Null Value (.NULL.) Is a Marker - Not a Data Type

ID: Q133740

The information in this article applies to:

Microsoft Visual FoxPro for Windows, version 3.0

SUMMARY

The null value (.NULL.) is not an actual value, it is a marker that denotes that a memory variable or a field value is missing or unknown. If .NULL. is assigned to a field or a variable, the field or variable's data type is not changed, but the value is changed to NULL.

MORE INFORMATION

Steps to Demonstration Behavior

The .NULL. value can be assigned to variables that are numeric or character as demonstrated by these steps:

1. From the Command window, issue these commands:

   nMyvariable=200        && and press ENTER
   ? TYPE("nMyvariable")  && and press ENTER
   ? nMyvariable

   The N is displayed for nMyvariable denoting a variable of numeric type.

2. To assign a null value (.NULL.) to nMyvariable, issue the following

   commands in the Command window:

   store .NULL. to nMyvariable        && and press ENTER
   ? TYPE("nMyvariable")              && and press ENTER
   ? nMyvariable                      && and press ENTER

   The value N is displayed followed by .NULL.; the type of the variable
   did not change but the value did.

3. To assign a character string to the variable and check the variable

   type, issue the following commands in the Command window:

   cMyvariable="This is a String"     && and press ENTER
   ? cMyvariable                      && and press ENTER

   The screen displays the string "This is a String."

4. Issue the following command in the Command window:

   ? TYPE("cMyvariable")              && and press ENTER

   The screen displays C denoting a character type.

5. Issue the following commands in the Command window:

   STORE .NULL. TO cMyvariable        && and press ENTER
   ?TYPE("cMyvariable")               && and press ENTER

   The screen displays C to show that the type has not been changed.

Additional reference words: 3.00 VFoxWin KBCategory: kbprg KBSubcategory: FxprgGeneral