Problem with a "for" loop in a sensor

the schoolpos seems to add up great inside of the for loop, but once I try calling the value of the vector after the math has been preformed inside the loop, it returns me the zero vector

sensor(integer num_detected)
    {
        
        integer i;
        for (i = 0; i <= (num_detected - 1); i++)
        {
            vector schoolpos = schoolpos + llDetectedPos(i);
            //llOwnerSay((string) schoolpos + " " + (string) i + " " + (string) llDetectedPos(i));
        }
        llOwnerSay((string)schoolpos);
     }

[edit] seems that if I declare the schoolpos as a vector inside the for loop the value of schoolpos is only kept while it's in the loop if I declare it as a vector globally and get rid of the vector statement from the loop it seems to work okay. I take it this is a standard way for variables to work in programming languages?

Yes. The curly braces define the scope of a variable, which basically define its life. A global variable is one that's outside of any functions, and is therefore available to all functions.

If you had:

 sensor(integer num_detected)
    {
        
        integer i;
        vector schoolpos;
        for (i = 0; i <= (num_detected - 1); i++)
        {
            schoolpos = schoolpos + llDetectedPos(i);
            //llOwnerSay((string) schoolpos + " " + (string) i + " " + (string) llDetectedPos(i));
        }
        llOwnerSay((string)schoolpos);
     } 

This would make the scope of the variable be the sensor event handler. You could have another schoolpos in a different function or event handler and it would be a completely different variable.

If you'd typed:

vector schoolpos;

default
{
...
 sensor(integer num_detected)
    {
        
        integer i;
        for (i = 0; i <= (num_detected - 1); i++)
        {
            schoolpos = schoolpos + llDetectedPos(i);
            //llOwnerSay((string) schoolpos + " " + (string) i + " " + (string) llDetectedPos(i));
        }
        llOwnerSay((string)schoolpos);
     } 

Now it's a global, and visible to the whole script. It's interesting that you get back the zero vector. C++ wouldn't compile your original script, complaining that the variable schoolpos isn't defined for the llOwnerSay call. Which is correct - scholpos is only defined inside the for loop block, it doesn't exist outside. I guess the LSL compiler doesn't fully check for scope issues like this.

Yes, declare it outside the default{} and it's global to the script. Declare it inside a state{} and it's local to that state only...leave that state and that variable is returned to the heap.

And, btw, you can simplify your for with; i<number_detected