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Computer Science & Engineering

Introduction to Computer Science & Engineering

Think about some of the different ways that people use computers. In school, students use computers for tasks such as writing papers, searching for articles, sending email, and participating in online classes. At work, people use computers to analyze data, make presentations, conduct business transactions, communicate with customers and coworkers, control machines in manufacturing facilities, and do many other things. At home, people use computers for tasks such as paying bills, shopping online, communicating with friends and family, and playing computer games. And don’t forget that cell phones, iPods®, BlackBerries®, car navigation systems, and many other devices are computers too. The uses of computers are almost limitless in our everyday lives.

Computers can do such a wide variety of things because they can be programmed. This means that computers are not designed to do just one job, but to do any job that their programs tell them to do. A program is a set of instructions that a computer follows to perform a task. For example, Figure 1-1 shows screens from two commonly used programs, Microsoft Word and Adobe Photoshop. Microsoft Word is a word processing program that allows you to create, edit, and print documents with your computer. Adobe Photoshop is an image editing program that allows you to work with graphic images, such as photos taken with your digital camera.

Programs are commonly referred to as software. Software is essential to a computer because it controls everything the computer does. All of the software that we use to make our computers useful is created by individuals working as programmers or software developers. A programmer, or software developer, is a person with the training and skills necessary to design, create, and test computer programs. Computer programming is an exciting and rewarding career. Today, you will find programmers’ work used in business, medicine, government, law enforcement, agriculture, academics, entertainment, and many other fields.

This book introduces you to the fundamental concepts of computer programming using the Python language. Before we begin exploring those concepts, you need to understand a few basic things about computers and how they work. This chapter will build a solid foundation of knowledge that you will continually rely on as you study computer science. First, we will discuss the physical components that computers are commonly made of. Next, we will look at how computers store data and execute programs. Finally, we will get a quick introduction to the software that you will use to write Python programs.

Hardware and Software

The physical devices that a computer is made of are referred to as the computer’s hardware. The programs that run on a computer are referred to as software.

Hardware

The term hardware refers to all of the physical devices, or components, that a computer is made of. A computer is not one single device, but a system of devices that all work together. Like the different instruments in a symphony orchestra, each device in a computer plays its own part. If you have ever shopped for a computer, you’ve probably seen sales literature listing components such as microprocessors, memory, disk drives, video displays, graphics cards, and so on. Unless you already know a lot about computers, or at least have a friend that does, understanding what these different components do might be challenging. As shown in Figure 1-2, a typical computer system consists of the following major components:

  • The central processing unit (CPU)
  • Main memory
  • Secondary storage devices
  • Input devices
  • Output devices

Let’s take a closer look at each of these components.

The CPU

When a computer is performing the tasks that a program tells it to do, we say that the computer is running or executing the program. The central processing unit, or CPU, is the part of a computer that actually runs programs. The CPU is the most important component in a computer because, without it, the computer could not run software.

In the earliest computers, CPUs were huge devices made of electrical and mechanical components such as vacuum tubes and switches. Figure 1-3 shows such a device. The two women in the photo are working with the historic ENIAC computer. The ENIAC, which is considered by many to be the world’s first programmable electronic computer, was built in 1945 to calculate artillery ballistic tables for the U.S. Army. This machine, which was primarily one big CPU, was 8 feet tall, 100 feet long, and weighed 30 tons.

Today, CPUs are small chips known as microprocessors. Figure 1-4 shows a photo of a lab technician holding a modern microprocessor. In addition to being much smaller than the old electromechanical CPUs in early computers, microprocessors are also much more powerful.

Main Memory

You can think of main memory as the computer’s work area. This is where the computer stores a program while the program is running, as well as the data that the program is working with. For example, suppose you are using a word processing program to write an essay for one of your classes. While you do this, both the word processing program and the
essay are stored in main memory.

Main memory is commonly known as random-access memory, or RAM. It is called this because the CPU is able to quickly access data stored at any random location in RAM. RAM is usually a volatile type of memory that is used only for temporary storage while a program is running. When the computer is turned off, the contents of RAM are erased. Inside your computer, RAM is stored in chips, similar to the ones shown in Figure 1-5.

