Finding a mode or modes in an array - c++

Question

EDIT Took a different approach and found the solution, updated the function to correctly find the mode or modes
I've been at this algorithm all day and night, I've looked at about 12 code examples 10x over but none of them seem to go above and beyond to address my problem.
Problem: Find the mode(s) in an array, if the array has more than one mode, display them all. (This is a homework assignment so I must use arrays/pointers)
Sample array:
-1, -1, 5, 6, 1, 1
Sample output:
This array has the following mode(s): -1, 1
The problem I'm having is trying to figure how to store and display just the highest mode OR the multiple modes if they exist.
I have used a lot of approaches and so I will post my most recent approach:
void getMode(int *arr, int size)
{
int *count = new int[size]; // to hold the number of times a value appears in the array
// fill the count array with zeros
for (int i = 0; i < size; i++)
count[i] = 0;
// find the possible modes
for (int x = 0; x < size; x++)
{
for (int y = 0; y < size; y++)
{
// don't count the values that will always occur at the same element
if (x == y)
continue;
if (arr[x] == arr[y])
count[x]++;
}
}
// find the the greatest count occurrences
int maxCount = getMaximum(count, size);
// store only unique values in the mode array
int *mode = new int[size]; // to store the mode(s) in the list
int modeCount = 0; // to count the number of modes
if (maxCount > 0)
{
for (int i = 0; i < size; i++)
{
if (count[i] == maxCount)
{
// call to function searchList
if (!searchList(mode, modeCount, arr[i]))
{
mode[modeCount] = arr[i];
modeCount++;
}
}
}
}
// display the modes
if (modeCount == 0)
cout << "The list has no mode\n";
else if (modeCount == 1)
{
cout << "The list has the following mode: " << mode[0] << endl;
}
else if (modeCount > 1)
{
cout << "The list has the following modes: ";
for (int i = 0; i < modeCount - 1; i++)
{
cout << mode[i] << ", ";
}
cout << mode[modeCount - 1] << endl;
}
// delete the dynamically allocated arrays
delete[]count;
delete[]mode;
count = NULL;
mode = NULL;
}
/*
definition of function searchList.
searchList accepts a pointer to an int array, its size, and a value to be searched for as its arguments.
if searchList finds the value to be searched for, searchList returns true.
*/
bool searchList(int *arr, int size, int value)
{
for (int x = 0; x < size; x++)
{
if (arr[x] == value)
{
return true;
}
}
return false;
}

Answer 1

It's best to build algorithms from smaller building blocks. The standard <algorithm> library is a great source of such components. Even if you're not using that, the program should be similarly structured with subroutines.
For homework at least, the reasoning behind the program should be fairly "obvious," especially given some comments.
Here's a version using <algorithm>, and std::unique_ptr instead of new, which you should never use. If it helps to satisfy the homework requirements, you might implement your own versions of the standard library facilities.
// Input: array "in" of size "count"
// Output: array "in" contains "count" modes of the input sequence
void filter_modes( int * in, int & count ) {
auto end = in + count;
std::sort( in, end ); // Sorting groups duplicate values.
// Use an ordered pair data type, <frequency, value>
typedef std::pair< int, int > value_frequency;
// Reserve memory for the analysis.
auto * frequencies = std::make_unique< value_frequency[] >( count );
int frequency_count = 0;
// Loop once per group of equal values in the input.
for ( auto group = in; group != end; ++ group ) {
auto group_start = group;
// Skip to the last equal value in this subsequence.
group = std::adjacent_find( group, end, std::not_equal_to<>{} );
frequencies[ frequency_count ++ ] = { // Record this group in the list.
group - group_start + 1, // One unique value plus # skipped values.
* group // The value.
};
}
// Sort <frequency, value> pairs in decreasing order (by frequency).
std::sort( frequencies.get(), frequencies.get() + frequency_count,
std::greater<>{} );
// Copy modes back to input array and set count appropriately.
for ( count = 0; frequencies[ count ].first == frequencies[ 0 ].first; ++ count ) {
in[ count ] = frequencies[ count ].second;
}
}

Answer 2

There's no real answer because of the way the mode is defined. Occasionally you see in British high school leaving exams the demand to identify the mode from a small distribution which is clearly amodal, but has one bin with excess count.
You need to bin the data, choosing bins so that the data has definite peaks and troughs. The modes are then the tips of the peaks. However little subsidiary peaks on the way up to the top are not modes, they're a sign that your binning has been too narrow. It's easy enough to eyeball the modes, a bit more difficult to work it out in a computer algorithm which has to be formal. One test is to move the bins by half a bin. If a mode disappears, it's noise rather than a real mode.