HowTo:Use conditional structures - Revision history

JHenderson: /* The #ifdef and #ifndef, #else and #end constructs */ - corrected typo in example

2023-07-31T18:05:56Z

The #ifdef and #ifndef, #else and #end constructs: - corrected typo in example

Wfpokorny: Moved nested loop example to new for_over_while howto. Fixed light color warning on move.

2016-11-23T13:30:51Z

Moved nested loop example to new for_over_while howto. Fixed light color warning on move.

Chrisb: /* The #switch, #case, #range and #break Directives */

2008-06-10T20:01:37Z

The #switch, #case, #range and #break Directives

Chrisb: /* The #switch, #case, #range and #break Directives */

2008-06-10T19:59:31Z

The #switch, #case, #range and #break Directives

Chrisb: Added #ifdef and #ifndef

2008-06-10T19:52:01Z

Added #ifdef and #ifndef

Chrisb: Added section on #while conditions

2008-06-10T19:48:37Z

Added section on #while conditions

Chrisb: /* The #if, #else, #end construct */

2008-06-10T17:44:17Z

The #if, #else, #end construct

Chrisb: Replaced incorrect #if assertions

2008-06-10T17:41:05Z

Replaced incorrect #if assertions

Chrisb: /* Using #switch with text values */

2008-06-10T15:32:26Z

Using #switch with text values

Chrisb: /* Using #switch with text values */

2008-06-10T15:31:40Z

Using #switch with text values

@@ Line 50: / Line 50: @@
 For example:
-<source lang=pov>
+<source lang="pov">
-#infdef(ObjectHeight) #declare ObjectHeight=1; #end
+#ifndef(ObjectHeight) #declare ObjectHeight=1; #end
 </source>

← Older revision		Revision as of 13:30, 23 November 2016
Line 196:		Line 196:
	This example places five copies of MyObject in a row spaced three units apart in the x-direction.		This example places five copies of MyObject in a row spaced three units apart in the x-direction.

−	As described above (for the #if statement), you can combine conditional terms to build more complex conditional expressions using '&' (And), '\|' (Or) and '!' (Not).	+	As described above (for the #if statement), you can combine conditional terms to build more complex looping conditional expressions using '&' (And), '\|' (Or) and '!' (Not).
−
−
−	~~===Nested Loops===~~
−	~~Loops can be nested. This capability is often used to lay out very large numbers of objects using an economical amount of code.~~
−
−	~~Here's a very simple but complete scene file containing a nested loop that you can play with:~~
−
−	~~<source lang=pov>~~
−	~~#include "colors.inc"~~
−	~~#include "woods.inc"~~
−
−	~~camera {location <-1.5,0.9,-0.08> look_at <0,0.7,0.5>}~~
−	~~light_source { <-20,75,-100> rgb White}~~
−	~~background {LightBlue}~~
−	~~plane {y,0 pigment {color GreenYellow}}~~
−
−	~~#declare RandomSeed = seed(1);~~
−	~~#declare XCount=0;~~
−	~~#while (XCount < 200)~~
−	~~#declare ZCount=0;~~
−	~~#while (ZCount < 200)~~
−	~~box {<-0.05,0,-0.05><0.05,1,0.05>~~
−	~~rotate <3rand(RandomSeed),5rand(RandomSeed),3*rand(RandomSeed)>~~
−	~~texture {T_Wood1 translate <rand(RandomSeed),ZCount,rand(RandomSeed)> rotate x*90 scale 0.07}~~
−	~~translate <XCount,0,ZCount>~~
−	}
−	~~#declare ZCount=ZCount+1;~~
−	~~#end~~
−	~~#declare XCount=XCount+1;~~
−	~~#end~~
−	~~</source>~~
−
−	This example uses 2 nested loops. Each nested loop has its own control variable. It starts with both XCount and YCount equal to zero. With XCount at zero, YCount loops from 0 to 199, then XCount is incremented to 1, YCount is reset to zero and the second loop repeats.
−
−	This example also introduces a degree of randomness in each iteration using the POV-Ray seed and rand functions. When generating a large number of objects using #while loops you'll find that a little randomness goes a long way. This example generates 40,000 wooden posts, each with a little bit of a wobble and each with a slightly different wood texture pattern. It should take about 10 or 20 seconds to render.

