# Groups#

Houdini supports complex group selection expressions

## Common Patterns#

Pattern Description
* In the Group SOP: all points/primitives. In other SOPs: every named group.
n Point/primitive number n.
n-m Points/primitives numbered from n to m (inclusive).
n-m:step Points/primitives numbered from n to m (inclusive) skipping every step. For example, 1-100:2 means every other number from 1 to 100.
n-m:keep,step Points/primitives numbered from n to m (inclusive). Use the first keep numbers and then skip every step after that.
!pattern Every point/primitive except the ones matching the pattern. For example, !1-10 means every point/primitive except the numbers 1 to 10.
pattern may be a numeric pattern, attribute pattern, or group name pattern.
^pattern Remove points/primitives matching the pattern from the results of the preceding pattern. For example, 0-100:2 ^10-20 means every other number from 1 to 100 except the numbers 10 to 20.
pattern may be a numeric pattern, attribute pattern, or group name pattern.
@attributeopvalue Selects the points/primitives where attribute matches value, based on the op, which can be one of =, == (same as =), !=, >, <, >=, or <=.
For example, @v>0 will create a group of all points whose v attribute is greater than 0.
You can specify components using [index] after the attribute name, for example @Cd[2] will get the blue channel of the diffuse color (Cd) attribute. As a convenience, you can also use .x, .y, and .z instead of [0], [1], and [2], for example P.x to get the X-axis component of the position (P) attribute. (If you don’t specify a component for a multi-component attribute type, the comparison will use the first component.)
For example, @P.y>0 will create a group of all points whose Y component is greater than 0.
For string attributes you must put quotation marks around the value if it contains spaces, for example @myattr=”foo bar”.
You can use wildcards (* and ?) in the value when using =, ==, and != on string attributes.
You can combine the attribute syntax and the range syntax: @id=5-10
You can also use the attribute syntax with a space separated list of integer values, but you need to enclose the list in quotes: @id="5 8 10 15"
group_name In SOPs other than the Group SOP, you can specify a named group of points/primitives (created with the Group SOP).
You can use pattern matching (, ?, and [ ]) in the group name. For example, arm includes all point/primitive groups whose names start with arm. ^ can be used in the pattern by enclosing the pattern in { }. For example, {arm* ^arm3*} includes all groups whose names start with arm, but not arm3.

## Cookbook#

• Every Other Point: vex>*:2
• All points but the last
0-npoints(opinputpath(“.”,0))-2

• Create a group within sphere radius bounds limited to closest npoints
int maxNumPts = chi("maxNumPts");
vector4 visFilterSphereCenterRadius = chp("visFilterSphereCenterRadius");

int closept[] = nearpoints(0, set(visFilterSphereCenterRadius.x,visFilterSphereCenterRadius.y,visFilterSphereCenterRadius.z),
visFilterSphereCenterRadius.w, maxNumPts);

foreach(int ptnm; closept) {
setpointgroup(0, 'visFilter', ptnm, 1, 'set');
}


## References#

Last update: November 18, 2021
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