What is the best way to select null values in a field? / How do I run a function on or create a subset of a field?

[#1]

So I started working with qlikview on Monday and 95% of everything seems really intuitive, but I can't figure out how to run a function on a constant selection. I have a computer science background and what I'd love to have is something in the script like this:

const datatype FOO(bar(field));          //field is an array of multiple data types, bar returns datatype.

But I can't figure out how to do that.

Let's start with a description of what I'm doing. I have a calendar object that sets a variable. When that variable changes, it triggers a selection on a set. I want the calendar to have a minimum and maximum date based on the contents of a field.

The field contains date values and null values, and is has the number format of "Date" in document settings. It represents report release dates with a date, and unreleased reports with a null value.

Now, I had it working just fine by setting the min and max on the calendar as "Min/Max({1} FieldName)." But then it broke when I implemented NULLASVALUE, with NullValue = 'NR' in the script, so that I could select unreleased reports.

I've tried changing the min/max to "Min/Max({1<FieldName = FieldName - {NULLVALUE}>} FieldName)," as well as "Min/Max({1<FieldName = FieldName - {NULLVALUE}>})" in hopes that the set expression would qualify as the expression/field in the function syntax.

I've also tried declaring variables in the script ahead of NULLASVALUE with:

LET vCalMax = Max(FieldName);

LET vCalMin = Min(FieldName);

but the variable overview shows the variable value as "Min/Max(FieldName)."

I know it's probably possible to create a new field that only contains the data I want, then set the min/max to "Min/Max({1} NewField).," but that seems inefficient.

So this leaves me with two big questions and one less important question:

What's the best way to select null values in a field?

How do I run a function on or create a subset of a field?

What's the proper way to set a variable using a function in the script?

tl;dr: Field contains dates and nulls. Max({1} FieldName) works until you use NULLASVALUE with NullValue = string. Help?

[#2 · Accepted solution]

Well I believe I've found a solution. I've set my Min/Max to "=Max({1} PurgeChar(DateField, 'NullValue'))," which I believe just finds the max of DateField like before, since PurgeChar with 'NullValue' returns DateField with NULL where it was before, which acts just like Max({1} DateField) as I had it before.

Someone correct me if I'm wrong.