Amortized Analysis

Amortized Analysis The average cost of a sequence of n operations on a given Data Structure. Aggregate Analysis Accounting Method Potential Method CS333 / Cutler Amortized Analysis

Amortized Analysis • Amortizedanalysis computes the average time required to perform a sequence of n operations on a data structure • Often worst case analysis is not tight and the amortized cost of an operation is less that its worst case. CS333 / Cutler Amortized Analysis

Applications of amortized analysis • Vectors/ tables • Disjoint sets • Priority queues • Heaps, Binomial heaps, Fibonacci heaps • Hashing CS333 / Cutler Amortized Analysis

Difference between amortized and average cost • To do averages we need to use probability • For amortized analysis no such assumptions are needed • We compute the average cost per operation for any mix of n operations CS333 / Cutler Amortized Analysis

Operations on Data Structures • A data structure has a set of operations associated with it. • Example: A stack with • push(), pop() and MultiPop(k). • Often some operations may be slow while others are fast. • push() and pop() are fast. • MultiPop(k) may be slow. • Sometimes the time of a single operation can vary CS333 / Cutler Amortized Analysis

Methods • Aggregate analysis- the total amount of time needed for the n operations is computed and divided by n • Accounting - operations are assigned an amortized cost. Objects of the data structure are assigned a credit • Potential – The prepaid work (money in the “bank”) is represented as “potential” energy that can be released to pay for future operations CS333 / Cutler Amortized Analysis

Aggregate analysis • n operations take T(n) time • Amortized cost of an operation is T(n)/n CS333 / Cutler Amortized Analysis

Stack - aggregate analysis • A stack with operations • Push, Pop and Multipop. Multipop(S, k) while notempty(S) and k>0 do Pop(S); k:=k-1 endwhile CS333 / Cutler Amortized Analysis

Stack - aggregate analysis • Push and Pop are O(1) (to move 1 data element) • Multipop is O(min(s, k)) where s is the size of the stack and k the number of elements to pop. • Assume a sequence of n Push, Pop and Multipop operations CS333 / Cutler Amortized Analysis

Stack - aggregate analysis • Each object can be popped only once for each time it is pushed • So the total number of times Pop can be called ( directly or from Multipop) is bound by the number of Pushes <=n. CS333 / Cutler Amortized Analysis

Stack - aggregate analysis • A sequence of nPush and Pop operations is therefore O(n) and the amortized cost of each is O(n)/n=O(1). All operations have same amortized cost. CS333 / Cutler Amortized Analysis

Stack Example: Op/Moves <=2 Operation Stack Stack Start a push a a b b push b a a c c b b push c a a b pop c Multipop(3) a pop b a pop a 6 Operation: 6 Moves 4 Operation: 6 Moves CS333 / Cutler Amortized Analysis

Accounting Method • Charge each operation an (invented) amortizedcost. • Often different from actual run time cost. Some operations may have an amortized cost larger than runtime, others may have less. • Unlike businesses we do not want to make a profit • We want to cover the actual cost • Amount charged but not used in performing an operation is stored with objects of the data structure CS333 / Cutler Amortized Analysis

Accounting method • Later operations can use stored amount to pay for their actual cost • Credit balance must not go negative(always enough to pay for performance of future operations) CS333 / Cutler Amortized Analysis

Stack - amortized analysis • We assign the amortized costs: $2 for Push $0 for both Pop and Multipop • For a sequence of nPush and Pop operations the total amortized cost is at most 2n or O(n) CS333 / Cutler Amortized Analysis

Stack - amortized analysis • Each time we do a Push we pay $1 for the actual cost of the Push and the element has a credit of $1. • Each time an element is popped we take the $1 credit to pay for it • Thus the balance is always nonnegative CS333 / Cutler Amortized Analysis

Increment (A) i = 0 while i < length[A] and A[i] = 1 A[i] = 0 i ++ if i < length[A] A[i] = 1 Initially the counter contains 0 Eventually it becomes 2k -1 Next it is reset to 0 k bit binary counter A k -1 1 0 CS333 / Cutler Amortized Analysis

Aggregate analysis 3 bit counterk=3 • Bit 2 1 0 • D No. • 0 0 0 0 • 0 01 • 2 0 1 0 • 3 0 1 1 • 4 1 0 0 • 5 1 0 1 • 6 1 1 0 • 1 1 1 • Flips 2 4 8 • Count number of times a bit is flipped. • Let number increments n = 2k (if n < 2k analysis similar) • A[0] flipped n times • A[1] flipped n/21 times • … • A[k - 1] flipped n/2k-1 times CS333 / Cutler Amortized Analysis

Accounting method • Charge amortized cost of $2 to set a bit to 1 • When a bit is set to 1, pay $1 for actual cost and store $1 with bit • Note: at all times a bit with value 1 has $1 • When a bit is reset to 0 use $1 to pay for actual cost • $2 per Increment operation CS333 / Cutler Amortized Analysis

Accounting method • Let the value stored in the counter be: • After increment and a payment of $2 = $1 +$1: $1 $1 $0 $0 $1 $0 $0 $1 $1 $1 1 1 0 0 1 0 0 1 1 1 $1 $1 $0 $0 $1 $0 $1 $0 $0 $0 1 1 0 0 1 0 1 0 0 0 CS333 / Cutler Amortized Analysis

Dynamic table (object table, hash table, vector, etc) • The table is dynamic • We can’t predict its maximum size • We would like to avoid allocating a lot of unused space (reasonable load balance) • May not be able to avoid table overflow. • Overflow should not cause run time failure CS333 / Cutler Amortized Analysis

Amortized Analysis