Using local variables

A variable that you declare inside an action is a local variable. You must declare a local variable (specifying its type) and initialize that variable before you can use it.

Although the correlator automatically initializes global variables that were not explicitly assigned a value, the correlator does not do this for local variables. For local variables, you must explicitly assign a value before you can use the variable.

If you try to inject an EPL file that declares a local variable and you have not initialized the value of that local variable before you try to use it, the correlator terminates injection of that file and generates a message such as the following: Local variable ‘var2’ might not have been initialized. EPL requires explicit assignment of values to local variables as a way of achieving the best performance.

When you declare a variable in an action, you can use that variable only in that action. You can declare a variable anywhere in an action, but you can use it only after you declare it and initialize it.

action anAction(integer a) returns integer {
integer i;
integer j;
i := 10;
j := a;
return j + i;
}

You can use the local action variables, i and j in the action, anAction(), after you initialize them. The following generates an error:

action anAction2(integer a) returns integer {
i := 10; // error, reference to undeclared variable i
j := a; // error, reference to undeclared variable j
integer i;
integer j;
i := 2;
j := 5;
return j + i;
}

Suppose that an action scope variable has the same name as a monitor scope variable. Within that action, after declaration of the action scope variable, any references to the variable resolve to the action scope variable. In other words, a local action variable always hides a global variable of the same name.

Consider again the definition for anAction2() in the previous code fragment, but with i and j variables declared in the monitor scope. The first use of i and j resolves successfully to the values of the i and j monitor scope variables. The second use occurs after the local declaration and initialization of i and j. That use resolves to the local (within the action) occurrence. This results in the following values:

Since you must explicitly initialize local variables before you can use them, the following example is invalid because j and i are not initialized to any value before they are used.

action anAction3(integer a) returns integer {
integer i;
integer j;
return j + i; // error, i and j were not initialised
}

It is possible to initialize a variable on the same line as its declaration, as follows:

action anAction4(integer a) returns integer {
integer i := 10;
integer j := a;
return j + i;
}

It is also possible to initialize a local variable by coassigning to it in an event listener. For example, the following is correct:

You can also initialize a local variable by coassigning to it from a stream. For example:

action onload() {
   float f;
   from x in all X() select x.f : f {
      log f.toString();
   }
}