Language Reference/Terms/If-then-else - Revision history

Thomas Linder Puls: /* if-then-else (expression) */ spelling

2015-03-28T14:39:19Z

if-then-else (expression): spelling

← Older revision		Revision as of 16:39, 28 March 2015
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	==== if-then-else (expression) ====		==== if-then-else (expression) ====

	The if-then-else expression ~~conditionaly~~ evaluates ~~excressions~~.		The if-then-else expression conditionally evaluates expressions.

	Syntactically it is same as the if-then-else statement, but and the terms in the branches must be expressions and the entire if-then-else expression will itself evaluate to a value.		Syntactically it is same as the if-then-else statement, but and the terms in the branches must be expressions and the entire if-then-else expression will itself evaluate to a value.

	The ~~shorhand~~ writings that leave out the else-part does not make sense for the expression.		The shorthand writings that leave out the else-part does not make sense for the expression.

	{{Example\|		{{Example\|

SergeMukhin: /* if-then-else (expression) */

2013-03-12T11:05:42Z

if-then-else (expression)

← Older revision		Revision as of 13:05, 12 March 2013
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	{{Example\|		{{Example\|
	<vip>clauses		<vip>clauses
	w(X, U) :-		w(X, Y) :-
	Min = if X < Y then X else Y end if,		Min = if X < Y then X else Y end if,
	writef("The minimum is : %\n", Min).		writef("The minimum is : %\n", Min).

Thomas Linder Puls: subarticle

2013-02-12T09:38:10Z

subarticle

← Older revision		Revision as of 11:38, 12 February 2013
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	The if-then-else expression above evaluates to <vp>X</vp> if <vp>X</vp> is less than <vp>Y</vp> else it evaluates to <vp>Y</vp>. <vp>Min</vp> is bound to the resulting value.}}		The if-then-else expression above evaluates to <vp>X</vp> if <vp>X</vp> is less than <vp>Y</vp> else it evaluates to <vp>Y</vp>. <vp>Min</vp> is bound to the resulting value.}}

	<noinclude>~~[[Category:Language Reference Subarticle]]~~</noinclude>		<noinclude>{{LanguageReferenceSubarticle\|Terms/If-then-else}}</noinclude>

Thomas Linder Puls: header level

2013-01-16T10:43:58Z

header level

← Older revision		Revision as of 12:43, 16 January 2013
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	if-then-else can be used both as a statement and as an expression.		if-then-else can be used both as a statement and as an expression.

	=== if-then-else (statement) ===		==== if-then-else (statement) ====

	The if-then-eslse statement conditionally executes a group of statements.		The if-then-eslse statement conditionally executes a group of statements.
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	<vipbnf>if <Cond> then <Term> else succeed end if</vipbnf>		<vipbnf>if <Cond> then <Term> else succeed end if</vipbnf>

	=== if-then-else (expression) ===		==== if-then-else (expression) ====

	The if-then-else expression conditionaly evaluates excressions.		The if-then-else expression conditionaly evaluates excressions.

Thomas Linder Puls: if-then-else expression/ implicit cut in condition

2013-01-16T10:40:25Z

if-then-else expression/ implicit cut in condition

← Older revision		Revision as of 12:40, 16 January 2013
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	~~Conditionally~~ executes a group of statements.		if-then-else can be used both as a statement and as an expression.

			=== if-then-else (statement) ===

			The if-then-eslse statement conditionally executes a group of statements.

	<vipbnf><IfThenElseTerm>:		<vipbnf><IfThenElseTerm>:
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	<vip>if Cond then T1 else T2 end if</vip>		<vip>if Cond then T1 else T2 end if</vip>

	First <vp>Cond</vp> is evaluated, if it succeeds then <vp>T1</vp> is evaluated otherwise <vp>T2</vp> is evaluated. <vp>Cond</vp> is ~~not allowed to leave backtrack points; that is~~, ~~it cannot be~~ <vp>multi</vp> or <vp>~~nondeterm~~</vp>. <vp>Cond</vp> is a cut-scope (see {{lang2\|Terms\|Cut_Scopes\|Cut Scopes}}).		First <vp>Cond</vp> is evaluated, if it succeeds then <vp>T1</vp> is evaluated otherwise <vp>T2</vp> is evaluated.

