=====Simulating RESUME and RESUME NEXT=====

//by Richard Russell, September 2007//\\ \\  BBC BASIC doesn't have the **RESUME** and **RESUME NEXT** statements sometimes found in other dialects of BASIC. Although there are alternative (and probably better) ways of achieving a similar effect when writing a program from scratch, their absence can be inconvenient when translating programs from another dialect. This article describes how a very similar effect can be achieved in //BBC BASIC for Windows// (version 5.60a or later).\\ \\  Firstly let's review what RESUME and RESUME NEXT do. They are intended for use in an error handler (i.e. a routine activated by **ON ERROR**) and cause execution to continue at the statement that resulted in the error (in the case of **RESUME**) or the statement following the one that resulted in the error (in the case of **RESUME NEXT**).\\ \\  Here is a simple **GW-BASIC** or **QBASIC** program which illustrates the use of RESUME (//don't// try to run it in **BBC BASIC**!):\\ \\ 
<code qbasic>
        PRINT "Program starting"
        ON ERROR GOTO 1000
        Divisor = 0
        PRINT "Attempting division..."
        PRINT 10 / Divisor
        PRINT "Program finished"
        END

  1000  PRINT "Error: "; ERR
        Divisor = 3
        RESUME
</code>
When run the program generates the following output:\\ \\ 
  Program starting
  Attempting division...
  Error:  11
   3.333333
  Program finished
Note that an error occurs (division by zero) because **Divisor** is initially zero, but in the error handler **Divisor** is changed to 3 and the division is repeated, this time successfully.\\ \\  Here is the same program in //BBC BASIC for Windows// using some sneaky code to reproduce the effect of **RESUME**:\\ \\ 
<code bb4w>
        PRINT "Program starting"
        ON ERROR GOTO 1000
        Divisor = 0
        PRINT "Attempting division..."
        PRINT 10 / Divisor
        PRINT "Program finished"
        END

  1000  PRINT "Error: "; ERR
        Divisor = 3
        REM RESUME
        P% = PAGE
        WHILE ?P% AND P% + ?P% < !408 P% += ?P% : ENDWHILE
        GOTO (P%)
</code>
This code produces the following output:\\ \\ 
  Program starting
  Attempting division...
  Error: 18
  3.33333333
  Program finished
Note that it has correctly re-executed the division statement but //not// the preceding "PRINT" statements.\\ \\  If we change the **QBASIC** program to use **RESUME NEXT** instead of **RESUME** this is the output we get:\\ \\ 
  Program starting
  Attempting division...
  Error:  11
  Program finished
Here the division isn't repeated, and the RESUME NEXT transfers control to the "PRINT "Program finished"" line.\\ \\  Here is the equivalent program in //BBC BASIC for Windows//:\\ \\ 
<code bb4w>
        PRINT "Program starting"
        ON ERROR GOTO 1000
        Divisor = 0
        PRINT "Attempting division..."
        PRINT 10 / Divisor
        PRINT "Program finished"
        END

  1000  PRINT "Error: "; ERR
        Divisor = 3
        REM RESUME NEXT
        P% = PAGE
        REPEAT P% += ?P% : UNTIL ?P% = 0 OR P% > !408
        GOTO (P%)
</code>
And here is the output:\\ \\ 
  Program starting
  Attempting division...
  Error: 18
  Program finished
There are a couple of respects in which the code replacements shown here don't work exactly the same as the traditional RESUME and RESUME NEXT statements:\\ \\ 

  - The BBC BASIC code transfers control to the **line** which caused the error or the following **line**, //not// to the statement causing the error and the following statement. Therefore it may not behave as required if there is more than one statement on a line.
  - If the error trapping occurs in a function, procedure or loop, **ON ERROR LOCAL** must be used to avoid the stack being cleared down.
\\  The code listed in this article requires //BBC BASIC for Windows// version 5.60a or later, and relies on undocumented behaviour which is not guaranteed to remain compatible in future versions. However it does work correctly with version 5.70a.