Secondary Storage Devices

Secondary storage is a type of memory that can hold data for long periods of time, even when there is no power to the computer. Programs are normally stored in secondary memory and loaded into main memory as needed. Important data, such as word processing documents, payroll data, and inventory records, is saved to secondary storage as well.

The most common type of secondary storage device is the disk drive. A disk drive stores data by magnetically encoding it onto a circular disk. Most computers have a disk drive mounted inside their case. External disk drives, which connect to one of the computer’s communication ports, are also available. External disk drives can be used to create backup copies of important data or to move data to another computer.

In addition to external disk drives, many types of devices have been created for copying data, and for moving it to other computers. For many years floppy disk drives were popular. A floppy disk drive records data onto a small floppy disk, which can be removed from the drive. Floppy disks have many disadvantages, however. They hold only a small amount of data, are slow to access data, and can be unreliable. The use of floppy disk drives has declined dramatically in recent years, in favor of superior devices such as USB drives. USB drives are small devices that plug into the computer’s USB (universal serial bus) port, and appear to the system as a disk drive. These drives do not actually contain a disk, however. They store data in a special type of memory known as flash memory. USB drives, which are also known as memory sticks and flash drives, are inexpensive, reliable, and small enough to be carried in your pocket.

Optical devices such as the CD (compact disc) and the DVD (digital versatile disc) are also popular for data storage. Data is not recorded magnetically on an optical disc, but is encoded as a series of pits on the disc surface. CD and DVD drives use a laser to detect the pits and thus read the encoded data. Optical discs hold large amounts of data, and because recordable CD and DVD drives are now commonplace, they are good mediums for creating backup copies of data.

Input Devices

Input is any data the computer collects from people and from other devices. The component that collects the data and sends it to the computer is called an input device. Common input devices are the keyboard, mouse, scanner, microphone, and digital camera. Disk drives and optical drives can also be considered input devices because programs and data are retrieved from them and loaded into the computer’s memory.

Output Devices

Output is any data the computer produces for people or for other devices. It might be a sales report, a list of names, or a graphic image. The data is sent to an output device, which formats and presents it. Common output devices are video displays and printers. Disk drives and CD recorders can also be considered output devices because the system sends data to them in order to be saved.

Software

If a computer is to function, software is not optional. Everything that a computer does, from the time you turn the power switch on until you shut the system down, is under the control of software. There are two general categories of software: system software and application software. Most computer programs clearly fit into one of these two categories. Let’s take a closer look at each.

System Software

The programs that control and manage the basic operations of a computer are generally referred to as system software. System software typically includes the following types of programs:

Operating Systems An operating system is the most fundamental set of programs on a computer. The operating system controls the internal operations of the computer’s hardware, manages all of the devices connected to the computer, allows data to be saved to and retrieved from storage devices, and allows other programs to run on the computer. Figure 1-6 shows screens from three popular operating systems: Windows Vista, Mac OS X, and Linux.

Utility Programs A utility program performs a specialized task that enhances the computer’s operation or safeguards data. Examples of utility programs are virus scanners, file compression programs, and data backup programs. Software Development Tools Software development tools are the programs that programmers use to create, modify, and test software. Assemblers, compilers, and interpreters are examples of programs that fall into this category.

Application Software

Programs that make a computer useful for everyday tasks are known as application software. These are the programs that people normally spend most of their time running on their computers. Figure 1-1, at the beginning of this chapter, shows screens from two commonly used applications: Microsoft Word, a word processing program, and Adobe Photoshop, an image editing program. Some other examples of application software are spreadsheet programs, email programs, web browsers, and game programs.

