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PrologCpp

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(Prolog) Prolog and C++ syntax comparison

This page translates C++ structures and syntaxes to their Prolog equivalents

Overview

• The Prolog equivalent of the C++ switch
• The Prolog equivalent of the C++ std::map

The Prolog equivalent of the C++ switch

Using the cut, !, you can make statements similar to a C++ switch statement.

(see also: Prolog switch)

% If the user enters a number from 1 to an including 3
% the right clauses are called.
% If you would remove the cuts,
% then the input '2' would results in calling both action(2) and action(_)
%
% Note that you can remove 'nondeterm' from the action's predicate,
% but I left it in, so you do can comment out the cuts
%
% Compiled under Visual Prolog 5.2
PREDICATES
  nondeterm action(integer) % The switch switches on an integer
	
CLAUSES
  action(1) :-              % Equivalent to 'case 1:' 
    !,
    nl,
    write("You typed one").
  action(2) :-              % Equivalent to 'case 2:' 
    !,
    nl,
    write("You typed two").
  action(3) :-              % Equivalent to 'case 3:' 
    !,
    nl,
    write("You typed three").
  action(_) :-              % Equivalent to 'default:' 
    !,
    nl,
    write("You typed an unknown number").
	
GOAL
  write("Type a number from 1 to 3: "),
  readInt(Num),
  action(Num),
  nl.

Note that the C++ 'falling through cases' has no direct Prolog equivalent, as the structure differs.

To give an example, the C++ code below CAN be translated to Prolog:

//C++ code
int x = 0; 
switch (x)
{
  case 0:  /* Something */ ; /* No break! Fall through to next case */
  case 1:  /* Something */ ; /* No break! Fall through to next case */
  default: /* Something */ ; 
}

% Compiled under Visual Prolog 5.2
PREDICATES
  nondeterm action(integer) % The switch switches on an integer
	
CLAUSES
  action(X) :-              
    X <= 0,
    % !, % Needed to remove the cuts
    nl,
    write("You typed zero").
  action(X) :-              
    X <= 1,
    % !, % Needed to remove the cuts
    nl,
    write("You typed one").
  action(_) :-              
    % !, % Needed to remove the cuts
    nl,
    write("You typed an other number").
	
GOAL
  write("Type a number 0,1 or different: "),
  readInt(Num),
  action(Num),
  nl.

The Prolog equivalent of the C++ std::map

% The Prolog predicate of a C++ std::map<int,int>
% Compiled under Visual Prolog 5.2
PREDICATES 
  % Declare the Prolog equivalent of a std::map<int,int> 
  % A std::map has a single key to a single value,
  % therefore the predicate is deterministic
  % input = key, output = value
  determ mapValues(Integer,Integer) - (i,o). % -(i,o) is non-standard flowcontrol, you can remove it.
CLAUSES 
  % Map the keys to their values
  mapValues(0,50).
  mapValues(1,40).
  mapValues(2,30).
  mapValues(3,20).
  mapValues(4,10).
  % mapValues(4,20). % NOT POSSIBLE! That would make the predicate 'mapValues' nondeterm!
  
GOAL 
  X = 2,                      % The value you want to look-up
  mapValues(X,Y),             % Get the value of the key X
  write("X: "), write(X), nl, % Write the key to screen
  write("Y: "), write(Y), nl. % Write the value to screen

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