Isset

Is there some fuction I can use to check if there's a variable set.

eg.

vector lavender = <0.2,0.2,0.4>;

if(isset("lavender"){
  return TRUE;
}

No, Tempus, LSL variables always have a value - they are never "unset".
What, specifically, are you trying to accomplish?

vector blah=ZERO_VECTOR;
...
if(blah!=ZERO_VECTOR) return TRUE;

Unless <0,0,0> is a valid vector in your application domain *shrug*

Tempus,
When you define a variable but don't give it a value, it has a "null value" by default.

String Null = ""
Integer Null = FALSE or 0
Float Null = FALSE or 0 (... I think)
Vector Null = ZERO_VECTOR
Rotation Null = ZERO_ROTATION
Key Null = NULL_KEY

Thus, if you've defined a vector called "lavender" and want to see if it's been given a value other than its null value, you could do:

if (lavender != ZERO_VECTOR) {
    llOwnerSay("Has a value.")
} else {
    llOwnerSay("Does not have a value.");
}

No. If you use a variable that hasn't been declared you get a compile error. If you display a variable that hasn't been assigned a value, you get the default value. For a vector, it's <0.0,0.0,0.0>. There's no way to unset a variable.

I want to compare predefined vector values set in the script but also being able to set userdefined vector values

so if the user type in my case /33 color lavender
the script look for this predefined variable based upon the parsed string command value the user entered "lavender" and set the variable lavender to the current vector value instead of putting 20

if(input_color == "lavedner"{
llSetColor
}
if(input_color = "red"{
llSetColor
}

At Harris hare, think that will work actually the end example

however, what I want to do is, take the string which the user entered eg. lavender call the predefined variable and it's vector value and set it as the vector value I want to recolor with.

I could just make two arrays, one containing the names and one containing the values and search index of the names and use this index to fetch the value also

Tempus,
Scripts cannot create a new variables after they've been compiled. What you need to do is use lists. That way you can have some predefined colors and still add new ones to the list while the script is running.

list colorNames = ["red","green","blue"];
list colorValues = [<1,0,0>,<0,1,0>,<0,0,1>"];

When someone wants to change to color to red, you use llListFindList to find out the index of the name "red" in the colorNames list and use that same index to find out what vector the color red is in the colorValues list.

If you want the user to add new colors, you need to have that be a different command. Instead of "/33 color red" you might make it "/33 newcolor black <0,0,0>" and then add the new name to the end of the colorNames list and the new value to the colorValues list.

That's a great idea. My intention was not to create new colors no, just to have a list of variables looking like this:

vector red = <1,0,0>
vector blue = <0,0,1>

etc.. but instead of using

if(color == "blue"{
llSetColor()
}
if color == "red"{
llSetColor()
}

etc..

I could check if the variable was predefined, else I could give an error message... but the newcolor idea, it's a good one.

Yes, you'll have to use lists:

list colorNames = [
"red",
"blue",
"green",
"lavender",
...
];

list colorVectors = [
<1.0, 0.0, 0.0>,
<0.0, 0.0, 1.0>,
...
];

Then your listen handler can be pretty smart. For instance, if someone just says /33 color lavender, you just need to search the names list to see if the color exists, and if it does, go to the vectors list, use the index you found in the names list, and there's your color. If the user types /33 color lavender <0.25, ...>, you can see if the named color exists, if it does, update the associated vector, if not, add a new color name and vector. Obviously, you'll have to take care that the two lists are always synchronized, so you can use the index in one list as the index in the other list. Alternatively, you could set up a '2-dimentional array' with a list too:

list colors = ["red", <1.0, 0.0, 0.0>, ...];

Then you search for the name, and the vector is the next position. Now you need to make sure you update the correct vector if someone redefines a color, add a color, vector pair if someone adds a new color, and so on.

You can read up on these Strided Lists here, Tempus.