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2 Computational Complexity

In the last chapter we developed a drawing program. To hold the drawing commands we built the PyList container class which is a lot like the built-in Python list class, but helps illustrate our first data structure. When we added a drawing command to the sequence we called the append method. It turns out that this method is called a lot. In fact, the flower picture in the first chapter took around 700 commands to draw. You can imagine that a complex picture with lots of free-hand drawing could contain thousands of drawing commands. When creating a free-hand drawing we want to append the next drawing command to the sequence quickly because there are so many commands being appended. How long does it take to append a drawing command to the sequence? Can we make a guess? Should we care about the exact amount of time?

In this chapter you'll learn how to answer these questions and you'll learn what questions are important for you as a computer programmer. First you'll read about some principles of computer architecture to understand something about how long it takes a computer to do some simple operations. With that knowledge you'll have the tools you'll need to make informed decisions about how much time it might take to execute some code you have written.

2.1 Chapter Goals

By the end of this chapter you should be able to answer these questions.

• What are some of the primitive operations that a computer can perform?

• How much time does it take to perform these primitive operations?

• What does the term computational complexity mean?

• Why do we care about computational complexity?

• When do we need to be concerned about the complexity of a piece of code?

• What can we do to improve the efficiency of a piece of code?

• What is the definition of Big-Oh notation?

• What is the definition of Theta notation?

• What is amortized complexity and what is its importance?

• How can we apply what we learned to make the PyList container class better?

2.2 Computer Architecture

A computer consists of a Central Processing Unit (i.e. the CPU) that interacts with Input/Output (i.e. I/O) devices like a keyboard, mouse, display, and network interface. When you run a program it is first read from a storage device like a hard drive into the Random Access Memory, or RAM, of the computer. RAM loses its contents when the power is shut off, so copies of programs are only stored in RAM while they are running. The permanent copy of a program is stored on the hard drive or some other permanent storage device.

The RAM of a computer holds a program as it is executing and also holds data that the program is manipulating. While a program is running, the CPU reads input from the input devices and stores data values in the RAM. The CPU also contains a very limited amount of memory, usually called registers. When an operation is performed by the CPU, such as adding two numbers together, the operands must be in registers in the CPU. Typical operations that are performed by the CPU are addition, subtraction, multiplication, division, and storing and retrieving values from the RAM.

2.2.1 Running a Program

When a user runs a program on a computer, the following actions occur:

1. The program is read from the disk or other storage device into RAM.

2. The operating system (typically Mac OS X, Microsoft Windows, or Linux) sets up two more areas of RAM called the run-time stack and the heap for use by the program.

3. The operating system starts the program executing by telling the CPU to start executing the first instruction of the computer.

4. The program reads data from the keyboard, mouse, disk, and other input sources.

5. Each instruction of the program retrieves small pieces of data from RAM, acts on them, and writes new data back to RAM.

6. Once the data is processed the result is provided as output on the screen or some other output device.

Because there is so little memory in the CPU, the normal mode of operation is to store values in the RAM until they are needed for a CPU operation. The RAM is a much bigger storage space than the CPU. But, because it is bigger, it is also slower than the CPU. Storing a value in RAM or retrieving a value from RAM can take as much time as several CPU operations. When needed, the values are copied from the

Fig. 2.1 Conceptual View of a Computer

RAM into the CPU, the operation is performed, and the result is then typically written back into the RAM. The RAM of a computer is accessed frequently as a program runs, so it is important that we understand what happens when it is accessed (Fig. 2.1). One analogy that is often used is that of a post office. The RAM of a computer is like a collection of post office boxes. Each box has an address and can hold a value. The values you can put in RAM are called bytes (i.e. eight bits grouped together). With eight bits, 256 different values can be stored. Usually bytes are interpreted as integers, so a byte can hold values from 0 to 255. If we want to store bigger values, we can group bytes together into words. The word size of a computer is either 32 bits (i.e. four bytes) or 64 bits, depending on the architecture of the computer's hardware. All modern computer hardware is capable of retrieving or storing a word at a time.

The post office box analogy helps us to visualize how the RAM of a computer is organized, but the analogy does not serve well to show us how the RAM of a computer behaves. If we were going to get something from a post office box, or store something in a post office box, there would have to be some kind of search done to find the post office box first. Then the letter or letters could be placed in it or taken from it. The more post office boxes in the post office, the longer that search would take. This helps us understand the fundamental problem we study in this text. As the size of a problem space grows, how does a program or algorithm behave? In terms of this analogy, as the number of post office boxes grows, how much longer does it take to store or retrieve a value?

The RAM of a computer does not behave like a post office. The computer does not need to find the right RAM location before the it can retrieve or store a value. A much better analogy is a group of people, each person representing a memory location within the RAM of the computer. Each person is assigned an address or name. To store a value in a location, you call out the name of the person and then tell them what value to remember. It does not take any time to find the right person because all the people are listening, just in case their name is called. To retrieve a value, you call the name of the person and they tell you the value they were told to remember. In this way it takes exactly the same amount of time to retrieve any value from any memory location. This is how the RAM of a computer works. It takes exactly the same amount of time to store a value in any location within the RAM. Likewise, retrieving a value takes the same amount of time whether it is in the first RAM location or the last.

 
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