Execute prints backwards

Question

Your task is to reverse the order in which some prints get executed.

---

Specs:
Your code will be in this form:

//some lines of code
/*code*/ print "Line1" /*code*/
/*code*/ print "Line2" /*code*/
/*code*/ print "Line3" /*code*/
/*code*/ print "Line4" /*code*/
//some lines of code

You will have to print (or echo, or write, or equivalent) those strings from the fourth to the first.

• You decide which lines of your program must print the strings, but they must be adjacent;

• Every line can contain only one print, and cannot exceed 60 bytes in length;

• Since this is popularity-contest, be creative and avoid to write just a goto or a simple for(i){if(i=4)print"Line1";if(i=3)...}

• The most upvoted answer in 2 weeks wins this.

• Your output MUST be Line4 Line3 Line2 Line1 OR Line4Line3Line2Line1 OR Line4\nLine3\nLine2\nLine1(where \n is a newline), and it must be generated only by executing those prints backwards.

Happy coding!

UPDATE: Contest is over! Thank you all :)

Answer 1

PHP

<?php

class D          {function D(){echo 'Line1';            }}
class C extends D{function C(){echo 'Line2';parent::D();}}
class B extends C{function B(){echo 'Line3';parent::C();}}
class A extends B{function A(){echo 'Line4';parent::B();}}
new A();

maximum of 60 bytes :)

Answer 2

Julia

macro rev(a)
    eval(a).args = flipud(eval(a).args);a
end

p=quote
    println("Line1")
    println("Line2")
    println("Line3")
    println("Line4")
  end

rp = @rev(p)
eval(rp)

It's overkill but I wanted to do something with macros and expressions. p is an expression object (an object representing parsed code), @rev operates on the expression to create a new expression rp with the lines reversed, then I eval the new expression. Should work to reverse execution for an arbitrary amount of code.

Answer 3

C

#include <stdio.h>

void myPrint(char *arg) {
  static int count = 0
  static char **lines = malloc(sizeof(char *) * 4);
  char[count] = arg;
  ++count;
  if (count >= 4) {
    for (int i = 3; i >= 0; --i) {
      puts(lines[i]);
    }
  }
}

#define print myPrint

int main(int argc, char **argv) {
  print("Line 1");
  print("Line 2");
  print("Line 3");
  print("Line 4");
  return -1;
}

#undef print

I didn't test it, so there may be syntax errors, segfaults or undefined behaviour, but I think the general idea is clear.

Answer 4

C#

A delegate that returns itself:

using System;

delegate A A(Action a);

static class Program
{
    static void Main()
    {
        Magic()
            (() => Console.WriteLine("Line1"))
            (() => Console.WriteLine("Line2"))
            (() => Console.WriteLine("Line3"))
            (() => Console.WriteLine("Line4"))
            (null);
    }

    static A Magic()
    {
        Action prev = () => { };
        A result = null;
        result = a =>
        {
            if (a == null)
                prev();
            else
            {
                var prevPrev = prev;
                prev = () =>
                {
                    a();
                    prevPrev();
                };
            }
            return result;
        };

        return result;
    }
}

Answer 5

Batch

I don't know if this technically allowed..

@echo off
for /f "tokens=1-4 delims=~" %%a in (
    "echo Line1
    ~echo Line2
    ~echo Line3
    ~echo Line4"
) do %%d & %%c & %%b & %%a

Given the requirement in the question, of no spaces when using line feed -

Line4\nLine3\nLine2\nLine1

The last line should be -

) do %%d&%%c&%%b&%%a

Answer 6

Bash

$ x=
$ print () {
>     x=$1$x
> }
$ print 1
$ print 2
$ print 3
$ print 4
$ echo $x

Define print as tacking the passed argument on to the beginning of a variable; clear it before and print it after.

