SCARLET - Relational Operators


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2.7.1 Relational Operators (RelOp)

SCARLET provides all the common relational operators on integer expressions IntExpression in just the form we are used to in daily life. In detail we are talking about

RelOp:
	`==`	equal to
	`<`	less than
	`>`	greater than
	`!=`	not equal to
	`>=`	greater than or equal to
	`<=`	less than or equal to

They compare two operands at a time and return a boolean value corresponding to the resulting logical value.

But relational operators may also be applied to string expressions (short: strings) StringExpression. In this case, the normal comparison for numbers is replaced by the lexical one, that means, two strings are compared character by character, starting at the left and obeying the following rules:

Two strings are equal to each other, if both have the same length, and all characters are identical, otherwise they are not equal.
A string S₁ with length n, , is greater than a second string S₂, if the first i-1, , characters are identical and either the ASCII number of the i-th character of S₁ is greater than the i-th character of S₂, or S₂ has only got i-1 characters.

If the exceptional features of the internal order of the ASCII code are not taken in account, the lexical comparison may sometimes lead to slightly surprising results:

Example 8:

 "bigger" > "big"       TRUE
 "mouse" > "MAMMOTHE"   TRUE
 "99" > "One"           FALSE

Parts of the ASCII code
21	§	45	-	65	A	124	\|
33	!	46	.	..	..	125	}
34	"	47	/	90	Z	126	~
35	#	48	0	91	[	129	ü
36	$	..	..	92	\	132	ä
37	%	57	9	93	]	142	Ä
38	&	58	:	94	^	148	ö
39	'	59	;	95	_	153	Ö
40	(	60	<	96	`	154	Ü
41	)	61	=	97	a	248	°
42	*	62	>	..	..	255
43	+	63	?	122	z
44	,	64	@	123	{

Because cell expressions CellExpression consist of one or more registers with the same priority, it does not normally make sense to ask whether one cell expression is greater (or less) than the other; but some of the relational operators can successfully be applied to them, too, namely == (equal) and != (not equal). Here a comparison can be done by comparing all corresponding registers with each other. Hence two cell expressions are equal, if the contents of each corresponding register pair are equal; otherwise they are not equal. Doing this, we take for granted that both cell expressions have the same register structure, otherwise they would not be comparable. (In fact this cannot happen, since the structure is uniformly fixed for all cell expressions within one program.)

If it is the issue, whether two register contents are equal while one of them is VOID, the result will always be TRUE; (in case of "unequal" it will be FALSE.) Indeed VOID enables partial comparisons of cell expressions.

Example 9:

 {"s", 25, VOID} == {"s", 25, "a"}      TRUE
 {"s", 25, VOID} != {"s", 25, "a"}      FALSE


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