What is wrong with my code?

Question

2.33/5 (3 votes)

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C#

List<int> list = new List<int>() { 1, 1, 2 };
            List<int> list1 = new List<int>() { 1, 2 };
            List<int> list2 = (list.Except(list1)).ToList();
            foreach (int item in list2)
            {
                Console.WriteLine(item);
            }

What I have tried:

what I am trying to do is take an input list from the user and checking if there are any duplicates in it and then print which elements are duplicates.

So basically My List1 is the user input and List2 is the List1.Distinct() (I am trying this by hardcoding just to understand how can I extract the duplicate values)

So basically if my input list is like: {1,2,3,4,2,1,5}
Output I am expecting is {1,2} since 1,2 have duplicate values

Posted 11-Jul-23 3:27am

Samriddha Chowdhury

Updated 12-Jul-23 21:52pm

PIEBALDconsult

v4

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Comments

PIEBALDconsult 11-Jul-23 9:36am

Aaaaand... what do you get?

Samriddha Chowdhury 11-Jul-23 9:40am

Hi the list2 is empty and prints nothing. How to achieve the result I want? Mind you that I am very new to programming and dont have much experience

PIEBALDconsult 11-Jul-23 11:17am

I think you need to improve the question to add more detail, such as what you want to have happen when the lists are { 2 , 1 , 1 } and { 1 , 2 } ? The second sequence does not appear in the first sequence.

BillWoodruff 11-Jul-23 11:45am

specify exactly the output you want to see

PIEBALDconsult 11-Jul-23 16:24pm

Oh, and what is the expected result of f ( { 1 , 1 , 2 , 2 } , { 1 , 2 } ) and why?

3 solutions

Solution 2

This is a fairly simple matter to fix:

C#

List<int> list1 = new List<int>() { 1, 1, 2 };
List<int> list2 = new List<int>() { 1, 2 };

foreach (int item in list2)
{
    if (list1.Contains(item))
    {
        list1.Remove(item);
    }
}

Posted 11-Jul-23 5:38am

Richard MacCutchan

Comments

PIEBALDconsult 11-Jul-23 11:54am

Unsure that's actually what is desired, because the question isn't clear enough.

Richard MacCutchan 11-Jul-23 12:24pm

It looks clear enough to me. He has [1,1,2] in the first list and [1,2] in the second. He wants to remove one item from the first list that matches each from the second: i.e. end up with [1].

PIEBALDconsult 11-Jul-23 12:28pm

I disagree, I think he wants to remove the _sequence_ { 1 , 2 } from { 1 , 1 , 2 }

Richard MacCutchan 11-Jul-23 12:45pm

That's what the code above does, and what I just said in my previous message.

PIEBALDconsult 11-Jul-23 13:37pm

What does yours do with the inputs { 2 , 1 , 1 } and { 1 , 2 } ?

Richard MacCutchan 11-Jul-23 14:03pm

Run the code and you will see.

PIEBALDconsult 11-Jul-23 14:06pm

Not going to. It looks like it will not return what I expect.

Richard MacCutchan 11-Jul-23 15:37pm

:))

Samriddha Chowdhury 12-Jul-23 2:28am

what I am trying to do is take an input list from the user and checking if there are any duplicates in it and then print which elements are duplicates.

So basically My List1 is the user input and List2 is the List1.Distinct() (I am trying this by hardcoding just to understand how can I extract the duplicate values)

So basically if my input list is like: {1,2,3,4,2,1,5}
Output I am expecting is {1,2} since 1,2 have duplicate values

Richard MacCutchan 12-Jul-23 3:27am

Well that is a different question from what you originally posted. However the following should do what you want:

List<int> list1 = new List<int>() { 1, 2, 3, 4, 2, 1, 5 };
List<int> list2 = new List<int>();
List<int> list3 = new List<int>();
foreach (int item in list1)
{
    if (!list2.Contains(item))
    {
        list2.Add(item);
    }
    else
    {
        list3.Add(item);
    }
}
foreach (int item in list3)
{
    Console.Write($"{item}, ");
}
Console.WriteLine(" ");

Samriddha Chowdhury 12-Jul-23 3:45am

@Richard MacCutchan - thank you very much sir. this exactly solves my problem. Sorry for the initial confusion

Richard MacCutchan 12-Jul-23 4:03am

You are welcome.

PIEBALDconsult 12-Jul-23 8:56am

Well, that was definitely not what you have in the question. Please use the Improve question button to update the question.

Richard MacCutchan 12-Jul-23 9:18am

He has already apologised, and accepted my answer.

PIEBALDconsult 12-Jul-23 9:27am

Still have to think of anyone who comes along and reads this in the future.

Samriddha Chowdhury 12-Jul-23 9:56am

Yes updated the tried section to elaborate the problem

Solution 3

Quote:
what I am trying to do is take an input list from the user and checking if there are any duplicates in it and then print which elements are duplicates.

So basically My List1 is the user input and List2 is the List1.Distinct() (I am trying this by hardcoding just to understand how can I extract the duplicate values)

So basically if my input list is like: {1,2,3,4,2,1,5}
Output I am expecting is {1,2} since 1,2 have duplicate values

Here is a different approach:

C#

List<int> list1 = new List<int>() { 1, 2, 3, 4, 2, 1, 5 };

List<int> duplicates = list1
    .GroupBy(x => x)
    .Where(x => x.Count() > 1)
    .Select(x => x.Key)
    .ToList();

Console.WriteLine(string.Join(',', duplicates));

Output:

1,2

What it does is
1. generate a grouping of unique values, with all of the same values
2. then use the count to identify duplicates
3. only select those duplicates
4. output the results as a list of integers

The last line joins all the results and outputs to the console.

