Freemarker: Removing Items from One Sequence from Another Sequence - sequence

Question

This may be something really simple, but I couldn't figure it out and been trying to find an example online to no avail. I'm basically trying to remove items found in one sequence from another sequence.
Example #1
Items added to the cart is in one sequence; items removed from cart is in another sequence:
<#assign Added_Items_to_Cart = "AAAA,BBBB,CCCC,DDDD,EEEE,FFFF">
<#assign Deleted_Items_from_Cart = "BBBB,DDDD">
The result I'm looking for is: AAAA,CCCC,EEEE,FFFF
Example #2
What if the all items added to and deleted from cart are in the same sequence?
<#assign Cart_Activity = "AAAA,BBBB,BBBB,CCCC,DDDD,EEEE,DDDD,FFFF,Add,Add,Delete,Add,Add,Add,Delete,Add">
The result I'm looking for is the same: AAAA,CCCC,EEEE,FFFF

Answer 1

First things first: You ask about sequence but the data you are dealing with are strings.
I know you are using the string to work as a sequence (and it works), but sequences are sequences and strings are strings, and they have diferente ways of dealing with. I just felt this was important to clarify if someone who is starting to learn how to program get to this answer.
Some assumptions since you're providing strings with data separated by comma:
You want a string with data separated by comma as a result.
You know how to properly create strings with data separated by comma.
You dont have commas in your items names.
Observations:
I'll give you the logic but not the code donne, as this can be a great chance for you to learn/practice freemarker (stackoverflow spirit, you know...)
You question is not about something specific of freemaker (it just happens to be the language you want to work with). Think about adding the logic tag to you question. :-)
Now to the answer on how to do what you want on a "string that is working as a sequence":
Example #1
Change your string to a real sequence :-)
1 - Use a built-in to split your string on commas. Do it for both Added_Items_to_Cart and Deleted_Items_from_Cart. Now you have two real sequences to work with.
2 - Create a new string tha twill be your result .
3 - Iterate over the sequence of added itens.
4 - For each item of the added list, you will check if the deleted list also contains this item.
4.1 - If the deleted list contains the item you do nothing.
4.2 - If the deleted list do not contains the item, you add that item to your string result
At the end of this nested iteration (thats another hint) you should get the result you're looking for.
Example #2
There are many ways of doing it and i'll just share the one that pops out of my mind right now.
I think it's noteworthy that in this approach you will always have an even sized list, as you always insert 2 infos each time: item and action.
So always the first half will be the 'item list' and the second half will be the 'action list'.
1 - Change that string to a sequence (yes, like on the other example).
2 - Get half of its size (in your example size = 16 so half of it is 8)
3 - Iterate over a range from 0 to half-1 (in your example 0 to 7)
4 - At each iteration you'll have a number. Lets call it num (yes I'm very creative):
4.1 - If at the position num + half you have the word "Add" you add the item of position num in your result string
4.2 - If at the position num + half you have the word "Delete" you remove the item of position num from your result string
And for the grand finale, some really usefull links that will help you in your freemarker life forever!!!
All built-ins from freemarker:
https://freemarker.apache.org/docs/ref_builtins.html
All directives from freemarker:
https://freemarker.apache.org/docs/ref_directive_alphaidx.html
Freemarekr cheatsheet :
https://freemarker.apache.org/docs/dgui_template_exp.html#exp_cheatsheet