# Interview Questions

# Computing average of stream of numbers

- June 11, 2012
- Posted by: Kamal Rawat
- Category: Algorithms

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

… …

… …

### Solution:

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); } }