@@ Line 92: / Line 92: @@
 </source>
-If the parameter to a #case clause matches the parameter to the #switch clause then everything up to the next #break, #else or #end clause is evaluated. Similarly, if the parameter to the #switch clause is between the minimum and maximum values of a #range clause then everything up to the next #break, #else or #end clause is evaluated. To explore the sensitivity to the accuracy of the floating point number comparison you can set a value using 'e' as follows:
+If the parameter to a #case clause matches the parameter to the #switch clause then everything up to the next #break, #else or #end clause is evaluated. Similarly, if the parameter to the #switch clause is between the minimum and maximum values of a #range clause then everything up to the next #break, #else or #end clause is evaluated.
 <source lang="pov">

@@ Line 63: / Line 63: @@
 <li>When processing a matching #case or #range clause, if a #break is not encountered it continues through successive #case statements whether they match or not.
 </ul>
 <source lang="pov">

@@ Line 41: / Line 41: @@
 As you've probably realised, there are many ways of expressing the same thing with conditional expressions and the art is in writing it in a way that is clear enough that you will understand it when you read it through again later. It can help to write the condition out as a comment before you build it into an expression. Leaving the comment in place helps anyone reading it later.
 ==The #switch, #case, #range and #break Directives==
@@ Line 157: / Line 169: @@
 This would match 'Rhino', 'Rhinoceros' and everything in between, but not 'R', 'Rhin' or <nowiki>''</nowiki>.
 == The #while, #end construct (Loops) ==

← Older revision		Revision as of 19:48, 10 June 2008
Line 157:		Line 157:

	This would match 'Rhino', 'Rhinoceros' and everything in between, but not 'R', 'Rhin' or <nowiki>''</nowiki>.		This would match 'Rhino', 'Rhinoceros' and everything in between, but not 'R', 'Rhin' or <nowiki>''</nowiki>.
		+
		+
		+
		+	== The #while, #end construct (Loops) ==
		+	See also - [[HowTo:Use_macros_and_loops]].
		+
		+	The #while, #end construct encapsulates a sequence of POV-Ray statements that are repeatedly executed for as long as the conditional expression specified is true. The conditional expression can be simple or complex and adheres to the same rules as apply to the #if statement described above.
		+
		+	As with the #if statement, the conditional expression must evaluate to a floating point number (often an integer value). This floating point number is interpreted as a boolean value, that is to say that if the expression evaluates to '0', it is considered 'false' and if it evaluates to anything else it is considered 'true'. If the condition is true then directives between the #while statement and a corresponding #end statement are evaluated. The conditional expression is then re-evaluated and, if still true the directives between the #while statement and the corresponding #end statement are repeated.
		+
		+	The statements repeat in a loop until the condition evaluates to false. It is up to you to control the loop by defining a suitable condition that will not loop indefinitely. If the condition never evaluates to false then the program will continue to evaluate the statements forever, until you hit the stop button, until the memory of your computer is exhausted or until you terminate the application (whichever happens first) see [[Wikipedia:Infinite_loop]].
		+
		+	Note: extremely small values of about 1e-10 are considered zero (false) as a result of floating point number accuracy limits.
		+
		+	Here is an example of a simple loop:
		+
		+	<source lang=pov>
		+	#declare Count=0;
		+	#while (Count < 5)
		+	object { MyObject translate x3Count }
		+	#declare Count=Count+1;
		+	#end
		+	</source>
		+	This example places five copies of MyObject in a row spaced three units apart in the x-direction.
		+
		+	As described above (for the #if statement), you can combine conditional terms to build more complex conditional expressions using '&' (And), '\|' (Or) and '!' (Not).
		+
		+
		+	===Nested Loops===
		+	Loops can be nested. This capability is often used to lay out very large numbers of objects using an economical amount of code.
		+
		+	Here's a very simple but complete scene file containing a nested loop that you can play with:
		+
		+	<source lang=pov>
		+	#include "colors.inc"
		+	#include "woods.inc"
		+
		+	camera {location <-1.5,0.9,-0.08> look_at <0,0.7,0.5>}
		+	light_source { <-20,75,-100> rgb White}
		+	background {LightBlue}
		+	plane {y,0 pigment {color GreenYellow}}
		+
		+	#declare RandomSeed = seed(1);
		+	#declare XCount=0;
		+	#while (XCount < 200)
		+	#declare ZCount=0;
		+	#while (ZCount < 200)
		+	box {<-0.05,0,-0.05><0.05,1,0.05>
		+	rotate <3rand(RandomSeed),5rand(RandomSeed),3*rand(RandomSeed)>
		+	texture {T_Wood1 translate <rand(RandomSeed),ZCount,rand(RandomSeed)> rotate x*90 scale 0.07}
		+	translate <XCount,0,ZCount>
		+	}
		+	#declare ZCount=ZCount+1;
		+	#end
		+	#declare XCount=XCount+1;
		+	#end
		+	</source>
		+
		+	This example uses 2 nested loops. Each nested loop has its own control variable. It starts with both XCount and YCount equal to zero. With XCount at zero, YCount loops from 0 to 199, then XCount is incremented to 1, YCount is reset to zero and the second loop repeats.
		+
		+	This example also introduces a degree of randomness in each iteration using the POV-Ray seed and rand functions. When generating a large number of objects using #while loops you'll find that a little randomness goes a long way. This example generates 40,000 wooden posts, each with a little bit of a wobble and each with a slightly different wood texture pattern. It should take about 10 or 20 seconds to render.

