How do I count the number of occurrences in an array?

Question

I have generated an array of 5 random integers from 1-5. Here is what the array looks like now: myArray[5] = {3, 3, 1, 4, 5}

I have now sorted the array of

Answer 1

Make a hash and initialize with zero.

int hash[10000]={0};
for(int i=0;i<n;i++)
{  
     hash[arr[i]]++;
}

the hash at index arr[i] will hold value which is the count of occurrence of that number. As hash[arr[i]]++ will increment the count at index equal to the value of arr[i]. This way we can check which value occurred how many times by checking hash[arr[i]] where arr[i] is value to be checked.

Answer 2

Use a std::map to map integers to their counts.

std::map<int, int> counts;
for (int i = 0; i < 5; i++) {
    counts[myArray[i]]++; // increment the counter for the current value
}

Now you can print the keys and values in counts. See How to loop through a C++ map of maps? for how to do this.

You can do it with an array instead of a map. The only difference is that it won't automatically expand to handle larger values (unless you use malloc and realloc to make it dynamically sized).

#define MAX_VALUE 9
int counts[MAX_VALUE+1] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
for (int i = 0; i < ARRAY_SIZE; i++) {
    if (myArray[i] <= MAX_VALUE) {
        counts[myArray[i]]++; // increment the counter for the current value
    }
}
for (int j = 0; j <= MAX_VALUE; j++) {
    cout << "Number: " << j << "Number of Occurances: " << counts[j] << endl;
}