Checkpoint

  1. What is a program?
  2. What is hardware?
  3. List the five major components of a computer system.
  4. What part of the computer actually runs programs?
  5. What part of the computer serves as a work area to store a program and its data while the program is running?
  6. What part of the computer holds data for long periods of time, even when there is no power to the computer?
  7. What part of the computer collects data from people and from other devices?
  8. What part of the computer formats and presents data for people or other devices?
  9. What fundamental set of programs control the internal operations of the computer’s hardware?
  10. What do you call a program that performs a specialized task, such as a virus scanner, a file compression program, or a data backup program?
  11. Word processing programs, spreadsheet programs, email programs, web browsers, and game programs belong to what category of software?

How Computers Store Data

All data that is stored in a computer is converted to sequences of 0s and 1s.

A computer’s memory is divided into tiny storage locations known as bytes. One byte is only enough memory to store a letter of the alphabet or a small number. In order to do anything meaningful, a computer has to have lots of bytes. Most computers today have millions, or even billions, of bytes of memory.

Each byte is divided into eight smaller storage locations known as bits. The term bit stands for binary digit. Computer scientists usually think of bits as tiny switches that can be either on or off. Bits aren’t actual “switches,” however, at least not in the conventional sense. In most computer systems, bits are tiny electrical components that can hold either a positive or a negative charge. Computer scientists think of a positive charge as a switch in the on position, and a negative charge as a switch in the off position. Figure 1-7 shows the way that a computer scientist might think of a byte of memory: as a collection of switches that are each flipped to either the on or off position.

When a piece of data is stored in a byte, the computer sets the eight bits to an on/off pattern that represents the data. For example, the pattern shown on the left in Figure 1-8 shows how the number 77 would be stored in a byte, and the pattern on the right shows how the letter A would be stored in a byte. We explain below how these patterns are determined.

Storing Numbers

A bit can be used in a very limited way to represent numbers. Depending on whether the bit is turned on or off, it can represent one of two different values. In computer systems, a bit that is turned off represents the number 0 and a bit that is turned on represents the number 1. This corresponds perfectly to the binary numbering system. In the binary numbering system (or binary, as it is usually called) all numeric values are written as sequences of 0s and 1s. Here is an example of a number that is written in binary:

           10011101

The position of each digit in a binary number has a value assigned to it. Starting with the rightmost digit and moving left, the position values are 20, 21, 22, 23, and so forth, as shown in Figure 1-9. Figure 1-10 shows the same diagram with the position values calculated. Starting with the rightmost digit and moving left, the position values are 1, 2, 4, 8, and so forth.

To determine the value of a binary number you simply add up the position values of all the 1s. For example, in the binary number 10011101, the position values of the 1s are 1, 4, 8, 16, and 128. This is shown in Figure 1-11. The sum of all of these position values is 157. So, the value of the binary number 10011101 is 157.

Figure 1-12 shows how you can picture the number 157 stored in a byte of memory. Each 1 is represented by a bit in the on position, and each 0 is represented by a bit in the off position.

When all of the bits in a byte are set to 0 (turned off), then the value of the byte is 0. When all of the bits in a byte are set to 1 (turned on), then the byte holds the largest value that can be stored in it. The largest value that can be stored in a byte is 1 2 4 8 16 32 64 128 255. This limit exists because there are only eight bits in a byte.

What if you need to store a number larger than 255? The answer is simple: use more than one byte. For example, suppose we put two bytes together. That gives us 16 bits. The position values of those 16 bits would be 20, 21, 22, 23, and so forth, up through 215. As shown in Figure 1-13, the maximum value that can be stored in two bytes is 65,535. If you need to store a number larger than this, then more bytes are necessary.

Storing Characters

Any piece of data that is stored in a computer’s memory must be stored as a binary number. That includes characters, such as letters and punctuation marks. When a character is stored in memory, it is first converted to a numeric code. The numeric code is then stored in memory as a binary number.

Over the years, different coding schemes have been developed to represent characters in computer memory. Historically, the most important of these coding schemes is ASCII, which stands for the American Standard Code for Information Interchange. ASCII is a set of 128 numeric codes that represent the English letters, various punctuation marks, and other characters. For example, the ASCII code for the uppercase letter A is 65. When you type an uppercase A on your computer keyboard, the number 65 is stored in memory (as a binary number, of course). This is shown in Figure 1-14.

