Problems with expressions

[omkar-jere]

global proc myCustomProc(string $ctrl) {
   expression -s "$ctrl = 1.5";

}

In the above example code I want to write an expression in which I need to specify an object(for e.g pCube1.tx) and $ctrl is a user input then how do I get that input into the expression ? I tried by directly putting $ctrl in the expression but it won't work. Any solutions ?

[ldunham1]

try:

eval("expression -s \""+$ctrl+" = 1.5\"");

you want maya to evaluate the value of the variables before they're written into the expression, eval will help with that.

you stipulate what the variables are, then their results can be written, rather than actually writing in the raw name of the variable itself.

[joojaa]

Please do not use eval, its like going duck hunting with a shoulder launch3ed heat seeking missile, not only is it expensive but inefficient. Bascally the real question is how do i concatenate strings, andf how do i make a sub expression:

expression -s ($ctrl+" = 1.5");

answer you concatenate strings with the + operator, and you use parenthesis to group them into one operator. 100 times faster than eval and 10 times safer. But basically its a VERY bad habit to use eval as theres only 2 cases where you MUST use eval and 1 case where you should consider it.

[ldunham1]

sir i tip my hat again, silly mistake on my part, especially when such a simple alternative was apparent.

 

[omkar-jere]

Please do not use eval, its like going duck hunting with a shoulder launch3ed heat seeking missile, not only is it expensive but inefficient. Bascally the real question is how do i concatenate strings, andf how do i make a sub expression:

expression -s ($ctrl+" = 1.5");

answer you concatenate strings with the + operator, and you use parenthesis to group them into one operator. 100 times faster than eval and 10 times safer. But basically its a VERY bad habit to use eval as theres only 2 cases where you MUST use eval and 1 case where you should consider it.

Pure Genius !!!!!

One more query the expession I want to write can also be done by connecting 5-6 utility nodes so which one would be faster evaluating 5-6 nodes or just 1 expression node ?

[joojaa]

Lol no really dont use eval, unless your doing one of the following then you dont need it:

1. need to defer evaluation to a later stage (command in user setup mostly, some deletion), but mainly also defer source directive to run on the line where it is.
2. Need to do some introspection, should be rare to need this but anyway
3. dynamic dispatch

[joojaa]

So syntax is now genius

One more query the expession I want to write can also be done by connecting
5-6 utility nodes so which one would be faster evaluating 5-6 nodes or just 1 expression node ?

Depends on the situation. Certainly if your script uses -alwaysEvaluate (default dont ask me why) then yes the utilities are likely to be faster, but theres a lot fo different things that get involved, it depends on many variables. Cartainly a buch of nodes is FAST, but the expression can be equally fast or sometimes faster depends on the algorithm used.

[omkar-jere]

The expression I want to write doesn't contain more than 2-3 variables and also 99.9 % of the time I use "On Demand" isntead of "Always Evaluate" when it comes to expressions So in this case I think I will stick with expressions.

[joojaa]

Yes but the speed does not come from the demand thats just a stupid penalty on top. Its comes form the fact that expressions evaluate as a whole and nodes dont, so if only one of the variables changes that the node connection with the least affecting variable change is definitely faster.

BUT! this is sort of a premature optimization