|Location||QL ROM (post JM), THOR XVI, Not SMSQ/E|
WHEN is used to identify the start of a SuperBASIC structure which is used to surround lines of SuperBASIC code which should be executed whenever the given condition is met. The condition is not checked when a variable is READ, or INPUT.
The syntax of the SuperBASIC structure can take two forms:
WHEN condition *[statements]* .. END WHEN
The condition can be anything which is accepted by the IF command, provided that it begins with the name of a variable (for example, WHEN a-10=b is acceptable, but WHEN 10-a=b is not). The variable cannot be an array.
When a program is run, the interpreter will make a note of the variable being tested and then jump to the statement following the END WHEN statement (unless the in-line format is used when control jumps to the next line if END WHEN does not appear on that line). Great care must however, be taken where the condition refers to more than one variable, as an ‘error in expression’ will be reported if a variable is not defined when the condition is tested, for example, the following stops with ‘error in expression’ at line 4:
4 WHEN x>1 AND y>1 5 x=x+1:PRINT 'hello' 6 END WHEN 7 PRINT 'Start' 8 : 100 FOR x=1 TO 2 110 FOR y=1 TO 2 120 PRINT x,y;' '; 130 END FOR y 140 END FOR x
This is because when line 100 is processed, the interpreter jumps to the WHEN clause. At this stage, y is undefined, hence the error. The program will work if you add the line:
Although blocks can be specified which check for various conditions of the same variable, if the conditions overlap, there is no guarantee as to which WHEN statement will be executed first. Blocks cannot be mixed together. In the following example, although if a$=’me’ the messages ‘hello’ and ‘who’ will be printed, and if a=2 the only message which will be printed is ‘A is 2’ - when the program is RUN, the first END WHEN command is matched with line 1, thus the message ‘who’ is also printed when the program is run (it is extremely bad programming practice in any event to mix program structures of this sort).
1 WHEN a$='me' 2 PRINT 'hello' 3 WHEN a=2 4 PRINT 'A is 2' 5 END WHEN 6 PRINT 'Who' 7 END WHEN
WHEN processing is turned off by the command WHEN anything, and also when the NEW, CLEAR, LOAD, LRUN, MERGE, and MRUN commands are issued. You can also switch off WHEN processing on a given variable (eg. b) by the command WHEN b (in the following example).
110 WHEN a>100 AND a<1000: PRINT 'A is now in the range 100-1000': a=a+100 120 WHEN b=a 130 PRINT 'B is now the same as A ': PRINT B,A: A=A+50 140 END WHEN 150 WHEN b MOD 100=0: b=b+200 155 : 160 LET a=100: b=a 170 a=10 180 REPeat Loop 190 a=a+1: b=b-1 200 AT 0,0: PRINT 'A='!a\\'B='!b 210 END REPeat Loop
This command does not work reliably on any QL versions other than Minerva v1.77 or later: although Toolkit II improves the reliability, problems include calling the block more than once, and reporting ‘bad name’ when the block is called. WHEN clauses will also remain in force despite NEW, CLEAR, LRUN, LOAD, MERGE and MRUN, unless Toolkit II is present.
A WHEN clause will not be called if it is already active, even though the program may have jumped out of the actual WHEN clause. For example:
100 WHEN a=100: PRINT 'A=100': GOTO 400 115 : 110 a=10 120 REPeat loop 130 a=a+10: PRINT a 140 END REPeat loop 150 STOP 160 : 400 FOR a=10 TO 200 STEP 30 410 PRINT a 420 END FOR a
On JS MG and THOR XVI ROMs, a maximum of 20 WHEN clauses can be active at any time.
END WHEN defines the end of a WHEN XXX structure.