*CompareValue* function is incredibly practical when you are writing comparers (functions that determine how some data structure is ordered). *System.Math *and *System.StrUtils* define a bunch of functions that can be used to compare integers, doubles, strings … There’s, however, no *CompareValue* for booleans.

A *CompareValue* function compares two parameters, traditionally named *left* and *right*, and returns *0* if they are the same, –*1* if *left *is smaller and *1* if *right* is smaller.

If we use the usual ordering of *false < true*, we can write the missing function as follows:

function CompareValue(left, right: boolean): integer; overload; begin if left < right then Result := -1 else if left > right then Result := 1 else Result := 0; end;

Your task for today – if you choose to accept it – is: Write this function without any *if* statements.

One possible solution:

function CompareValue(left, right: boolean): integer; overload; begin Result := Ord(left and not right) - Ord(right and not left); end;

A proof – in a form of an Excel spreadsheet, nonetheless:

left | right | left and not right | right and not left | Ord(l & ^r) | Ord(r & ^l) | Ord(l & ^r) - Ord(r & ^l) |

FALSE | FALSE | FALSE | FALSE | 0 | 0 | 0 |

FALSE | TRUE | FALSE | TRUE | 0 | 1 | -1 |

TRUE | FALSE | TRUE | FALSE | 1 | 0 | 1 |

TRUE | TRUE | FALSE | FALSE | 0 | 0 | 0 |

> Write this function without any if statements

ReplyDeleteBut why would you do that...?

I'd bet that if you compare the generated assembly of the two functions, the first version is more efficient.

For fun, of course!

DeleteUm, why not just do:

ReplyDeleteResult := Ord(Left) - Ord(Right);

For Booolean, False is 0 and True is 1, unless I've missed something.

Certainly better than my solution. I never claimed I have published the best one ;)

DeleteYou missed that value might come from outside. Like in Microsoft compilers TRUE is -1 not +1 as in Borland compilers. Also, there can be any garbage like 42. Or -42.

DeleteAlso, true(10) and true(100), which is larger?

So, my approach would be Result := Sign(cardinal(Left)) - Sign(Cardinal(Right);

Why not byte(...) ? Because there are also LongBool and WordBool data types.

Or equivalent assembler code if we could have inline assembler functions :-D

Funny thing, Intel also makes True being +1, in hardware (see SETcc opcodes introduced into 80386 processor in early 1990-s). Dunno why, for -1 always seemed more natural TRUE to me, being the complement thus unifying bitwise and logic operations.

Delete@Arioch, do the non-bit Boolean types cast down implicitly? I'm not sure.

DeleteYour approach should work though. As long as casting to Cardinal doesn't cause problems for LongBools. Aren't LongBools signed?

@Arioch:

Deletesign() requires math.pas and will end up slow. And <0 is true in boolean!

Result := Ord(left <> false) - Ord(right <> false);

This also gives you the simplest assembly, better than "ifs" even.

Or maybe even

DeleteResult := Ord(Ord(left)<>0) - Ord(Ord(right)<>0)

In case Delphi ever decides to start optimizing "left <> false" to simply "left".

I know I'm a bit late to the game, but:

ReplyDeletefunction CompareBoolean(const ALeft, ARight: boolean): integer;

const

CComparisonMap: array[boolean, boolean] of integer = ((0, -1), (1, 0));

begin

result := CComparisonMap[ALeft, ARight];

end;

N@