program slova;

uses
   System.SysUtils;

procedure ReadFromFile(var Sentence: String);
var
   FileName: String;
   IsCorrect: Boolean;
   InFile: Text;
begin
   repeat
      Writeln('Введите имя файла, из которого хотите считать: ');
      Readln(FileName);
      FileName := FileName + '.txt';
      IsCorrect := True;
      Assign(InFile, FileName);
      {$I-}
      Reset(InFile);
      {$I+}
      if IOResult <> 0 then
      begin
         Writeln('Файл не существует!');
         IsCorrect := False;
      end
      else
      begin
         Read(InFile, Sentence);
         if length(Sentence) = 0 then
         begin
            Writeln('Файл пуст!');
            IsCorrect := False;
         end;
      end;
   until IsCorrect;
   Close(InFile);
end;

procedure Input(var Sentence: String);
var
   IsCorrect: Boolean;
begin
repeat
   Writeln('Введите предложение: ');
   Readln(Sentence);
   IsCorrect := True;
   if length(Sentence) = 0 then
   begin
      Writeln('Строка не может быть пустой!');
      IsCorrect := False;
   end;
until IsCorrect;
end;

function FlipTwo(Source: String; FirstSymbol, Space: Integer): String;
var
   FirstWord, SecondWord: String;
   Post: Integer;
   f: TReplaceFlags;
begin
   Source := StringReplace(Source,'#',',',f);
   Source := StringReplace(Source,' ',',',f);
   Post := pos(' ',Source);
   if (Post = 0) then
      Post := length(Source) + 1;
   FirstWord := Copy(Source, FirstSymbol, Space - FirstSymbol);
   SecondWord := Copy(Source, Space + 1, Post - (Space + 1));
   Delete(Source, Space + 1,Post - (Space + 1));
   Insert(FirstWord, Source, Space + 1);
   Delete(Source, FirstSymbol, Space - FirstSymbol);
   Insert(SecondWord, Source, FirstSymbol);
   FlipTwo := Source;
end;

procedure Swap(var Sentence: String);
var
   Prev, Current: Integer;
    f: TReplaceFlags;
begin
   Prev := 1;
   Current := pos(' ',Sentence);
   Sentence := FlipTwo(Sentence,Prev,Current);
   Sentence := StringReplace(Sentence,' ','#',f);
   while pos(' ',Sentence) > 0 do
   begin
      Prev := pos('#',Sentence)+1;
      Current := pos(' ',Sentence);
      Sentence := FlipTwo(Sentence,Prev,Current);
      Sentence := StringReplace(Sentence,' ','#',f);
   end;
   f := [rfReplaceAll];
   Sentence := StringReplace(Sentence,'#',' ',f);
   Sentence := StringReplace(Sentence,',',' ',f);
   Writeln(Sentence);
end;

procedure OutInFile(Sentence: String);
var
   FileName: String;
   OutFile: Text;
begin
   Writeln('Введите имя файла для записи: ');
   Readln(FileName);
   FileName := FileName + '.txt';
   Assign(OutFile, FileName);
   Rewrite(OutFile);
   Write(OutFile, Sentence);
   Close(OutFile);
end;

procedure Main();
var
   Choose: Char;
   Sentence: String;
   IsCorrect: Boolean;
begin
   Writeln('Тема: В заданном предложении, состоящем из нескольких слов, поменять местами слова, стоящие на четных местах со словами, стоящими на нечетных местах.');
   Writeln('Если хотите считать предложение из файла, введите "y" или "n":');
   repeat
      Readln(Choose);
      if (Choose <> 'y') and (Choose <> 'n') then
         begin
            Writeln('Неверный ввод');
            IsCorrect := False;
         end
      else
         IsCorrect := True;
   until IsCorrect;
   case Choose of
      'y': ReadFromFile(Sentence);
      'n': Input(Sentence);
   end;
   Swap(Sentence);
   OutInFile(Sentence);
   Readln;
end;

begin
   Main();
end.