Answer 7

Ruby

["print 'Line1'",
"print 'Line2'",
"print 'Line3'",
"print 'Line4'"].reverse.each {|l| eval "#{l}"}

#=> Line4Line3Line2Line1

Answer 8

PHP

    $a = array(
        function(){print "Line 1\n";},
        function(){print "Line 2\n";},
        function(){print "Line 3\n";},
        function(){print "Line 4\n";},
    );
    foreach (array_reverse($a) as $l) { $l(); }

Answer 9

Javascript - Without relying on the event loop*

var p = function (v, c) { if (c) { c(); } console.log(v); }
p('Line 1', function() { p ('Line 2', function () { p ('Line 3', function () { p('Line 4'); })})});

Result:

Line 4
Line 3
Line 2
Line 1

Answer 10

Javascript 1.8

See Javascript 1.8 and browser support.

console.log([
    'print "Line1"',
    'print "Line2"',
    'print "Line3"',
    'print "Line4"'
    ].reduceRight(
        function(p,s) p + s.match(/"(.*)"/)[1],
        ""
    )
)

Answer 11

q

Very easy in a language with "left-of-right" evaluation.

(show "Line1"),
(show "Line2"),
(show "Line3"),
show "Line4"

"Line4"

"Line3"

"Line2"

"Line1"

edit: formatted code to fit one print per line rule, though q ignores white space so I'm not sure this counts

Answer 12

main-while-loop and if switches (Python)

next = 4
while next:
    if next == 1: print "Line1"
    if next == 2: print "Line2"
    if next == 3: print "Line3"
    if next == 4: print "Line4"
    next -= 1

Answer 13

C++

#include <iostream>
#define print(s) std::cout << s << "\n"; return g; }
#define f []{ auto g = 

int main() {
    f f f f []{};
    print ("Line 1");
    print ("Line 2");
    print ("Line 3");
    print ("Line 4")
    ()()()()();
}

Answer 14

Simple T-SQL Solution

The code here is not "organic" (not part of the raw code body), though, unlike my other batch solution.

set nocount on

declare @t table (stmt nvarchar(500))
declare @stmt nvarchar(500)

insert into @t values 
('print ''Line1'''),
('print ''Line2'''),
('print ''Line3'''),
('print ''Line4''')

select top 1 @stmt = stmt from @t order by stmt desc

while @@rowcount > 0 begin
    exec(@stmt)
    delete from @t where stmt = @stmt
    select top 1 @stmt = stmt from @t order by stmt desc
end

Answer 15

PowerShell

(   {Write-Host 'Line 1'},
    {Write-Host 'Line 2'},
    {Write-Host 'Line 3'},
    {Write-Host 'Line 4'}
)[3..0]|ForEach-Object {Invoke-Command $_}

Answer 16

Python 2.7

for l in """
print "Line1"
print "Line2"
print "Line3"
print "Line4"
""".splitlines()[::-1]:
    exec(l)

you can see the result here: http://ideone.com/QypGiB

a[::-1] represents the reversed list a.

Answer 17

Python 3

import sys

plist = []
cout = 0

def print(s):
    global cout
    plist.append(s+"\n")
    cout += 1
    if cout == 4:
        for i in reversed(plist):
            sys.stdout.write(i)

print("Line 1")
print("Line 2")
print("Line 3")
print("Line 4")

Output (escaped):

"Line4\nLine3\nLine2\nLine1\n"

Answer 18

More fun with BASH arrays

#!/bin/bash
a=("echo Line1"  
   "echo Line2"
   "echo Line3"
   "echo Line4")
for (( i=${#a[@]}-1; i>=0; i-- )) ; do
  eval ${a[i]}
done

Output:

Line4
Line3
Line2
Line1

Answer 19

Bash

#!/bin/bash
eval `tac $0 | head -n 4` && exit
echo 'Line1' ;
echo 'Line2' ;
echo 'Line3' ;
echo 'Line4' ;

Output:

Line4
Line3
Line2
Line1

Answer 20

One more from the Bash command-line:

echo "Line1" "Line2" "Line3" "Line4" | { read a b c d; echo $d $c $b $a; }

Output:

Line4 Line3 Line2 Line1

Here is a version expanded to multiple lines to match the exact syntax of the spec:

echo "Line1" < \
echo "Line2" < \
echo "Line3" < \
echo "Line4" | { read a b c d; echo $d; echo $c; echo $b; echo $a; }

Answer 21

Smalltalk (79/66/143 chars)

version 1 (79):

{
    [ 'line1' print ].
    [ 'line2' print ].
    [ 'line3' print ].
    [ 'line4' print ]
} reverse map:#value

little explanation:
[..] is a block (aka lambda closure);
{..} is an array constructor;
reverse and map are obvious;
#value evaluates a block closure.

output:

line4line3line2line1

version 2 (66):

Here is a version using reflection; it fetches the string constants of the block and prints them:

