Syntax Query
Question 1
Is it acceptable to omit the curly braces after if and for statements if
there is only one child object?
Question 2
Should union() always be specified or is it OK to rely on implicit union
operations?
// example 1
if(true) cube(20);
// example 2
difference(){
translate([0,0,0]) {cube(30,center=true);cube(30);}
sphere(r=18);
// example 3 - combining 1 & 2
if(true) translate([10,0,0]) {cube(30,center=true);cube(30);}
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Q1. Yes. Note it is often good to use round braces in the if expression, as
you have. e.g.
if (thisBooleanVariable) cube(); // in this exact case without () you get an
error
else sphere();
Q2.
implicit union() is only on the top level of the main program or a module
AFAIK.
using {} without a union(), in your ex. 2&3, can be problematic if it
contains geometry.
I think {} turns into group{} (not documented), I find it is inconsistent.
Don't use it.
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Q1) round braces around the "if" expression are not just a good idea, they
are mandatory! :)
Q2) a statement such as translate () {...} used to create a group is fine,
but will perform implicit union. I normally use group() in that sort of
case, but group() and union() are equivalent.
"{A; B;}" does not become "group() {A;B;}" , it becomes "A; B;". Probably
not what is desired.
viz
difference ()
{
{ cube(); sphere(); }
cube();
}
=> CSG Tree
group() {
difference() {
cube(size = [1, 1, 1], center = false);
sphere($fn = 0, $fa = 12, $fs = 2, r = 1);
cube(size = [1, 1, 1], center = false);
}
}
There is another quirk in the syntax I noticed:
cube () { cube(); }
compiles and renders without error/warning, but the second cube is ignored.
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Bob,
Please do not confuse a beginner. You did not respond to his Q1.
He asked about the “{}” around a single statement after an if() definition,
and you responded about the “()” of the test itself.
Just my humble $0.02,
Jean-Paul
AC9GH
On Aug 6, 2015, at 8:28 AM, bobc bobcousins42@googlemail.com wrote:
Q1) round braces around the "if" expression are not just a good idea, they
are mandatory! :)
Q2) a statement such as translate () {...} used to create a group is fine,
but will perform implicit union. I normally use group() in that sort of
case, but group() and union() are equivalent.
"{A; B;}" does not become "group() {A;B;}" , it becomes "A; B;". Probably
not what is desired.
viz
difference ()
{
{ cube(); sphere(); }
cube();
}
=> CSG Tree
group() {
difference() {
cube(size = [1, 1, 1], center = false);
sphere($fn = 0, $fa = 12, $fs = 2, r = 1);
cube(size = [1, 1, 1], center = false);
}
}
There is another quirk in the syntax I noticed:
cube () { cube(); }
compiles and renders without error/warning, but the second cube is ignored.
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Sorry, yes you are right! I was responding to Michael's comment, he confused
me talking about round braces.
For the OP, I believe that
if (1) cube ();
if (1) { cube (); }
if (1) group () { cube(); }
if (1) union () { cube(); }
are all identical in function.
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OK thanks, in summary instead of writing:
if() {translate() union{cube();cube();}}
it can simply be written as:
if() translate() {cube();cube();}
I hope that the above is correct...
The only item I am now unsure of is group(), which does not appear to be
documented, but OpenSCAD clearly recognises. Is it a deprecated boolean
operator?
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Trygon wrote
The only item I am now unsure of is group(), which does not appear to be
documented, but OpenSCAD clearly recognises. Is it a deprecated boolean
operator?
Go to menu [View][Show CSG tree], where openscad display how shapes are
organized internally, you'll see lots of group() but no union(). Obviously
it's an internal thing.
$ Runsun Pan, PhD
$ -- libs: doctest , faces ( git ), offliner ( git );
tips: hash( 1 , 2 ), sweep , var
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bobc wrote
Sorry, yes you are right! I was responding to Michael's comment, he
confused me talking about round braces.
For the OP, I believe that
if (1) cube ();
if (1) { cube (); }
if (1) group () { cube(); }
if (1) union () { cube(); }
are all identical in function.
The reason I said to avoid plain { } is the code below (cut down example),
with the commented out union() does not produce the same geometry as with
the uncommented union(). Both for F5 & F6. I came across this yesterday
morning, so it was fresh in my mind.
intersection() {
cube();
//union()
{
sphere();
cylinder(r=0.5,h=5,true);
}
}
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Yes, that seems like a bug, if not an unexpected feature. the "{..}" gets
flattened in the CSG tree:
group() {
intersection() {
cube(size = [1, 1, 1], center = false);
sphere($fn = 0, $fa = 12, $fs = 2, r = 1);
cylinder($fn = 0, $fa = 12, $fs = 2, h = 1, r1 = 0.5, r2 = 0.5, center
= false);
}
}
There is no implicit union generated in this case, unlike "if () {...}".
I'm not sure whether this is an unintended side-effect of the syntax, or if
it is needed in some other case?
I don't recall seeing an issue for it, I'm not sure what keywords to search
for. I've probably stumbled across this a few times myself without
realising.
On 6 August 2015 at 22:15, MichaelAtOz oz.at.michael@gmail.com wrote:
bobc wrote
Sorry, yes you are right! I was responding to Michael's comment, he
confused me talking about round braces.
For the OP, I believe that
if (1) cube ();
if (1) { cube (); }
if (1) group () { cube(); }
if (1) union () { cube(); }
are all identical in function.
The reason I said to avoid plain { } is the code below (cut down example),
with the commented out union() does not produce the same geometry as with
the uncommented union(). Both for F5 & F6. I came across this yesterday
morning, so it was fresh in my mind.
intersection() {
cube();
//union()
{
sphere();
cylinder(r=0.5,h=5,true);
}
}
Unless specifically shown otherwise above, my contribution is in the
Public Domain; To the extent possible under law, I have waived all
copyright and related or neighbouring rights to this work. This work is
published globally via the internet. :) Inclusion of works of previous
authors is not included in the above.
The TPP is no simple “trade agreement.” Fight it!
http://www.ourfairdeal.org/
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http://forum.openscad.org/Syntax-Query-tp13457p13465.html
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Given group() and the use of plain {} is undocumented, I just steer clear
most of the time.
I just use {} to group related variables so they can be collapsed in the
editor.
Unless specifically shown otherwise above, my contribution is in the Public Domain; To the extent possible under law, I have waived all copyright and related or neighbouring rights to this work. This work is published globally via the internet. :) Inclusion of works of previous authors is not included in the above.
The TPP is no simple “trade agreement.” Fight it! http://www.ourfairdeal.org/
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