In case you are curious, the ASCII code for uppercase B is 66, for uppercase C is 67, and so forth. Appendix C shows all of the ASCII codes and the characters they represent.

The ASCII character set was developed in the early 1960s, and was eventually adopted by most all computer manufacturers. ASCII is limited however, because it defines codes for only 128 characters. To remedy this, the Unicode character set was developed in the early 1990s. Unicode is an extensive encoding scheme that is compatible with ASCII, but can also represent characters for many of the languages in the world. Today, Unicode is quickly becoming the standard character set used in the computer industry.

Advanced Number Storage

Earlier you read about numbers and how they are stored in memory. While reading that section, perhaps it occurred to you that the binary numbering system can be used to represent only integer numbers, beginning with 0. Negative numbers and real numbers (such as 3.14159) cannot be represented using the simple binary numbering technique we discussed.

Computers are able to store negative numbers and real numbers in memory, but to do so they use encoding schemes along with the binary numbering system. Negative numbers are encoded using a technique known as two’s complement, and real numbers are encoded in floating-point notation. You don’t need to know how these encoding schemes work, only that they are used to convert negative numbers and real numbers to binary format.

Other Types of Data

Computers are often referred to as digital devices. The term digital can be used to describe anything that uses binary numbers. Digital data is data that is stored in binary, and a digital device is any device that works with binary data. In this section, we have discussed how numbers and characters are stored in binary, but computers also work with many other types of digital data.

For example, consider the pictures that you take with your digital camera. These images are composed of tiny dots of color known as pixels. (The term pixel stands for picture element.) As shown in Figure 1-15, each pixel in an image is converted to a numeric code that represents the pixel’s color. The numeric code is stored in memory as a binary number.

The music that you play on your CD player, iPod or MP3 player is also digital. A digital song is broken into small pieces known as samples. Each sample is converted to a binary number, which can be stored in memory. The more samples that a song is divided into,
the more it sounds like the original music when it is played back. A CD quality song is divided into more than 44,000 samples per second!

Checkpoint

  1. What amount of memory is enough to store a letter of the alphabet or a small number?
  2. What do you call a tiny “switch” that can be set to either on or off?
  3. In what numbering system are all numeric values written as sequences of 0s and 1s?
  4. What is the purpose of ASCII?
  5. What encoding scheme is extensive enough to represent the characters of many of the languages in the world?
  6. What do the terms “digital data” and “digital device” mean?

How a Program Works

A computer’s CPU can only understand instructions that are written in machine language. Because people find it very difficult to write entire programs in machine language, other programming languages have been invented.

Earlier, we stated that the CPU is the most important component in a computer because it is the part of the computer that runs programs. Sometimes the CPU is called the “computer’s brain,” and is described as being “smart.” Although these are common metaphors, you should understand that the CPU is not a brain, and it is not smart. The CPU is an electronic device that is designed to do specific things. In particular, the CPU is designed to perform operations such as the following:

• Reading a piece of data from main memory
• Adding two numbers
• Subtracting one number from another number
• Multiplying two numbers
• Dividing one number by another number
• Moving a piece of data from one memory location to another
• Determining whether one value is equal to another value

As you can see from this list, the CPU performs simple operations on pieces of data. The CPU does nothing on its own, however. It has to be told what to do, and that’s the purpose of a program. A program is nothing more than a list of instructions that cause the CPU to perform operations.

Each instruction in a program is a command that tells the CPU to perform a specific operation. Here’s an example of an instruction that might appear in a program:

            10110000

To you and me, this is only a series of 0s and 1s. To a CPU, however, this is an instruction to perform an operation.1 It is written in 0s and 1s because CPUs only understand instructions that are written in machine language, and machine language instructions always have an underlying binary structure.

A machine language instruction exists for each operation that a CPU is capable of performing. For example, there is an instruction for adding numbers, there is an instruction for subtracting one number from another, and so forth. The entire set of instructions that a CPU can execute is known as the CPU’s instruction set.

