Statements and Control Structures

The following grammar shows the basic building blocks of hoc programs

stmt:   expr ;
     |  select NAME;
     |  access NAME;
     |  generate NAME;
     |  set_priority NUMBER;
     |  dim NAME indexlist ;
     |  variable = expr ;
     |  procedure ( arglist ) ;
     |  for( expr ; expr ; expr ) ;
     |  while ( expr ) stmt ;
     |  break;
     |  continue;
     |  if ( expr ) stmt ;
     |  if ( expr ) stmt; else stmt ;
     |  return optional-expr ;
     |  { stmtlist }

stmtlist: /* empty */
     | stmtlist stmt

indexlist: /* empty */
     | indexlist index

index: [ expr ]

Syntax and semantic of these control-structures are just like in C.

The select statement has the special meaning discussed in the previous chapter. It tells the YODA when to call this task, and prepares the availability of the online data via the variable (or array, depending on the selector database) NAME.

The access statement is similar to the select statement in that it makes data available as an array. However, it does not decide wether this task is called or not.

Quite special is the generate statement. Here a subevent (specified by its NAME in the selector database) is declared to be generated by the callback. An array NAME and a variable NAME_LENGTH holding the dimension given in the database are automatically created. The user then can change the contents of the array and the length of the subevent which is actually written to the eventstream after the analyze procedure has been called.

Closely related to this feature is the command set_priority which allows to change the default priority of 50 to any value in the range 1-99. A lower number means that the callback is called earlier. This allows to create subevents by one callback whcih can then be selected or accessed by callbacks (soft or hard) of higher priority. The priority of a soft callback cannot be changed via a task reload. The task must be unloaded and then loaded again in order to make the new priority active.

Variables need not to be declared. All variables are of type double and are global (i.e. every function or procedure of the same task can access the variable for reading and writing). To define simple arrays (of type double, as all variables are), use the dim statement. It defines the following NAME to be an array with the dimension expr. Note that arrays are indexed just as in C, starting with zero with the last valid index is dimension-1. Note also that whole arrays cannot be passed to procedures or functions.

YODA also supports multidimensional arrays. They are declared like dim x[10][5].