@@ Line 11: / Line 11: @@
 <source lang=pov>
- #declare CameraDistance=25.8;
+#declare CameraDistance=25.8;
   ... Definition of some complex object that looks like a sphere at great distances. ...
- #if (CameraDistance>100)
+#if (CameraDistance>100)
     sphere { <0,0,0>, 1.2 pigment {rgb 1}}
- #else
+#else
     object {MyComplexObject}
- #end
+#end
 </source>
@@ Line 29: / Line 29: @@
 Here's an example that might take a little thinking about:
 <source lang=pov>
- #declare Clothing="Jeans";
+#declare Clothing="Jeans";
- #declare JeansPermitted=false;
+#declare JeansPermitted=false;
- // Check to see if Jeans are permitted
+// Check to see if Jeans are permitted
- #if (strcmp(Clothing,"Jeans")=0 & !JeansPermitted)
+#if (strcmp(Clothing,"Jeans")=0 & !JeansPermitted)
     #debug "Sorry. Jeans are not permitted\n"
- #end
+#end
 </source>
@@ Line 41: / Line 41: @@
 As you've probably realised, there are many ways of expressing the same thing with conditional expressions and the art is in writing it in a way that is clear enough that you will understand it when you read it through again later. It can help to write the condition out as a comment before you build it into an expression. Leaving the comment in place helps anyone reading it later.
 ==The #switch, #case, #range and #break Directives==

@@ Line 2: / Line 2: @@
-==#if==
+==The #if, #else, #end construct==
-The '''#if''' statement can only evaluate floating point expressions. There is no boolean logic using '''and''', '''or''' or '''not'''. The equivalent must be achieved using arithmetic.
+The #else directive is optional but, if specified and the conditional expression was false, then directives between the #else and the corresponding #end statement are evaluated.
-Here's an example of '''and''' where a rotation (for a texture, say) depends on both the row and column. Enclosing both the conditions in brackets isolates them and each produces either 0 or 1. Summing them will give 0, 1 or 2. The rotation will occur when both are true, producing 2.
+For example:
-<source lang="pov">
+ #if (CameraDistance>100)
-#local fRotateY = 0;
+   sphere { <0,0,0>, 1.2 pigment {rgb 1}}
-// If Row = 1 and Col = 4
+ #else
-#if ((uiRow = 1) + (uiCol = 4) = 2)
+   object {MyComplexObject}
-    // Rotates only row 1, column 4.
+ #end
 </source>
-And here's an example of '''or''' where the rotation depends on the column having either of two values. Again, enclosing both the conditions in brackets isolates them and each produces either a 0 or a 1. Summing them will give 0, 1 or 2. The rotation will occur when either or both are true and they produce 1 or 2. Note that the '''> 0''' is optional and just a matter of style.
+Here the 'greater than' comparitive operator '>' is used to see whether the value of the CameraDistance variable is greater than 100. In this example, the expression 'CameraDistance>100' evaluates to 0 ('false') because the CameraDistance variable is less than 100.
-−
-<source lang="pov">
+===Conditional Conjunctions ===
-#local fRotateY = 0;
+You can combine conditional terms to build more complex conditional expressions using '&' (And), '|' (Or) and '!' (Not). Use brackets to control the sequence of evaluation. You can also use POV-Ray functions within terms so long as they can be resolved to numeric values. The built-in float identifiers on, off, yes, no, true, and false are designed for use as boolean constants and simply evaluate to 0 (off, no and false) or 1 (on, yes and true).
-// If Row = 1 or Col = 6
-#if ((uiRow = 1) + (uiCol = 6) > 0)
+Here's an example that might take a little thinking about:
-    // Rotates all of row 1 and all of column 6
+<source lang=pov>
-    #local fRotateY = 45;
+ #declare Clothing="Jeans";
-#end
+ #declare JeansPermitted=false;
 </source>
-And here's a truly ugly looking case which uses both. This is when you start wishing that they'd introduce a boolean type. ;-)
+In this example, the strcmp function returns a zero because there is no difference between the two strings it has compared.

@@ Line 152: / Line 152: @@
 </source>
-This would match 'Rhino' or 'Rhinoceros', but you clearly need to be a little careful with this as it would also match 'Rhinoc', 'R' and ''. To set a minimum abbreviation length use the max function:
+This would match 'Rhino' or 'Rhinoceros', but you clearly need to be a little careful with this as it would also match 'Rhinoc', 'R' and <nowiki>''</nowiki>. To set a minimum abbreviation length use the max function:
 <source lang="pov">

← Older revision		Revision as of 15:31, 10 June 2008
Line 158:		Line 158:
	</source>		</source>

−	This would match 'Rhino', 'Rhinoceros' and everything in between, but not 'R', 'Rhin' or ''.	+	This would match 'Rhino', 'Rhinoceros' and everything in between, but not 'R', 'Rhin' or <nowiki>''</nowiki>.