			<vp>Cond</vp> is followed by an implicit cut, which turns:
			* a <vp>nondeterm</vp> condition into a <vp>determ</vp> condition and
			* a <vp>multi</vp> condition into a <vp>procedure</vp>.

			<vp>Cond</vp> is a cut-scope (see {{lang2\|Terms\|Cut_Scopes\|Cut Scopes}}).

	{{Example\|		{{Example\|
	<vip>clauses		<vip>clauses
	w(X) :-		w(X) :-
	if X div 2 = 0 and X > 3 then		if X div 2 = 0 and X > 3 then
	write("X is good")		write("X is good")
	else		else
	write("X is bad"),		write("X is bad"),
	nl		nl
	end if,		end if,
	nl.</vip>		nl.</vip>

	There are several things to notice about this example:		There are several things to notice about this example:
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	<vipbnf>if <Cond> then <Term> else succeed end if</vipbnf>		<vipbnf>if <Cond> then <Term> else succeed end if</vipbnf>

			=== if-then-else (expression) ===

			The if-then-else expression conditionaly evaluates excressions.

			Syntactically it is same as the if-then-else statement, but and the terms in the branches must be expressions and the entire if-then-else expression will itself evaluate to a value.

			The shorhand writings that leave out the else-part does not make sense for the expression.

			{{Example\|
			<vip>clauses
			w(X, U) :-
			Min = if X < Y then X else Y end if,
			writef("The minimum is : %\n", Min).
			</vip>
			The if-then-else expression above evaluates to <vp>X</vp> if <vp>X</vp> is less than <vp>Y</vp> else it evaluates to <vp>Y</vp>. <vp>Min</vp> is bound to the resulting value.}}

	<noinclude>[[Category:Language Reference Subarticle]]</noinclude>		<noinclude>[[Category:Language Reference Subarticle]]</noinclude>

Thomas Linder Puls: line

2012-10-25T15:49:57Z

line

← Older revision		Revision as of 17:49, 25 October 2012
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	<vipbnf>if <Cond> then <Term> else succeed end if</vipbnf>		<vipbnf>if <Cond> then <Term> else succeed end if</vipbnf>

	<noinclude>[[Category:Language Reference Subarticle]]</noinclude>		<noinclude>[[Category:Language Reference Subarticle]]</noinclude>

Thomas Linder Puls: Example Template

2008-11-05T09:48:46Z

Example Template

← Older revision		Revision as of 11:48, 5 November 2008
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	First <vp>Cond</vp> is evaluated, if it succeeds then <vp>T1</vp> is evaluated otherwise <vp>T2</vp> is evaluated. <vp>Cond</vp> is not allowed to leave backtrack points; that is, it cannot be <vp>multi</vp> or <vp>nondeterm</vp>. <vp>Cond</vp> is a cut-scope (see {{lang2\|Terms\|Cut_Scopes\|Cut Scopes}}).		First <vp>Cond</vp> is evaluated, if it succeeds then <vp>T1</vp> is evaluated otherwise <vp>T2</vp> is evaluated. <vp>Cond</vp> is not allowed to leave backtrack points; that is, it cannot be <vp>multi</vp> or <vp>nondeterm</vp>. <vp>Cond</vp> is a cut-scope (see {{lang2\|Terms\|Cut_Scopes\|Cut Scopes}}).

