Altered version of function to validate numeric input with bugs fixed

Mac found some more bugs in the above code. Numbers starting with 0 or with -0 are flagged as non-numbers. Such strings entered as negative real numbers or as positive serial numbers should be valid. Also, decimal numbers ending with 0 should also be valid.

I added extra code in the function between the IF UCASE$ line and the stnum$ = LTRIM$ line. I also changed the arguments in the main program to pass by value since snum$ is changed by the function.

The altered code may now be longer than the original version, but here it is:

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DECLARE FUNCTION IsNumber% (snum$, real)
DIM snum AS STRING
CLS : INPUT "Enter a real number:  ", snum$
IF IsNumber%((snum$), 1) THEN
    PRINT snum$; " is a real number."
    PRINT "The numeric value is:  "; VAL(snum$)
ELSE
    PRINT snum$; " is NOT a real number."
END IF
PRINT : INPUT "Enter a positive integer:  ", snum$
IF IsNumber%((snum$), 0) THEN
    PRINT snum$; " is a number."
    PRINT "The numeric value is: "; VAL(snum$)
ELSE
    PRINT snum$; " is NOT a positive integer."
END IF
END

FUNCTION IsNumber% (snum$, real)
CONST FALSE = 0, TRUE = NOT FALSE
DIM num AS DOUBLE, stnum AS STRING
DIM x AS INTEGER, s AS INTEGER, nnum AS STRING
IF UCASE$(snum$) <> LCASE$(snum$) THEN EXIT FUNCTION
IF LEFT$(snum$, 1) = "-" THEN
    snum$ = MID$(snum$, 2)
    s = 1
END IF
N = LEN(snum$)
nnum$ = snum$
FOR x = 1 TO N
    IF MID$(snum$, x, 1) = "0" THEN
        nnum$ = MID$(snum$, x + 1)
    ELSE
        EXIT FOR
    END IF
NEXT x
snum$ = nnum$
IF s = 1 THEN snum$ = "-" + snum$
IF real THEN
    num = VAL(snum$)
    N = LEN(snum$)
    FOR x = N TO 1 STEP -1
        IF RIGHT$(nnum$, 1) = "0" THEN
            nnum$ = MID$(snum$, 1, x - 1)
        ELSE
            EXIT FOR
        END IF
    NEXT x
    snum$ = nnum$
ELSE
    num = INT(ABS(VAL(snum$)))
END IF
stnum$ = LTRIM$(STR$(num))
IF snum$ = stnum$ THEN
    IsNumber = TRUE
ELSE
    IsNumber = FALSE
END IF
END FUNCTION