Getting unique values using dictionary - would like to understand more - vba

Question

I have this code I made from studying multiple posts.
https://www.youtube.com/watch?v=j2RfI75Yfg8
https://www.mrexcel.com/board/threads/storing-unique-values-from-advanced-filter-to-an-array.1048617/
Option Explicit
Sub GetTheUniqueValues()
Dim dict As New Scripting.Dictionary
Dim rng_col_a As Range
Dim col_a_last_row As Long
Dim source_array As Variant
Dim i As Long
Dim j As Long
Dim new_array As Variant
dict.CompareMode = TextCompare
col_a_last_row = ActiveSheet.Range("A1048576").End(xlUp).row
Set rng_col_a = Range(ActiveSheet.Range("A2"), ActiveSheet.Range("A" & col_a_last_row))
source_array = rng_col_a
For i = LBound(source_array) To UBound(source_array)
If source_array(i, 1) <> "" Then dict(source_array(i, 1)) = Empty
Next i
new_array = dict.Keys
For j = LBound(new_array) To UBound(new_array)
ActiveSheet.Range("H" & j + 2).Value = new_array(j)
Next j
End Sub
I would like to understand more about
If source_array(i, 1) <> "" Then dict(source_array(i, 1)) = Empty
I am new to VBA and programming in general so may I now what the "dict(source_array(i, 1)) = Empty" does and why this particular line of code is effective of only putting unique values in the dictionary.
What does it do if the condition is true?
What does it do if the condition is false?
Thank you.

Answer 1

Using dict("a") = Empty tells the dict dictionary that its element with key a has no value (is Empty).
If the dict dictionary doesn't have a key "a", then the code dict("a") = Empty will create it.
That means when cycling through all the values in the source_array, it won't (can't) create duplicates. If key "a" already exists it'll just assign Empty to it again, if it doesn't exist, it'll create it.
This is better than trying to add keys e.g. using
dict.Add "a", Empty
Will only work if key "a" doesn't already exist, but e.g.
dict("a") = Empty
Will either assign Empty to key "a" or create it. It can't error like the first method.
The If source_array(i, 1) <> "" is simply checking that there is a value to create in the first place.
If it's true then it'll try to create the key, if it's false it'll just move to the next i value.

Answer 2

If source_array(i, 1) <> "" Then dict(source_array(i, 1)) = Empty only create a new key and do not allocate any value. If instead of Empty will be 1, the final result will be the same, due to the fact that only the dictionary (unique) keys count...
dict.Keys is an array (independent of the key item values) and only it is used by this code. A shorter version of dropping the dictionary keys would be (without iteration between the array elements):
ActiveSheet.Range("H2").Resize(UBound(new_array) + 1, 1).Value = WorksheetFunction.Transpose(new_array)
A shorter (but interesting, I think) version, for the whole code, would be the next one (it does not need a reference to 'Microsoft Scripting Runtime'):
Sub GetTheUniqueValues()
Dim source_array As Variant, sh As Worksheet, j As Long, col_a_last_row As Long
Set sh = ActiveSheet
col_a_last_row = sh.Range("A" & Rows.count).End(xlUp).row
source_array = sh.Range(sh.Range("A2"), sh.Range("A" & col_a_last_row)).Value
With CreateObject("scripting.dictionary")
For j = 1 To UBound(source_array)
.Item(source_array(j, 1)) = Application.Index(source_array, j, 0)
Next
sh.Range("H2").Resize(.count, UBound(source_array, 2)) = Application.Index(.Items, 0, 0)
End With
End Sub
It would be a bigger challenge to understand it. But still not something very complicated...