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Q69162: VAL Function Returns "Type Mismatch" for Some Invalid Numbers

Article: Q69162
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BASICCOM SR# S910121-320 | mspl13_basic
Last Modified: 14-FEB-1991

The VAL function may return the error message "Type mismatch" in
specific cases if the string value is not a valid number. In most
other cases, VAL normally returns 0 (zero) for invalid numbers.

This information applies to Microsoft QuickBASIC 4.00, 4.00b, and
4.50, to Microsoft BASIC Compiler 6.00 and 6.00b, and to Microsoft
BASIC Professional Development System (PDS) 7.00 and 7.10.

Microsoft is researching this problem and will post new information
here as it becomes available.

The VAL function returns the numeric value of a string expression that
represents a number. VAL interprets the sequence of characters until a
character is found that cannot be recognized as part of a number. If
the string is not a valid number (such as a string of alphabetic
characters), the VAL function returns zero. However, in certain cases,
a string of alphabetic characters will return the "Type mismatch"
error.

The problem often occurs with a string expression that starts out with
a valid numeric expression, such as "&" (for example, &H for
hexadecimal notation), "-" (for example, a negative number), or "+"
(for example, a positive number), but follows that with an invalid
character. When parsing such a string, the VAL function will encounter
an invalid numeric format and give "Type mismatch" when it should
return zero.

Examples
--------

PRINT VAL("12.32")            '12.32 prints correctly
PRINT VAL("s%k&r3ds")         '0 prints correctly
PRINT VAL("&+%we3")        'Gives "Type mismatch", but should be 0

The string expressions listed below generate the "Type mismatch" error
when passed to the VAL function. This is not a complete list.

Each line in the table below shows combinations of characters, which
are grouped by the following rules:

   [] = Encloses symbol required to pass to VAL
   |  = Logical or (separates choices in a list of characters)
   char = CHR$(32) through CHR$(126)
   real = A valid number which contains a decimal point
   NULL = A byte value equal to 0 (zero)
   integer = A valid number with no decimal point

List of known string of symbols:

[ & | D | E | NULL ] +% [ NULL | char ]
[ & | D | E | NULL ] +& [ NULL | char ]
[ & | D | E | NULL ] +@ [ NULL | char ]
[ & | D | E | NULL ] -% [ NULL | char ]
[ & | D | E | NULL ] -& [ NULL | char ]
[ & | D | E | NULL ] -@ [ NULL | char ]

[ + | - | . | integer | NULL ] .% [ NULL | char ]
[ + | - | . | integer | NULL ] .& [ NULL | char ]

[ + | - | . | real | NULL ] D% [ NULL | char ]
[ + | - | . | real | NULL ] D& [ NULL | char ]
[ + | - | . | real | NULL ] D@ [ NULL | char ]
[ + | - | . | real | NULL ] E% [ NULL | char ]
[ + | - | . | real | NULL ] E& [ NULL | char ]
[ + | - | . | real | NULL ] E@ [ NULL | char ]

[ . ][ real ][ % | & ]
[ E | D ][ integer ][ % | & | @ ]

For other examples where the VAL function gives a "Type mismatch"
error, query on the following words:

   BASIC and VAL and type and mismatch

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