计算数组中的位数（c ++）

Question

let's say I have an array arr[5]={5,2,3,2,5} and i wrote following program for it

#include <iostream>
using namespace std;
int main()
{
  int n;
  cout<<"Enter Length of Elements= ";
  cin>>n;
  int arr[50];
  for(int i=0;i<n;i++)
  {
      cout<<"Enter Number=";
      cin>>arr[i];
  }
  for(int i=0;i<n;i++)
  {
      int countNum=1;


      for(int j=i+1;j<n;j++)
      {
          if(arr[i]==arr[j])
          {
              if(i>0)
              {
                  int countNum2=0;


                  for(int k=0;k>i;k++)
                  {
                      //bool repeat=false;

                      if(arr[i]==arr[k])
                      {
                        //repeat=false;
                      }
                      else
                      {
                          countNum2++;
                      }
                  }
                  if(countNum2==i)
                  {
                     countNum++;
                  }
              }
              else
              {
                countNum++;
              }
          }
          else
          {
              for(int k=0;k<i;k++)
              {
                  if(arr[k]==arr[i])
                  {

                  }
                  else
                  {
                      countNum=1;
                  }

              }
          }

      }
      cout<<arr[i]<<" has appeared "<<countNum<< "Times"<<endl;

  }

    return 0;
}

but why I am getting 5 has appeared 2 Times

2 has appeared 1 Time

3 has appeared 1 Time

2 has appeared 1 Time

5 has appeared 1 Time

instead of

5 has appeared 2 Times

2 has appeared 2 Times

3 has appeared 1 Times

so how to fix my program help!

Answer 1

The problem with your code is that you don't remove the duplicates or assign an array which effectively stores the count of each unique element in your array.

Also the use of so many loops is completely unnecessary.

You just need to implement two loops, outer one going through all the elements and the inner one checking for dupes first (using an array to check frequency/occurence status) and counting appearance of each element seperately with a variable used as a counter.

Set a counter array (with the corresponding size of your taken array) with a specific value (say zero) and change that value when same element occurs while traversing the array, to trigger not to count for that value again.

Then transfer the count value from the counter variable to the counter array (the one which we set and which distinguishes between duplicates) each time the inner loop finishes iterating over the whole array. (ie place it after the values are counted)

With a little bit of modification, your code will work as you would want it to:

#include <iostream>
using namespace std;
int main()
{
  int n;
  cout<<"Enter Length of Elements = ";
  cin>>n;
  int arr[50];
  for(int i=0;i<n;i++)
  {
      cout<<"Enter Number = ";
      cin>>arr[i];
  }

  int counter[50];
  for(int i=0; i<n; i++)
    counter[i]=0;

    // Our counter variable, but counts will be transferred to count[] later on:
    int tempcount;

    for(int i=0; i<n; i++)
    {   // Each distinct element occurs once atleast so initialize to one:
        tempcount = 1;
        for(int j=i+1; j<n; j++)
        {
            // If dupe is found: 
            if(arr[i]==arr[j])
            {
                tempcount++;

                // Ensuring not to count frequency of same element again:
                counter[j] = 1;
            }
        }

        // If occurence of current element is not counted before:
        if(counter[i] != 1)
            counter[i] = tempcount;
    }

    for(int i=0; i<n; i++)
    {
        if(counter[i] != 0)
            printf("%d has appeared %d times.\n", arr[i], counter[i]);
    }
 return 0;
}

I used a variable tempcount to count occurence of each element and a zero-initialized array count to get the dupes checked (by setting it to 1 for a duplicate entry, and not counting it if it qualifies as 1) first. Then I transferred the counted occurence values to counter[] from tempcount at each outer loop iteration. (for all the unique elements)

Answer 2

That's what you exactly need (amount of each number in array):

// we'll store amounts of numbers like key-value pairs.
// std::map does exactly what we need. As a key we will
// store a number and as a key - corresponding counter
std::map<int, size_t> digit_count;

// it is simpler for explanation to have our
// array on stack, because it helps us not to
// think about some language-specific things
// like memory management and focus on the algorithm
const int arr[] = { 5, 2, 3, 2, 5 };

// iterate over each element in array
for(const auto elem : arr) 
{
    // operator[] of std::map creates default-initialized
    // element at the first access. For size_t it is 0.
    // So we can just add 1 at each appearance of the number 
    // in array to its counter.
    digit_count[elem] += 1;
}

// Now just iterate over all elements in our container and
// print result. std::map's iterator is a pair, which first element
// is a key (our number in array) and second element is a value
// (corresponding counter)
for(const auto& elem : digit_count) {
    std::cout << elem.first << " appeared " << elem.second << " times\n";
}

https://godbolt.org/z/_WTvAm

Well, let's write some basic code, but firstly let's consider an algorithm (it is not the most efficient one, but more understandable):

The most understandable way is to iterate over each number in array and increment some corresponding counter by one. Let it be a pair with the first element to be our number and the second to be a counter:

struct pair {
    int number;
    int counter;
};

Other part of algorithm will be explained in code below

// Say that we know an array length and its elements
size_t length = // user-defined, typed by user, etc.
int* arr = new int[length];
// input elements

// There will be no more, than length different numbers
pair* counts = new pair[length];

// Initialize counters
// Each counte will be initialized to zero explicitly (but it is not obligatory,
// because in struct each field is initialized by it's default
// value implicitly)
for(size_t i = 0; i < length; i++) {
    counts[i].counter = 0;
}

// Iterate over each element in array: arr[i]
for(size_t i = 0; i < length; i++)
{
    // Now we need to find corresponding counter in our counters.
    size_t index_of_counter = 0;

    // Corresponding counter is the first counter with 0 value (in case when
    // we meet current number for the first time) or the counter that have
    // the corresponding value equal to arr[i]
    for(; counts[index_of_counter].counter != 0 && counts[index_of_counter].number != arr[i]; index_of_counter++)
        ; // Do nothing here - index_of_counter is incrementing in loop-statement

    // We found an index of our counter
    // Let's assign the value (it will assign a value
    // to newly initialized pair and won't change anything
    // in case of already existing value).
    counts[index_of_counter].number = arr[i];

    // Increment our counter. It'll became 1 in case of new
    // counter, because of zero assigned to it a bit above.
    counts[index_of_counter].counter += 1;
}

// Now let's iterate over all counters until we reach the first
// containing zero (it means that this counter and all after it are not used)
for(size_t i = 0; i < length && counts[i].counter > 0; i++) {
    std::cout << counts[i].number << " appeared " << counts[i].counter << " times\n";
}

// correctly delete dynamically allocated memory
delete[] counts;
delete[] arr;

https://godbolt.org/z/hN33Pn

Moreover it is exactly the same solution like with std::map (the same idea), so I hope it can help you to understand, how the first solution works inside