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22C:21 Problem 2 (Set 1)

22C:21 Problem 2 (Set 1). Solution outline. Original INSERT code. public void insert (Record rec) { // Check for overflow if (numRecords == maxRecords) { return; // Doing nothing }. Modified INSERT code. public void insert (Record rec) {

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22C:21 Problem 2 (Set 1)

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  1. 22C:21 Problem 2 (Set 1) Solution outline

  2. Original INSERT code public void insert (Record rec) { // Check for overflow if (numRecords == maxRecords) { return; // Doing nothing }

  3. Modified INSERT code public void insert (Record rec) { // Check for overflow. Double when necessary. if (numRecords == maxRecords) { maxRecords *= 2; Record tempList[] = new Record[maxRecords]; System.arraycopy(recordList,0,tempList,0,recordList.length); recordList = tempList; }

  4. Output • --------------------------- • Capacity:2 • --------------------------- • 0 3 • Record inserted at position 0: 3 • --------------------------- • Capacity:2 • --------------------------- • 0 3 • 1 22 • Record inserted at position 1: 22 • --------------------------- • Capacity:4 • --------------------------- • 0 3 • 1 13 • 2 22 • Record inserted at position 1: 13 • (and so on)

  5. Amortized Analysis • Average running time per operation over a sequence of worst-case operations.

  6. am·or·tize[am-er-tahyz, uh-mawr-tahyz] –verb (used with object), -tized, -tiz·ing. 1.Finance. a. to liquidate or extinguish (a mortgage, debt, or other obligation), esp. by periodic payments to the creditor or to a sinking fund. b. to write off a cost of (an asset) gradually. 2.Old English Law. to convey to a corporation or church group; alienate in mortmain.

  7. Basic idea • Knowledge of which sequence of operations is possible. • Data structures that have states that persists between operations. • Worst-case operation can alter the state in a way that worst-case doesn’t occur again for a long time. Thus amortizing the cost!

  8. Some intuition … so on $2 $2 $2 $2 $2 $2 Now t = 4, Total cost $2t = $8 t = 2, Total cost $2t = $4

  9. Formal analysis • Consider DynamicRecordDB with N slots and n records. • INSERT operations doubles the size before adding another item if n = N. • Any operation that doesn’t involve doubling takes O(1) time unit – say, at most 1 seconds. • Resizing takes 2n seconds.

  10. Analysis (contd.) • We start from empty list and perform i INSERT operations. So, n = i and N is the smallest power of 2 ≥ i. • Total seconds for all the resizing operations = 2 + 4 + 8 + … +N/4 + N/2 + N = 2N – 2. In reference to the code: n = numRecords, N = maxRecords. We start with N = 2. Then N becomes 4 and finally 8.

  11. Analysis (almost done!) • Total seconds for i INSERTs = i + 2N – 2 • Now, N ≤ 2n = 2i. So the i INSERTs take O(5i – 2) or O(i) time. This is worst case! • So, on average, each INSERT takes O(i)/i = O(1) time. This is the amortized running time of insertion.

  12. Bottom line(s) • Amortized analysis is a way of proving that even if an operation is occasionally expensive, its cost is made up for by other, cheaper occurrences of the same operation.

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