The machine language instruction that was previously shown is an example of only one instruction. It takes a lot more than one instruction, however, for the computer to do anything meaningful. Because the operations that a CPU knows how to perform are so basic in nature, a meaningful task can be accomplished only if the CPU performs many operations. For example, if you want your computer to calculate the amount of interest that you will earn from your savings account this year, the CPU will have to perform a large number of instructions, carried out in the proper sequence. It is not unusual for a program to contain thousands or even millions of machine language instructions.

Programs are usually stored on a secondary storage device such as a disk drive. When you install a program on your computer, the program is typically copied to your computer’s disk drive from a CD-ROM, or perhaps downloaded from a website.

Although a program can be stored on a secondary storage device such as a disk drive, it has to be copied into main memory, or RAM, each time the CPU executes it. For example, suppose you have a word processing program on your computer’s disk. To execute the program you use the mouse to double-click the program’s icon. This causes the program to be copied from the disk into main memory. Then, the computer’s CPU executes the copy of the program that is in main memory. This process is illustrated in Figure 1-16.

When a CPU executes the instructions in a program, it is engaged in a process that is known as the fetch-decode-execute cycle. This cycle, which consists of three steps, is repeated for each instruction in the program. The steps are:

1. Fetch A program is a long sequence of machine language instructions. The first step of the cycle is to fetch, or read, the next instruction from memory into the CPU.

2. Decode A machine language instruction is a binary number that represents a command that tells the CPU to perform an operation. In this step the CPU decodes the instruction that was just fetched from memory, to determine which operation it should perform.

3. Execute The last step in the cycle is to execute, or perform, the operation.

From Machine Language to Assembly Language

Computers can only execute programs that are written in machine language. As previously mentioned, a program can have thousands or even millions of binary instructions, and writing such a program would be very tedious and time consuming. Programming in machine language would also be very difficult because putting a 0 or a 1 in the wrong place will cause an error.

Although a computer’s CPU only understands machine language, it is impractical for people to write programs in machine language. For this reason, assembly language was created in the early days of computing2 as an alternative to machine language. Instead of using binary numbers for instructions, assembly language uses short words that are known as mnemonics. For example, in assembly language, the mnemonic add typically means to add numbers, mul typically means to multiply numbers, and mov typically means to move a value to a location in memory. When a programmer uses assembly language to write a program, he or she can write short mnemonics instead of binary numbers.

Assembly language programs cannot be executed by the CPU, however. The CPU only understands machine language, so a special program known as an assembler is used to translate an assembly language program to a machine language program. This process is shown in Figure 1-18. The machine language program that is created by the assembler can then be executed by the CPU.

High-Level Languages

Although assembly language makes it unnecessary to write binary machine language instructions, it is not without difficulties. Assembly language is primarily a direct substitute for machine language, and like machine language, it requires that you know a lot about the CPU. Assembly language also requires that you write a large number of instructions for even the simplest program. Because assembly language is so close in nature to machine language, it is referred to as a low-level language.

In the 1950s, a new generation of programming languages known as high-level languages began to appear. A high-level language allows you to create powerful and complex programs without knowing how the CPU works, and without writing large numbers of low-level instructions. In addition, most high-level languages use words that are easy to understand. For example, if a programmer were using COBOL (which was one of the early high-level languages created in the 1950s), he or she would write the following instruction to display the message Hello world on the computer screen:

     DISPLAY “Hello world”

Python is a modern, high-level programming language that we will use in this book. In Python you would display the message Hello world with the following instruction:

     print ‘Hello world’

Doing the same thing in assembly language would require several instructions, and intimate knowledge of how the CPU interacts with the computer’s output device. As you can see from this example, high-level languages allow programmers to concentrate on the tasks they want to perform with their programs rather than the details of how the CPU will execute those programs.

Since the 1950s, thousands of high-level languages have been created. Table 1-1 lists several of the more well-known languages.

Key Words, Operators, and Syntax: an Overview

Each high-level language has its own set of predefined words that the programmer must use to write a program. The words that make up a high-level programming language are known as key words or reserved words. Each key word has a specific meaning, and cannot be used for any other purpose. You previously saw an example of a Python statement that uses the key word print to print a message on the screen. Table 1-2 shows all of the Python key words.

