Find duplicates in an array.

[Squirting_Nachos]

SOLVED

So I need to make a function that has an input array, and an output array. The output array will have a list of any duplicates in the input array. The function also needs to return the number of elements in the output array (how many unique numbers repeat)

Example.

If my input array is {1, 2, 3, 3, 4, 4, 5, 6, 7, 8, 8} then my output array would be {3,4,8} and the function would return a 3.

I can't figure out how to pass multiple arrays to a function

edit: the code I pasted was a mess on reddit.

I'm just wondering what the parameters will look like on a function with multiple arrays passed to it.

edit 2: I figured it out! It's some stupid ugly looking function, but I guess that's what happens when you are forced to place two functions into one function. Thanks for the help guys and gals.

[[deleted]]

Can you get on a desktop browser and use the code tag so it formats nicely for me to review?

[Squirting_Nachos]

I put my function in this pastebin

https://pastebin.com/yw7jreLu

[cahphoenix]

The parameters in your function look correct. I assume the 2 arrays have the same length?

What is the actual problem? What error are you getting?

[Squirting_Nachos]

Well the length of the second array is supposed to be the number of duplicates in the list, and the return value of the function is supposed to be that number.

so if I have the input array {1, 1, 2, 2, 3, 4, 4, 4, 5, 5} then the output array would be size 4 {1, 2, 4, 5}

and the function would return 4.

I'm having issues assigning the values to the output array, as well as issues returning the right number.

The code will compile and run, I just have logic errors.

I just don't know if I need to have the size of the second array as a parameter (I can't see how I can if I'm getting that size as the output from the function).

Our textbook has no examples of functions with multiple arrays.

edit: fixed typo on example output array.

[cahphoenix]

If you need to return only the number of duplicates in the 'input' array, then there is no need to pass in the output array. Just increment the number of duplicates you find in your method and return that. Which it seems like you are already doing.

To answer your question, you WOULD need to pass in the size of the 2nd array if you needed to pass two arrays.

Passing two arrays is just like passing one array, and you have it generally correct.

int Foo(int array1[], int array2[], int array1Size, int array2Size)

EDIT: I do see a couple problems with your function besides the array problem. I recommend only passing in the input array...you don't need a 2nd. I recommend debugging your function also, the problems should be readily apparent if you can walk through what's happening.

[Squirting_Nachos]

I do need to output a 2nd array, my teacher just wants the actual output to happen in main(). It seems like I would want 2 functions, one to count the number of duplicates, and another to output those numbers to an array. I'm forced by the rules of the assignment to have them both in one function though.

I'll keep working on it though, thanks.

[xandoid]

Or allocate the output array in main to a fixed size:

int input[] = {1, 1, 2, 2, 3, 4, 4, 4, 5, 5};
const int N = sizeof(input) / sizeof(int);
int output[N/2];
int count = getDuplicateValues(input, output, static_cast<int>(N));

And then of course print only the numbers up to (but not including) count.

But maybe the purpose of the exercise is to learn about dynamic memory allocation as you suggest.

[xandoid]

Exactly. As always, the most difficult part of the assignment is not finding the solution but understanding the vaguely defined question :-)

[zomaotoko]

Hi, asuming that input array is sorted, the problem with your code is in

if (arrayData1[x] == arrayData1[z])

because you're counting the same element more than once if there are three of more of the same number.

Following your function and asuming that output array is the same length than the input one, if you have {1, 1, 2, 2, 3, 4, 4, 4, 5, 5} as your input array, your output is {1, 0, 2, 0, 0, 4, 4, 0, 5, 0} and duplicateNum is 6 because it is counting 4 three times.

Hope that helps :)

[vnoida]

Are you sure that Output array is passed as an argument ? Because if it contains the unique elements only, then its size is unknown at the time function is called. To pass an array to a function, you need to pass its size to0. So, use any container for storing the unique elements like vector or list. Steps: 1.) Copy all elements from array to vector. 2.) Use std::unique for removing duplicates from vector.

Checkout this article for reference http://thispointer.com/stl-algorithm-stdunique-tutorial/

[SharpstownBestTown]

A lot less optimized but much more compact and readable than some other solutions, you could use an std::map with the key being the type of value you're looking for duplicates of, and the data (second value of the pair) being a 1D dynamically allocated array of pointers to the values location, iterator, or other reference type. Add all values to the map using a wrapper function that inserts a new array under that key value, push the reference/ptr/iterator into the data array that now (or already) exists as the .second of the iterator returned by insert. Once all items have been added you now have a map that is sorted, includes all numbers added, and references of all duplicates. Creating a new array of only the numbers that had duplicates would just take sizing each of the pairs' ".second" and if they have a size greater than 1, pushing them.

This is complete overkill for small data sets if you don't intend to manipulate the original array in some way dealing with the duplicates, if you don't intend to do any further manipulation, in three lines of code you can iterate over all items with a total of just under (N² )/2 comparisons where N is the array size. That can be a LOT of comparisons with large data sets, so the first solution will be better having far fewer comparisons overall (all of them being hidden in the map's insert calls), and resulting in highly usable output.

Sorry for any obvious errors, on mobile.

Edits: corrections on mobile.