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From: Robert Byrne <byrner@db.erau.edu>
Subject: RECURSION PROBLEM
Date: 1997/10/28
Message-ID: <34558FD4.2781@db.erau.edu>#1/1
X-Deja-AN: 286843118
Organization: Embry-Riddle Aeronautical University
Newsgroups: comp.lang.ada
Date: 1997-10-28T00:00:00+00:00
List-Id: <comp.lang.ada>


I need to use recursion to display all pairs (combinations) of
characters from a set of characters read from a file given on the
command line.  This is easy without recursion; the below program works
correctly.  I am thinking that the easiest way to modify this program
would be to change the inner for loop in the procedure
DISPLAY_COMBINATIONS to a recursive procedure.  Does anyone have any
idea how to go about doing this?  I would appreciate any help.  Thanks.

byrner@db.erau.edu
------------------------------------------------------------------------

with ADA.TEXT_IO, ADA.COMMAND_LINE;
use ADA.TEXT_IO;

procedure PROG12 is 

  INPUT_STRING: FILE_TYPE;    --file to read from
  LENGTH: NATURAL:= 0;        --length of string containing all
characters
  TEMP_LENGTH: NATURAL:= 0;   --length of string for each line
  LETTER: STRING(1..80);      --string containing all characters
  TEMP_LETTER: STRING(1..80); --string for each line
  N: INTEGER:= 1;             --counter for # of lines read
   

procedure DISPLAY_COMBINATIONS(LETTER: in STRING; LENGTH: in NATURAL)
is  
  
begin --DISPLAY_COMBINATIONS

for I in 1..(LENGTH-1) loop 
  for J in (I+1)..LENGTH loop
    PUT("("); PUT(LETTER(I)); PUT(", "); PUT(LETTER(J)); PUT(")");
    NEW_LINE;          
  end loop;  
end loop;

end DISPLAY_COMBINATIONS;


begin --PROG12
  
  --open file to read from
  OPEN(INPUT_STRING, IN_FILE, ADA.COMMAND_LINE.ARGUMENT(1));
    
  --input string from file INPUT_STRING
  while not END_OF_FILE(INPUT_STRING) loop       --check for end of
file    
    GET_LINE(INPUT_STRING, TEMP_LETTER, TEMP_LENGTH);  
    if TEMP_LENGTH /= 0 then                     --check for characters
in line
      LENGTH:= LENGTH + TEMP_LENGTH;
      if N = 1 then 
        LETTER(1..LENGTH):= TEMP_LETTER(1..LENGTH);
      else
        LETTER(1..LENGTH):= LETTER(1..LENGTH-TEMP_LENGTH) &
TEMP_LETTER(1..TEMP_LENGTH);
      end if;
      N:= N+1;
    end if;  
  end loop;
    
  --close input file  
  CLOSE(INPUT_STRING); 
    
  --determine and display all combinations of 2 characters
  NEW_LINE;
  DISPLAY_COMBINATIONS(LETTER, LENGTH);

end PROG12;
------------------------------------------------------------------------