In addition to key words, programming languages have operators that perform various operations on data. For example, all programming languages have math operators that perform arithmetic. In Python, as well as most other languages, the sign is an operator that adds two numbers. The following adds 12 and 75:
12 + 75

There are numerous other operators in the Python language, many of which you will learn about as you progress through this text.

In addition to key words and operators, each language also has its own syntax, which is a set of rules that must be strictly followed when writing a program.

The syntax rules dictate how key words, operators, and various punctuation characters must be used in a program. When you are learning a programming language, you must learn the syntax rules for that particular language. The individual instructions that you use to write a program in a high-level programming language are called statements. A programming statement can consist of key words, operators, punctuation, and other allowable programming elements, arranged in the proper sequence to perform an operation.

Compilers and Interpreters

Because the CPU understands only machine language instructions, programs that are written in a high-level language must be translated into machine language. Depending on the language that a program has been written in, the programmer will use either a compiler or an interpreter to make the translation.

A compiler is a program that translates a high-level language program into a separate machine language program. The machine language program can then be executed at any time it is needed. This is shown in Figure 1-19. As shown in the figure, compiling and executing are two different processes.

The Python language uses an interpreter, which is a program that both translates and executes the instructions in a high-level language program. As the interpreter reads each individual instruction in the program, it converts it to machine language instructions and then immediately executes them. This process repeats for every instruction in the program. This process is illustrated in Figure 1-20. Because interpreters combine translation and execution, they typically do not create separate machine language programs.

The statements that a programmer writes in a high-level language are called source code, or simply code. Typically, the programmer types a program’s code into a text editor and then saves the code in a file on the computer’s disk. Next, the programmer uses a compiler to translate the code into a machine language program, or an interpreter to translate and execute the code. If the code contains a syntax error, however, it cannot be translated. A syntax error is a mistake such as a misspelled key word, a missing punctuation character, or the incorrect use of an operator. When this happens the compiler or interpreter displays an error message indicating that the program contains a syntax error. The programmer corrects the error and then attempts once again to translate the program.

Checkpoint

  1. A CPU understands instructions that are written only in what language?
  2. A program has to be copied into what type of memory each time the CPU executes it?
  3. When a CPU executes the instructions in a program, it is engaged in what process?
  4. What is assembly language?
  5. What type of programming language allows you to create powerful and complex programs without knowing how the CPU works?
  6. Each language has a set of rules that must be strictly followed when writing a program. What is this set of rules called?
  7. What do you call a program that translates a high-level language program into a separate machine language program?
  8. What do you call a program that both translates and executes the instructions in a high-level language program?
  9. What type of mistake is usually caused by a misspelled key word, a missing punctuation character, or the incorrect use of an operator?

Using Python

The Python interpreter can run Python programs that are saved in files, or interactively execute Python statements that are typed at the keyboard. Python comes with a program named IDLE that simplifies the process of writing, executing, and testing programs.

Installing Python

Before you can try any of the programs shown in this book, or write any programs of your own, you need to make sure that Python is installed on your computer and properly configured. If you are working in a computer lab, this has probably been done already. If you are using your own computer, you can follow the instructions in Appendix A to install Python from the accompanying CD.

The Python Interpreter

You learned earlier that Python is an interpreted language. When you install the Python language on your computer, one of the items that is installed is the Python interpreter. The Python interpreter is a program that can read Python programming statements and execute them. (Sometimes we will refer to the Python interpreter simply as the interpreter.)

You can use the interpreter in two modes: interactive mode and script mode. In interactive mode, the interpreter waits for you to type Python statements on the keyboard. Once you type a statement, the interpreter executes it and then waits for you to type another statement. In script mode, the interpreter reads the contents of a file that contains Python statements. Such a file is known as a Python program or a Python script. The interpreter executes each statement in the Python program as it reads it.

