadLine()
while (line <> null) do printfn "%d> %s" i line ; line <- stream.ReadLine() ; i <- i + 1 done
stream.Close()
let proc_a_line (filename : string) proc =
let stream = new StreamReader(filename)
let rec proc_a_line' count line =
match line with
| null -> stream.Close()
| _ -> proc count line ; proc_a_line' (count + 1) (stream.ReadLine())
proc_a_line' 1 (stream.ReadLine())

// ------

let _ = proc_a_line "test.txt" (fun i line -> printfn "%d> %s" i line)
let reader(filename : string) = seq {
use sr = new StreamReader(filename)
while not sr.EndOfStream do
let line = sr.ReadLine()
yield line
done
}

// ------

reader("test.txt") |> Seq.iteri (fun i line -> printfn "%d> %s" (i + 1) line)
File.ReadAllLines("test.txt") |> Array.iteri (fun i line -> printfn "%d> %s" (i + 1) line)
// Unlike ReadAllLines, ReadLines (new in .NET 4) only reads the file
// one line at a time, rather than reading the entire file into an array first.

open System.IO
File.ReadLines("test.txt") |> Seq.iteri (fun i line -> printfn "%d> %s" (i + 1) line)
fantom
File(`input.text`).readAllLines.each |Str s, Int i| { echo("${i+1}> $s") }
Writing

Write a string to a file

python
open('test.txt', 'wt').write('Hello World!')
erlang
Line = "This line overwites file contents!\n",
{ok, IODevice} = file:open("test.txt", [write]), file:write(IODevice, Line), file:close(IODevice).
clojure
(with-out-writer "output.txt" (println "Hello file!"))
cpp
IO::StreamWriter^ stream;

try
{
stream = gcnew IO::StreamWriter("test.txt", false);
stream->WriteLine("This line overwites file contents!");
}
fsharp
let stream = new StreamWriter("test.txt", false)
stream.WriteLine("This line overwrites file contents!")
fantom
File(`out.txt`).out.writeChars("some text").flush

Append to a file

python
open('test.txt', 'at').write('Hello World!\n')
erlang
Line = "This line appended to file!\n",
{ok, IODevice} = file:open("test.txt", [append]), file:write(IODevice, Line), file:close(IODevice).
clojure
(with-out-append-writer "output.txt" (println "This is appended to the file"))
cpp
IO::StreamWriter^ stream;

try
{
stream = gcnew IO::StreamWriter("test.txt", true);
stream->WriteLine("This line appended to file!");
}
fsharp
let stream = new StreamWriter("test.txt", true)
stream.WriteLine("This line appended to file!")
fantom
File(`out.txt`).out(true).writeChars("some text").flush
Directories

Process each file in a directory

python
import os
results = (process(f) for f in os.listdir(".") if os.path.isfile(f))
erlang
% File basenames only - many tasks require absolute paths to work
lists:foreach(fun (FileOrDirPath) -> Worker(FileOrDirPath) end, file:list_dir(Directory)).
% Absolute paths provided - will accomodate most tasks
lists:foreach(fun (FileOrDirPath) -> Worker(FileOrDirPath) end, list_dir_path(Directory)).
clojure
; (defn process-file [f] "process one file" body...)
(map process-file (.listFiles (File. ".")))
cpp
for each(String^ filename in IO::Directory::GetFiles(dirname)) process(filename);
fsharp
let dirname = "c:\\"

let processFile filename = printfn "%s" filename
for filename in Directory.GetFiles(dirname) do processFile filename done
let dirname = "c:\\"

Directory.GetFiles(dirname) |> Array.iter (fun filename -> printfn "%s" filename)
fantom
File(`./`).list.each { process(it) }

Process each file in a directory recursively

python
import os
results = (process(os.path.join(p, n)) for p,d,l in os.walk(".") for n in l)
erlang
filelib:fold_files(Directory, ".*", true, fun (FileOrDirPath, Acc) -> Worker(FileOrDirPath), Acc end, []).
process_dir(Directory, Worker).
clojure
; (defn process-file [f] "process one file" body...)
(map process-file (file-seq (File. ".")))
cpp
void processFile(String^ filename) { Console::WriteLine("{0}", filename); }

void processDirectory(String^ dirname)
{
for each(String^ filename in IO::Directory::GetFiles(dirname)) processFile(filename);
for each(String^ subdirname in IO::Directory::GetDirectories(dirname)) processDirectory(subdirname);
}

int main()
{
processDirectory("c:\\");
}
fsharp
let processDirectory dirname proc =
let rec processDirectory' dirname' =
Directory.GetFiles(dirname') |> Array.iter proc
Directory.GetDirectories(dirname') |> Array.iter processDirectory'
processDirectory' dirname

// ------

let dirname = "c:\\"

processDirectory dirname (fun filename -> printfn "%s" filename)
fantom
File(`./`).walk { process(it) }