Posted 12-Jul-23 21:52pm

Graeme_Grant

Comments

Richard MacCutchan 13-Jul-23 4:09am

A much more elegant solution than mine. I did try using LINQ but I am so poor at it ...

Graeme_Grant 13-Jul-23 4:45am

Thanks, to me it suits the requirements.

Samriddha Chowdhury 13-Jul-23 11:29am

Thank you sir. Since I am quite new so I don't feel comfortable with LINQ yet. So I am using the foreach loop solution since I understand it more easily

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Richard Deeming · Accepted Answer · 2023-07-11T03:41:00

Solution 1

There's nothing wrong with your code; it's your understanding of the Except method that's at fault. :)

Except returns "the set difference of the elements of two sequences". That means it returns a distinct set of elements which only exist in one of the input sequences.

In your example, all elements from the first list exist in the second, and all elements from the second list exist in the first. Therefore, the difference is the empty set, and there will be no elements to output.

Posted 11-Jul-23 3:41am

Richard Deeming

Comments

Samriddha Chowdhury 11-Jul-23 9:44am

Thank you sir. Could you please help me to rectify the code so that I can get the desired output?

Richard Deeming 11-Jul-23 9:49am

It's not a standard LINQ operator; you'll need to remove the items one-by-one:

List<int> list2 = new List<int>(list);
foreach (int i in list1)
{
	list2.Remove(i);
}

PIEBALDconsult 11-Jul-23 10:36am

Won't that have the same effect?
He doesn't want to remove all 1s and 2s.

Richard Deeming 11-Jul-23 10:37am

No; Remove will remove the first instance of the specified value from the list.

Demo[^]

Samriddha Chowdhury 11-Jul-23 10:41am

yes that is correct. I have changed my code like this

List<int> list = new List<int>() { 1, 1, 2 };
List<int> list1 = new List<int>() { 1, 2 };
List<int> list2 = new List<int>(list);
foreach (int i in list1)
{
list2.Remove(i);
}

and it is also producing empty list

Could you please make the changes in my code itself?

Richard Deeming 11-Jul-23 10:43am

No, it's not: Demo[^]

If you look at the output, the original method using Except produces nothing; the updated method using Remove produces a single 1 as its output.

Samriddha Chowdhury 11-Jul-23 10:46am

no the 1 is not coming up. empty list only

Richard Deeming 11-Jul-23 10:48am

Click on the demo link, and look at the output:

Except:

Remove:
1

The output from the Remove version clearly includes a single 1.

So either you're looking at the wrong output, or the code you're running is not what you think it is.

Samriddha Chowdhury 11-Jul-23 10:49am

Please ignore my previous comment. Yes only 1 is coming up now. I didn't print the list to look at the items
Many thanks sir :)

Richard MacCutchan 11-Jul-23 15:38pm

See my answer below.

Samriddha Chowdhury 11-Jul-23 11:04am

Hi now I am trying to apply this logic to the following problem : "Take 2 integer lists from user and check if any duplicates present in them". I have written the below code. When I am comparing the lists with each other then I find and print the duplicates but while checking the list itself I am unable to print the duplicates in the list itself.

static void Main(string[] args)
{

/* Inputting 1st list from user */

int x = Convert.ToInt32(Console.ReadLine());

List<int> list = new List<int>();

for (int i = 0; i < x; i++)
{
list.Add(Convert.ToInt32(Console.ReadLine()));
}

/* Inputting 2nd list from user */

int y = Convert.ToInt32(Console.ReadLine());

List<int> ints = new List<int>();

for (int i = 0; i < y; i++)
{
ints.Add(Convert.ToInt32(Console.ReadLine()));
}

/* Comparing both lists and adding duplicates in 3rd list */

List<int> r = new List<int>();

foreach (int i in list)
{
if (ints.Contains(i))
{
if (!r.Contains(i))
{
r.Add(i);
}
}
}

/* Printing message if/if not duplicates found */

/* Checking for duplicates in 1st List */
if (list.Count() != list.Distinct().Count())
{
Console.Write("Duplicates present in list 1");

List<int> temp = list.Distinct().ToList();
List<int> list2 = new List<int>(list);
foreach (int i in list)
{
list2.Remove(i);
}

//Console.WriteLine("Remove:");
foreach (int item in list2)
{
Console.Write(" "+item);
}

}
Console.WriteLine();

/* Checking for duplicates in 2nd List */
if (ints.Count() != ints.Distinct().Count())
{
Console.Write("Duplicates present in list 2");

List<int> l = (ints.Except(ints.Distinct())).ToList();

foreach (var item in l)
{
Console.Write(" " + item);
}
}
Console.WriteLine();

if (r.Count() == 0)
{
Console.WriteLine("No duplicates in comparing 2 lists");
}
else
{
Console.Write("Duplicates found: ");
foreach (var item in r)
{
Console.Write(" " + item);
}
}
Console.WriteLine();
}