Question

I’m using PowerBuilder and have a problem with the if condition. I want to check if a variable is not null or it is not empty.

So first I have following if condition to test if the variable is null or not:

IF IsNull(ls_name) THEN
    messagebox("ls_name", "is null")  //true
else
    messagebox("ls_name", "is not null")
end if

From the above condition, I know that ls_name is null. Now I test if its empty or not:

if ls_name = "" then
    messagebox ("ls_name", "is empty")
else
    messagebox("ls_name", "is not empty") //true
end if

And i get that ls_name is not empty.

Now when I use an if condition to check if ls_name is NOT Null or it is NOT empty, I put the following if condition:

IF not IsNull(ls_name) or ls_name <> "" THEN
    messagebox("test", "condition true")
else
    messagebox("test", "condition false")  //this becomes true.
end if

It goes to the else part of the condition. Shouldn’t the if part become true? Am I writing the if condition wrong?

Answer 1

Thinking of “null” as “unknown” has always gotten me by in understanding how expressions evaluated.

So, when ls_name is null,

not IsNull(ls_name) or ls_name <> ""

evaluates to

NOT TRUE OR UNKNOWN

or

FALSE OR UNKNOWN

Well, for an OR to evaluate to TRUE, at least one condition must be TRUE. That doesn’t apply in this case.

The way I usually test for this is:

IF NOT (IsNull (ls_name) OR ls_name = "") THEN

which, when ls_name is null, resolves to

NOT (TRUE OR UNKNOWN)

then

NOT (TRUE)

and

FALSE

In other words, the variable is “empty”.

Good luck,

Terry