Zbex procedures

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A procedure is a section of code which can be entered (called) from different points in a program and which, when completed, will return control to the calling point. Procedures may call other procedures, but Zbex is not designed for recursive procedure calls (that is when a procedure calls itself). The codes for procedures must come after the main program.

A procedure is identified by the procedure statement. A procedure is called using the perform statement. Return from a procedure is accomplished with the return statement. Example:

              Program 
        --------------------- 
        int i,j   
     
        loop for i = 1 to 5 
          perform square 
          putc ~i  ~j 
        repeat 
        stop 
     
        procedure square 
          j = i * i 
        return 
        run 

                     Execution 
        ------------------------------------- 
        ** S=10, P=29, L=235, M=410 ** 
        1 1 
        2 4 
        3 9 
        4 16 
        5 25 
        Ready for program 

It is possible to declare variables inside a procedure. Variables so declared can be referenced only within the procedure. If a variable of the same name was declared in the main program, it will be masked (unaccessable) inside the procedure. Example, using the variable i:

              Program 
        --------------------- 
        int i,j,k 
   
        loop for i = 1 to 5 
          j = i 
          perform factorial 
          putc ~i  ~k 
        repeat 
        stop 
    
        procedure factorial 
          int i
          k = 1 
          loop for i = 1 to j 
            k = k * i 
          repeat 
        return 
        run 

                     Execution 
        ------------------------------------- 
        ** S=15, P=41, L=239, M=410 ** 
        1 1 
        2 2 
        3 6 
        4 24 
        5 120 
        Ready for program 

It is possible to pass data to a procedure and for a procedure to pass data back to the calling point. The buffers for this must be included in the procedure statement, and the data types for the buffers must be declared inside the procedure. Values must be loaded into the buffer at the time the procedure is called. The actual transfer of data is handled by the getvalue and passback statements inside the procedure. Example:

              Program 
        --------------------- 
        int i,j 
    
        loop for i = 6 to 10 
          perform factorial (i,j) 
          putc .w2 ~i  .t4w8 ~j 
        repeat 
        stop 
    
        procedure factorial (a,b) 
          int a,b,i 
          getvalue a 
          b = 1 
          loop for i = 1 to a 
            b = b * i 
          repeat 
          passback b 
        return 
        run 

                     Execution 
        ------------------------------------- 
        ** S=16, P=53, L=241, M=410 ** 
         6      720 
         7     5040 
         8    40320 
         9   362880 
        10  3628800 
        Ready for program 

There are times when you will want to return directly to a point in the main program, irrespective of how many levels of procedure calls you have made. An example of this would be a language compiler. If you are processing a line of text and a procedure within a procedure within a procedure identifies a syntax error, you would not want to trace your way back to the main program through these procedures; instead, you would want to jump directly to the error handling portion of the program with a flag telling that portion what went wrong. Zbex provides you a way to do this using the special trp variable and the special trap label. If you include an integer number with a return statement, the record of procedure calls will be erased and control will pass directly to the trap label in the main program. The special integer variable trp will be assigned the value attached to the return statement. Example:

              Program 
        --------------------- 
           int i 
     
           trp = 0 
        trap: 
           i = 0 
           perform pro1 
           stop 
     
           procedure pro1 
             putc pro1  ...  
             i = i + 1 
             if i > trp 
               putc 
               return 1 
             end 
             perform pro2 
             putc returning from pro1 
           return 
    
           procedure pro2 
             putc pro2  ...  
             i = i + 1 
             if i > trp 
               putc 
               return 2 
             end 
             perform pro3 
             putc returning from pro2 
           return 
     
           procedure pro3 
             putc pro3 
             putc returning from pro3 
           return 
           run 

                     Execution 
        ------------------------------------- 
        ** S=31, P=83, L=233, M=410 ** 
        pro1 
        pro1  pro2 
        pro1  pro2  pro3 
        returning from pro3 
        returning from pro2 
        returning from pro1 
        Ready for program 

There are only two ways to interrupt an Zbex program once it has been started. One of these is by pressing Ctrl+Break (the other is by responding to a getc with two exclaimation points (!!)). On occasion you may want Ctrl+Break to actually do something in your Zbex program. For example, if your program is writing to the screen, and you want it to stop, you must push Ctrl+Break. In some cases, you may not want your Zbex program to halt, you may simply want to get control of it. Zbex provides you a means for doing this. It is a special procedure called break. If you include a procedure with this name, control will automatically pass to it when you press Ctrl+Break. A normal return from this procedure will put you back into your Zbex program exactly at the point where Ctrl+Break was pressed. Example:

              Program 
        --------------------- 
        int i,j 
     
        loop for i = 1 to 100000 
          loop for j = 1 to 300000    /* (for a delay) 
          repeat 
          putc ~i 
        repeat 
        stop 
    
        procedure break 
          getc 
        return 
        run 

                     Execution 
        ------------------------------------- 
        ** S=11, P=26, L=235, M=410 ** 
        1 
        2 
        3 
                (here, Ctrl+Break was pressed and a blank line entered) 
        4 
        5 
        !!      (here, Ctrl+Break was pressed and !! was entered) 
        Ready for program 

There is an important precaution you must observe with the procedure break. You need to provide some mechanism for terminating the program inside the procedure, or else you may not be able to terminate your program at all (except by exiting Dmuse)! The program below, once started, cannot be stopped, except by exiting Dmuse.

        int i,j 
     
        loop 
          i = i + 1 
          putc ~i 
        repeat 
        stop 

        procedure break 
        return 
        run

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