bool  FileSeek(
   int                  file_handle,     // File handle
   long                 offset,          // In bytes
   ENUM_FILE_POSITION   origin           // Position for reference
   );

//+------------------------------------------------------------------+
//|                                                Demo_FileSeek.mq5 |
//|                        Copyright 2013, MetaQuotes Software Corp. |
//|                                              https://www.mql5.com |
//+------------------------------------------------------------------+
#property copyright "Copyright 2013, MetaQuotes Software Corp."
#property link      "https://www.mql5.com"
#property version   "1.00"
//--- display the window of input parameters when launching the script
#property script_show_inputs
//--- input parameters
input string InpFileName="file.txt";    // file name
input string InpDirectoryName="Data";   // directory name
input int    InpEncodingType=FILE_ANSI; // ANSI=32 or UNICODE=64
//+------------------------------------------------------------------+
//| Script program start function                                    |
//+------------------------------------------------------------------+
void OnStart()
  {
//--- specify the value of the variable for generating random numbers
   _RandomSeed=GetTickCount();
//--- variables for positions of the strings' start points
   ulong pos[];
   int   size;
//--- reset the error value
   ResetLastError();
//--- open the file
   int file_handle=FileOpen(InpDirectoryName+"//"+InpFileName,FILE_READ|FILE_TXT|InpEncodingType);
   if(file_handle!=INVALID_HANDLE)
     {
      PrintFormat("%s file is available for reading",InpFileName);
      //--- receive start position for each string in the file
      GetStringPositions(file_handle,pos);
      //--- define the number of strings in the file
      size=ArraySize(pos);
      if(!size)
        {
         //--- stop if the file does not have strings
         PrintFormat("%s file is empty!",InpFileName);
         FileClose(file_handle);
         return;
        }
      //--- make a random selection of a string number
      int ind=MathRand()%size;
      //--- shift position to the starting point of the string
      if(FileSeek(file_handle,pos[ind],SEEK_SET)==true)
        {
      //--- read and print the string with ind number
         PrintFormat("String text with %d number: \"%s\"",ind,FileReadString(file_handle));
        }
      //--- close the file
      FileClose(file_handle);
      PrintFormat("%s file is closed",InpFileName);
     }
   else
      PrintFormat("Failed to open %s file, Error code = %d",InpFileName,GetLastError());
  }
//+-------------------------------------------------------------------------------+
//| The function defines starting points for each of the strings in the file and  |
//| places them in arr array                                                      |
//+-------------------------------------------------------------------------------+
void GetStringPositions(const int handle,ulong &arr[])
  {
//--- default array size
   int def_size=127;
//--- allocate memory for the array
   ArrayResize(arr,def_size);
//--- string counter
   int i=0;
//--- if this is not the file's end, then there is at least one string
   if(!FileIsEnding(handle))
     {
      arr[i]=FileTell(handle);
      i++;
     }
   else
      return; // the file is empty, exit
//--- define the shift in bytes depending on encoding
   int shift;
   if(FileGetInteger(handle,FILE_IS_ANSI))
      shift=1;
   else
      shift=2;
//--- go through the strings in the loop
   while(1)
     {
      //--- read the string
      FileReadString(handle);
      //--- check for the file end
      if(!FileIsEnding(handle))
        {
         //--- store the next string's position
         arr[i]=FileTell(handle)+shift;
         i++;
         //--- increase the size of the array if it is overflown
         if(i==def_size)
           {
            def_size+=def_size+1;
            ArrayResize(arr,def_size);
           }
        }
      else
         break; // end of the file, exit
     }
//--- define the actual size of the array
   ArrayResize(arr,i);
  }