ObeidaZakzak
Hello,

Using the Solver SOP is of course the way to do what you are looking for, but the Attribute Wrangle inside is missing something to make it work as intended.

If you want to keep the previously written value, then you have to store it in a variable and return it when the "if" statement condition is not satisfied.

// get current pos attrib value
float pos = detail(0, "pos");

// initialize offset value to add to pos attrib
// when "if" statement is not satisfied then offset is 0 and will not affect pos attrib value
float offset = 0;

// sin function values are always between -1 and 1
// enter "if" statement only when sin value is between 0 and 1
float signal = sin(@Time * chf('speed'));
if (signal >= 0) {
    offset = chf('units') * signal;
}

setdetailattrib(0, "pos", pos + offset, "set");

Not sure why you need velocity from another geometry, you can find an example with velocity as well in the hip file.

animatrix_
preserve history over time

ObeidaZakzak
Hello,

A parameter expression should be used to set that parameter's value. You are attempting to read this parameter's vlaue in an expression that's is meant to set it, so no wonder that it's finishing with an infinit loop.

Take a look at parameter callbacks instead as they sound to fit your need of triggering an event when clicking on the toggle parm.

viklc
Hi, an expression always has a return value, since you feed the same parm with its value you will get an infinite loop.

If you have more than one line of code, you need a return statement, and you don't need to import hou inside expressions, that's done by default. You can simply write pwd().parm('...')...