|Location||CONTROL (DIY Toolkit Vol E)|
Coercion is the process of converting a string which holds a number into the actual number. It is a powerful in-built feature of SuperBASIC. This allows you to create input routines such as:
100 dage% = RND(10 TO 110) 110 INPUT "Your age [" & dage% & "]?" ! age$; 120 IF age$ = "" THEN 130 age% = dage%: PRINT age% 140 ELSE 150 age% = age$: PRINT 160 END IF
Although SuperBASIC coercion is very powerful, it does have its limits when non-numeric strings are entered. If age$ was “44”, age%=age$ will assign 44 to age%. Even if the string was not really a number, eg. “44x5”, SuperBASIC will simply ignore everything behind legal characters (ie. age%=age$ would assign 44 to age% still). However, if age$ contained something like “no thanks” it cannot be coerced and the program will fail with an ‘error in expression’ (-17).
The function CHECK% exploits the fact that SuperBASIC is obviously able to see the difference between a valid number or what comes close to that and nonsense. CHECK% carries out an explicit coercion for integer numbers: it will try to make a number from the supplied parameter in the same way as SuperBASIC would. However, CHECK% will not stop with an error for unusable strings, instead it returns -32768.
Although “-32768” is converted correctly to -32768, this value must be reserved because the program cannot know whether the input was illegal or -32768.
Let’s rewrite the above example for coercion with CHECK%. We have to replace the implicit coercion age%=age$ with age%=CHECK%(age$) and put INPUT into a loop:
100 dage% = RND(10 TO 110) 110 REPeat asking 120 INPUT "Your age [" & dage% & "]?" ! age$; 130 IF age$ = "" THEN 140 age% = dage%: PRINT age% 150 ELSE 160 age% = CHECK%(age$): PRINT 170 IF age% > -32768 THEN EXIT asking 180 END IF 190 END REPeat asking
WHEN ERRor can trap the coercion failure.
See the Coercion Appendix also.