Math Terminology: Can you tell me?

raddevus

Background I'm attempting to solve a problem in the most efficient way possible. Attempting to solve the problem put me on the path to attempting to explain the problem. That put me on the path of wondering if there is a term for this. Problem Definition I have two lists of strings. * origItems * newItems I want to find all the strings in newItems that are not contained in origItems. I want to make a new list that contains only those new items. The C# code to solve the problem looks like the following:

List origItems = new List();
origItems.Add("abc");
origItems.Add("def");
origItems.Add("ghi");

List newItems = new List();
newItems.Add("abc");
newItems.Add("def");
newItems.Add("super");
newItems.Add("extra");

List itemsOnlyInNew = newItems.Except(origItems).ToList();
foreach (String s in itemsOnlyInNew){
	Console.WriteLine(s);
}

The output is : super extra Is there a mathematical term that covers this (searching one list based upon another list of items)? (I'm thinking like the term permutation but I don't believe this is a permutation.) Please Feel Free To Make Up A Term If there isn't already a term (in English), please make one up. If there is a term in another language please share that and we'll incorporate it. Just thought this was interesting. Thanks much.

NeverJustHere

Isn't it set theory: newItems - origItems? Think of the two sets as a Venn Diagram, intersection, union, etc.

0x01AA

If I really understand what you are asking, it is simple set theory? Set theory - Wikipedia[^] And there especally I think your question is related to intersection (or the difference against it) Intersection (set theory) - Wikipedia[^]

PIEBALDconsult

Are duplicates possible? (Not that it really matters.)

raddevus

NeverJustHere wrote:

Think of the two sets as a Venn Diagram, intersection, union, etc.

After posting. I got up. Walked out to make coffee and was talking to the wife unit when that hit me. :-O Sometimes I just post dumb questions. :confused: Oops.

raddevus

After posting. I got up. Walked out to make coffee and was talking to the wife unit when that hit me. Blush | :O Thank you for your informative and kind answer. :thumbsup: I think I must've been thinking of the specific instance and related code so much I confused my simple brain. I'm kinda dumb sometimes. :rolleyes:

raddevus

Thanks for your question. Duplicates in orig and new are possible. Duplicates in final set are not allowed.

PIEBALDconsult

Yeah, in C# I'd do that with HashSets, slick as snot. HashSet<T>.ExceptWith(IEnumerable<T>) Method (System.Collections.Generic) | Microsoft Docs[^]

megaadam

I agree with the previous posters: Set Theory In C++ there is the container std::set something similar exists in Python so I assume C# also has something like it. In C++ it looks like this: set_difference - C++ Reference[^]

"If we don't change direction, we'll end up where we're going"

0x01AA

I just missed that you answered already with set theory. Was not my intention to plagiarize ;)

0x01AA

Don't worry, happens to me nearly everyday and sometimes (usually?) more than one time per day :-O

raddevus

Thank you :thumbsup: I'm crawling under my desk for while.

0x01AA

No need ;) Or then I also have to do it. A customer send me the information the angle is 390 deegree. I was that much confused and answered this can't be possible. On the other hand I have no problem when somebody tells me the angle is 2.5 π rad whatelse :laugh:

obermd

In Set Theory, the opposite of "Intersection" is "Difference". You're describing a Difference operation.

englebart

Be careful on order if you use a library. A.diff(B) != B.diff(A) I have seen Difference used for this operation. It looks like Complement could be another term. There is also Symmetric Difference which contains outliers from both sets.

Leo56

OK, not a Maths term per se, but when anyone invokes the 'M' word in my hearing I always think "Freudenschade"...... :-D

Bruce Patin

You mean SchadenFreude?

John Godin

In case it might be useful to you. I have ubuntu Linux running on my Win 10 laptop (WSL) and it has this handy program called comm. I use it when I need to do a task like you describe. HTH Usage: comm [OPTION]... FILE1 FILE2 Compare sorted files FILE1 and FILE2 line by line. When FILE1 or FILE2 (not both) is -, read standard input. With no options, produce three-column output. Column one contains lines unique to FILE1, column two contains lines unique to FILE2, and column three contains lines common to both files.