# 17.2 The accounting method

## 17.2-1

Suppose we perform a sequence of stack operations on a stack whose size never exceeds $k$. After every $k$ operations, we make a copy of the entire stack for backup purposes. Show that the cost of $n$ stack operations, including copying the stack, is $O(n)$ by assigning suitable amortized costs to the various stack operations.

For every stack operation, we charge twice.

• First, we charge the actual cost of the stack operation.
• Second, we charge the cost of copying an element later on.

Since we have the size of the stack never exceed $k$, and there are always $k$ operations between backups, we always overpay by at least enough. Therefore, the amortized cost of the operation is constant, and the cost of the $n$ operation is $O(n)$.

## 17.2-2

Redo Exercise 17.1-3 using an accounting method of analysis.

Let $c_i =$ csot of $i$th operation.

$$c_i = \begin{cases} i & \text{if i is an exact power of 2}, \\ 1 & \text{otherwise}. \end{cases}$$

Charge each operation $3$ (amotized cost $\hat c_i$).

• If $i$ is not an exact power of $2$, pay $\$1$, and store$\$2$ as credit.
• If $i$ is an exact power of $2$, pay $\$i$, using stored credit. $$\begin{array}{cccc} \text{Operation} & \text{Cost} & \text{Actual cost} & \text{Credit remaining} \\ \hline 1 & 3 & 1 & 2 \\ 2 & 3 & 2 & 3 \\ 3 & 3 & 1 & 5 \\ 4 & 3 & 4 & 4 \\ 5 & 3 & 1 & 6 \\ 6 & 3 & 1 & 8 \\ 7 & 3 & 1 & 10 \\ 8 & 3 & 8 & 5 \\ 9 & 3 & 1 & 7 \\ 10 & 3 & 1 & 9 \\ \vdots & \vdots & \vdots & \vdots \end{array}$$ Since the amortized cost is$\$3$ per operation, $\sum\limits_{i = 1}^n \hat c_i = 3n$.

We know from Exercise 17.1-3 that $\sum\limits_{i = 1}^n \hat c_i < 3n$.

Then we have

$$\sum_{i = 1}^n \hat c_i \ge \sum_{i = 1}^n c_i \Rightarrow \text{credit} = \text{amortized cose} - \text{actual cost} \ge 0.$$

Since the amortized cost of each operation is $O(1)$, and the amount of credit never goes negative, the total cost of $n$ operations is $O(n)$.

## 17.2-3

Suppose we wish not only to increment a counter but also to reset it to zero (i.e., make all bits in it $0$). Counting the time to examine or modify a bit as $\Theta(1)$, show how to implement a counter as an array of bits so that any sequence of $n$ $\text{INCREMENT}$ and $\text{RESET}$ operations takes time $O(n)$ on an initially zero counter. ($\textit{Hint:}$ Keep a pointer to the high-order $1$.)

(Removed)