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Q27631: Fixed String Initializing, Space & Null Bytes, LEN, and RTRIM$

Article: Q27631
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 9-FEB-1990

An uninitialized fixed-length string is filled with null bytes. When
you initialize a fixed-length string to be equal to "" (the "null
string"), the string will be filled with blank bytes. This behavior of
fixed-length strings differs from variable-length strings, and may
affect your program's use of functions like RTRIM$ in ways you don't
expect (see code examples below).

Note that a null byte has an ASCII value of zero, the same as is
returned by the CHR$(0) function, whereas a blank byte has an ASCII
value of 32, the same as is returned by the CHR$(32) function.

The length shown by the LEN function for a fixed-length string x will
always be the fixed size n determined in the DIM x AS STRING * n
statement.

This information applies to Microsoft QuickBASIC Versions 4.00, 4.00b,
and 4.50, to Microsoft BASIC Compiler Versions 6.00 and 6.00b for
MS-DOS and MS OS/2, and to Microsoft BASIC Professional Development
System (PDS) Version 7.00 for MS-DOS and MS OS/2.

Note that fixed-length strings are not supported in QuickBASIC
Versions 3.00 and earlier.

Note that LEN(RTRIM$(x)) returns the actual length of just the
non-null, nonblank contents of x if x is an INITIALIZED fixed-length
string. (This is because RTRIM$ actually returns a variable-length
string.) If x is an UNINITIALIZED fixed-length string, LEN(RTRIM$(x))
always returns the fixed, dimensioned length, since RTRIM$ is ignored
in this case.

The following code example demonstrates the difference between
assigned and unassigned fixed-length strings:

   DIM s AS STRING * 10                          'initially nulls
   PRINT LEN(s),LEN(RTRIM$(s)),ASC(RIGHT$(s,1))  '10,10 and 0 (null)
   s = ""                                        'pad with spaces
   PRINT LEN(s),LEN(RTRIM$(s)),ASC(RIGHT$(s,1))  '10,0 and 32 (space)

In the following sample program, A is a fixed-length string with a
length of 10 bytes, and B$ is a variable-length string. Both A and B$
are assigned a string of ten blank characters (ASCII value 32).

   DIM a AS STRING * 10
   B$ = SPACE$(10)
   PRINT LEN(RTRIM$(a))  ' Prints 10 since fixed string is uninitialized.
   PRINT LEN(RTRIM$(b$))  ' Prints 0 for variable-length string.
   a = ""
   PRINT LEN(RTRIM$(a))  ' prints 0 as the length of the trimmed string
                         ' returned by RTRIM$ since fixed string is all
                         ' blanks.

Note that the RTRIM$ function returns a variable-length string, even
when a fixed-length string is its argument in parentheses. In the
above program, the LEN function is operating on the variable-length
string returned by the RTRIM$ function.

The following code example illustrates one case where the incorrect
assumption that fixed-length strings are automatically initialized to
spaces will cause unexpected results. The INPUT statement will never
get executed, unless the fixed length STRING "s" is initialized to all
blanks:

DIM s AS STRING * 10        'Desired result: wait until something entered.
's = ""                     'Uncomment this line for expected results.
WHILE RTRIM$(s) = ""        'Fails first time since RTRIM$(s)=10 nulls.
   INPUT "Input data: ",s   'Never executed
WEND

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