Given any block containing string-prints:

b := [
    'line1' print.
    'line2' print.
    'line3' print.
    'line4' print
].

where 'normal' evaluation with:

b value.

gives: line1line2line3line4l

The following extracts the string literals and prints them reverse:

(b method literals select:[:l|l class==String]) reverse map:#print

gives: line4line3line2line1

the code is a little unsafe (it requires that the block's containing method does not contain other string literals; but we are playing golf - not trusted software development ;-)

version 3 (143)

defines a 'function' which redirects stdout to some internal stream, evaluates a block closure and finally prints the collected output in reverse:

w := [:b | |sav red|
  [
      sav := Stdout.
      Stdout := red := '' writeStream.
      b value.
  ] ensure:[
      Stdout := sav.
      red contents asCollectionOfLines reverse map:#printNL
  ]
].

then we evaluate any code within that wrapper:

w value:[
 'line1' printNL.
 'line2' printNL.
 'line3' printNL.
 'line4' printNL
]

its not thread save, though.

(all character counts are w.o omittable whitespace)

Answer 22

Coffeescript

revision 2: not as coffeescriptic but smaller

do(c=console)->c.log "Line1" if!c
.log "Line2" if!c
.log "Line3" if!c
.log "Line4"

revision 1:

do(c=console)->c.log "Line1" unless c
.log "Line2" unless c
.log "Line3" unless c
.log "Line4"

Single line variant:

do(c=console)->c.log "Line1" unless c.log "Line2" unless c.log "Line3" unless c.log "Line4"

Answer 23

F#

(Ab)using custom keywords in a computation expression.

type Reverse = Reverse with
    [<CustomOperation "print">]
    member this.Print(l, x) = x :: l
    member this.Yield(_) = []
    member this.Run(l) = List.iter (printfn "%s") l

Reverse {
    print "Line1"
    print "Line2"
    print "Line3"
    print "Line4"
}

Answer 24

Shell again

cat <<XXX | sort -r | sh -
echo Line 1
echo Line 2
echo Line 3
echo Line 4
XXX

Answer 25

Python 2

Using the much maligned inline if-else

from sys import stdout
stdout.write('Line 1\n') if (stdout.write('Line 2\n') if (stdout.write('Line 3\n') if stdout.write('Line 4\n') or 1 else 0) or 1 else 0) or 1 else 0

Answer 26

Java

This is an alternative to the System.out abuse in the other submission, by abusing the PrintStream instead (the out).

class MyPrintStream extends PrintStream {
    String t = "";

    public MyPrintStream() {
        super(System.out);
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                MyPrintStream.super.println(t);
            }
        });
    }

    public void println(String s) {
        t = s + "\n" + t;
    }
}

public class Temp {
    static {
        System.setOut(new MyPrintStream());
    }

    public static void main(String[] args) throws InterruptedException {
        System.out.println("Line 1");
        System.out.println("Line 2");
        System.out.println("Line 3");
        System.out.println("Line 4");
    }
}

Answer 27

output_list = []
class newstdout():
    def write(self, s):
        output_list.append(s)

stdout, sys.stdout = sys.stdout, newstdout()

print("Line 1")
print("Line 2")
print("Line 3")

sys.stdout = stdout
_ = [print(s, end='') for s in output_list[::-1]]

Answer 28

Objective-C

#define NSLog(a) NSLog( [self changeNumber:(NSString*)a])
...
    NSLog(@"Line1");
    NSLog(@"Line2");
    NSLog(@"Line3");
    NSLog(@"Line4");
...
- (NSString *)changeNumber:(NSString *)string
{
    int number = [string substringFromIndex:string.length-1].intValue;
    int newnumber = -number + 5;
    NSString *newString = [string stringByReplacingOccurrencesOfString:[NSString stringWithFormat:@"%d", number] withString:[NSString stringWithFormat:@"%d", newnumber]];
    return newString;
}

Answer 29

Clojure

Using clojure macro.

(defmacro do-reverse [& v]
 `(do 
    ~@(reverse v)))

(do-reverse
 (println "Line 1")
 (println "Line 2")
 (println "Line 3")
 (println "Line 4"))

Answer 30

Python 2

import inspect

def main():
    print "Line 1"
    print "Line 2"
    print "Line 3"
    print "Line 4"

for line in inspect.getsourcelines(main)[0][:0:-1]:
    exec(line.strip())