Interactive Mode

Once Python has been installed and set up on your system, you start the interpreter in interactive mode by going to the operating system’s command line and typing the following command:
python

If you are using Windows, you can alternatively click the Start button, then All Programs. You should see a program group named something like Python 2.5. (The “2.5” is the version of Python that is installed. At the time this is being written, Python 2.5 is the latest version.) Inside this program group, you should see an item named Python (command line). Clicking this menu item will start the Python interpreter in interactive mode.

When the Python interpreter starts in interactive mode, you will see something like the following displayed in a console window:
Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08) [MSC                    v.1310 32 bit (Intel)] on win32 Type “help”, “copyright”,                     “credits” or “license” for more information. >>>

The >>> that you see is a prompt that indicates the interpreter is waiting for you to type a Python statement. Let’s try it out. One of the simplest statements that you can write in Python is a print statement, which causes a message to be displayed on the screen. For example, the following statement causes the message Python programming is fun! to be displayed:
print ‘Python programming is fun!’

Notice that after the word print, we have written Python programming is fun! inside a set of single-quote marks. The quote marks are necessary, but they will not be displayed. They simply mark the beginning and the end of the text that we wish to display.
Here is an example of how you would type this print statement at the interpreter’s prompt:
>>> print ‘Python programming is fun!’

After typing the statement you press the Enter key and the Python interpreter executes the statement, as shown here:
>>> print ‘Python programming is fun!’ Python programming          is fun!>>>

After the message is displayed, the >>> prompt appears again, indicating that the interpreter is waiting for you to enter another statement. Let’s look at another example. In the following sample session we have entered two print statements.
>>> print ‘To be or not to be’
To be or not to be
>>> print ‘That is the question.’
That is the question.
>>>

If you incorrectly type a statement in interactive mode, the interpreter will display an error message. This will make interactive mode useful to you while you learn Python. As you learn new parts of the Python language, you can try them out in interactive mode and get immediate feedback from the interpreter.

To quit the Python interpreter in interactive mode on a Windows computer, press Ctrl-Z (pressing both keys together) followed by Enter. On a Mac, Linux, or UNIX computer, press Ctrl-D.

Writing Python Programs and Running Them in Script Mode

Although interactive mode is useful for testing code, the statements that you enter in interactive mode are not saved as a program. They are simply executed and their results displayed on the screen. If you want to save a set of Python statements as a program, you save those statements in a file. Then, to execute the program, you use the Python interpreter in script mode.

For example, suppose you want to write a Python program that displays the following three lines of text:

Nudge nudge
Wink wink
Know what I mean?

To write the program you would use a simple text editor like Notepad (which is installed on all Windows computers) to create a file containing the following statements:

print ‘Nudge nudge’
print ‘Wink wink’
print ‘Know what I mean?’

When you save a Python program, you give it a name that ends with the .py extension, which identifies it as a Python program. For example, you might save the program previously shown with the name test.py. To run the program you would go to the directory in which the file is saved and type the following command at the operating system command line:
python test.py

This starts the Python interpreter in script mode and causes it to execute the statements in the file test.py. When the program finishes executing, the Python interpreter exits.

The IDLE Programming Environment

The previous sections described how the Python interpreter can be started in interactive mode or script mode at the operating system command line. As an alternative, you can use an integrated development environment, which is a single program that gives you all of the tools you need to write, execute, and test a program.

Recent versions of Python include a program named IDLE, which is automatically installed when the Python language is installed. (IDLE stands for Integrated DeveLopment Environment.) When you run IDLE, the window shown in Figure 1-21 appears. Notice that the >>> prompt appears in the IDLE window, indicating that the interpreter is running in interactive mode. You can type Python statements at this prompt and see them executed in the IDLE window.

IDLE also has a built-in text editor with features specifically designed to help you write Python programs. For example, the IDLE editor “colorizes” code so that key words and other parts of a program are displayed in their own distinct colors. This helps make programs easier to read. In IDLE you can write programs, save them to disk, and execute them. Appendix B provides a quick introduction to IDLE, and leads you through the process of creating, saving, and executing a Python program.

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