Interview Question: Average of stream of numbers

Given a stream of numbers (keep reading numbers from the keyboard). Write code to print the average at each point. For example: user is entering the numbers, at each point you should print the average of all the numbers entered till now, as shown below

numbers                   Average
------------              -------------
10                        10
10, 20                    15
10, 20, 30                20
10, 20, 30, 40            25
10, 20, 30, 40, 50        30
...                       ...
...                       ...

One brute force approach is to compute average at each time, when a number is entered. Each average calculation will be O(n). Hence the total time taken will be O(n²).

Better Approach:

When a new number is entered, we consider the previous average and compute the new average in constant time, i.e O(1).

Hence, at every point when a number is entered, we consider the previous average and calculate the new average in O(1) time.

// Prints average of a stream of numbers
void streamAvg(double arr[], int n)
{
   float avg = 0;
   for(int i = 0; i < n; i++)
   {
      avg = (avg * i + arr[i])/(i+1);

      printf("Average of %d numbers is %f \n", i+1, avg);
   }
}

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