Syntax CHECK% (integer$)
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


CHECKF does the same as CHECK% but converts strings containing floating point numbers.

WHEN ERRor can trap the coercion failure.

See the Coercion Appendix also.