	~~'''~~Example~~'''~~		{{Example\|

	<vip>clauses		<vip>clauses
	w(X) :-		w(X) :-
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	*You can use "<vp>and</vp>" and "<vp>or</vp>" logical operators and other "complex" terms in all three sub-terms.		*You can use "<vp>and</vp>" and "<vp>or</vp>" logical operators and other "complex" terms in all three sub-terms.
	*There is no comma before the keywords "<vp>then</vp>", "<vp>elseif</vp>", "<vp>else</vp>", and "<vp>end if</vp>".		*There is no comma before the keywords "<vp>then</vp>", "<vp>elseif</vp>", "<vp>else</vp>", and "<vp>end if</vp>".
			}}

	For readability sake, we always recommend using "<vp>or</vp>" instead of "<vp>;</vp>". Likewise we also recommend using "<vp>and</vp>" (instead of "<vp>,</vp>") when it (as in the condition above) represents a logical "condition" rather than a "sequentation".		For readability sake, we always recommend using "<vp>or</vp>" instead of "<vp>;</vp>". Likewise we also recommend using "<vp>and</vp>" (instead of "<vp>,</vp>") when it (as in the condition above) represents a logical "condition" rather than a "sequentation".
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	Leaving out the <vp>else</vp>-part is just shorthand for writing that <vp>else</vp> succeed, i.e.		Leaving out the <vp>else</vp>-part is just shorthand for writing that <vp>else</vp> succeed, i.e.

	<~~vip~~>if Cond then T1 end if</~~vip~~>		<vipbnf>if <Cond> then <Term> end if</vipbnf>

	is short-hand for		is short-hand for

	<~~vip~~>if Cond then T1 else succeed end if</~~vip~~>		<vipbnf>if <Cond> then <Term> else succeed end if</vipbnf>

	<noinclude>[[Category:Language Reference Subarticle]]</noinclude>		<noinclude>[[Category:Language Reference Subarticle]]</noinclude>

Thomas Linder Puls: Categorized

2008-10-29T20:01:30Z

Categorized

← Older revision		Revision as of 22:01, 29 October 2008
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	<vip>if Cond then T1 else succeed end if</vip>		<vip>if Cond then T1 else succeed end if</vip>

			<noinclude>[[Category:Language Reference Subarticle]]</noinclude>

Thomas Linder Puls: 1 revision(s)

2008-09-15T14:22:23Z

1 revision(s)

← Older revision	Revision as of 16:22, 15 September 2008
(No difference)

Thomas Linder Puls: /* If-then-else Statement */

2008-09-15T13:14:15Z

If-then-else Statement

New page

Conditionally executes a group of statements.

<vipbnf><IfThenElseTerm>:
if <Condition> then <Term> <Elseif>-list-opt <Else>-opt end if

<Elseif>:
elseif <Condition> then <Term>

<Else>:
else <Term></vipbnf>

The following two terms are equivalents.

<vip>if Cond1 then T1 elseif Cond2 then T2 else T3 end if</vip>

<vip>if Cond1 then T1 else
if Cond2 then T2 else T3 end if
end if</vip>

Consider the schematic <vp>if then else</vp> term:

<vip>if Cond then T1 else T2 end if</vip>

First <vp>Cond</vp> is evaluated, if it succeeds then <vp>T1</vp> is evaluated otherwise <vp>T2</vp> is evaluated. <vp>Cond</vp> is not allowed to leave backtrack points; that is, it cannot be <vp>multi</vp> or <vp>nondeterm</vp>. <vp>Cond</vp> is a cut-scope (see {{lang2|Terms|Cut_Scopes|Cut Scopes}}).

'''Example'''

<vip>clauses
w(X) :-
if X div 2 = 0 and X > 3 then
write("X is good")
else
write("X is bad"),
nl
end if,
nl.</vip>

There are several things to notice about this example:

*You can use "<vp>and</vp>" and "<vp>or</vp>" logical operators and other "complex" terms in all three sub-terms.
*There is no comma before the keywords "<vp>then</vp>", "<vp>elseif</vp>", "<vp>else</vp>", and "<vp>end if</vp>".

For readability sake, we always recommend using "<vp>or</vp>" instead of "<vp>;</vp>". Likewise we also recommend using "<vp>and</vp>" (instead of "<vp>,</vp>") when it (as in the condition above) represents a logical "condition" rather than a "sequentation".

Leaving out the <vp>else</vp>-part is just shorthand for writing that <vp>else</vp> succeed, i.e.

<vip>if Cond then T1 end if</vip>

is short-hand for

<vip>if Cond then T1 else succeed end if</vip>