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Chapter 1

What Is a Program?

In This Chapter

▶ Understanding programs

▶ Writing your first “program”

▶ Looking at computer languages

n this chapter, you will learn what a program is and what it means to write

a program. You’ll practice on a Human Computer. You’ll then see some

program snippets written for a real computer. Finally, you’ll see your first

code snippet written in C++.

Up until now all of the programs running on your computer were written by

someone else. Very soon now, that won’t be true anymore. You will be join-

ing the ranks of the few, the proud: the programmers.

How Does My Son Differ

from a Computer?

A computer is an amazingly fast but incredibly stupid machine. A computer

can do anything you tell it (within reason), but it does exactly what it’s told —

nothing more and nothing less.

In this respect, a computer is almost the exact opposite of a human: humans

respond intuitively. When I was learning a second language, I learned that it

wasn’t enough to understand what was being said — it’s just as important

and considerably more difficult to understand what was left unsaid. This is

information that the speaker shares with the listener through common expe-

rience or education — things that don’t need to be said.

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Summary of content (12 pages)

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Chapter 1 AL What Is a Program? In This Chapter RI ▶ Understanding programs ▶ Writing your first “program” MA TE ▶ Looking at computer languages I TE D n this chapter, you will learn what a program is and what it means to write a program. You’ll practice on a Human Computer. You’ll then see some program snippets written for a real computer. Finally, you’ll see your first code snippet written in C++. GH Up until now all of the programs running on your computer were written by someone else.
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10 Part I: Let’s Get Started For example, I say things to my son like, “Wash the dishes” (for all the good it does me). This seems like clear enough instructions, but the vast majority of the information contained in that sentence is implied and unspoken. Let’s assume that my son knows what dishes are and that dirty dishes are normally in the sink. But what about knives and forks? After all, I only said dishes, I didn’t say anything about eating utensils, and don’t even get me started on glassware.
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Chapter 1: What Is a Program? On the other hand, humans are very good at certain types of processing that computers do poorly, if at all. For example, humans are very good at pulling the meaning out of a sentence garbled by large amounts of background noise. By contrast, digital cell phones have the infuriating habit of just going silent whenever the noise level gets above a built-in threshold.
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12 Part I: Let’s Get Started As detailed as these instructions might seem to be, this is not a program. This is called an algorithm. An algorithm is a description of the steps to be performed, usually at a high level of abstraction. An algorithm is detailed but general. I could use this algorithm to repair any of the flat tires that I have experienced or ever will experience. But an algorithm does not contain sufficient detail for even our intentionally obtuse human computer to perform the task.
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Chapter 1: What Is a Program? The program Now it’s time to convert the algorithm, written in everyday English, into a program written in Tire Changing Language. Take the phrase, “Remove the lug nuts.” Actually, quite a bit is left unstated in that sentence. The word “remove” is not in the processor’s vocabulary. In addition, no mention is made of the wrench at all. The following steps implement the phrase “Remove a lug nut” using only the verbs and nouns contained in Tire Changing Language: 1. Grab wrench.
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14 Part I: Let’s Get Started 3. { 4. Move wrench to lug nut. 5. Turn wrench counterclockwise five times. 6. } 7. Move wrench to toolbox. 8. Release wrench. The program starts with Step 1 just as before and grabs a wrench. In the second step, however, before the program waves the wrench uselessly around an empty bolt, it checks to see if a lug nut is present. If so, flow continues on with Steps 3, 4, and 5 as before.
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Chapter 1: What Is a Program? 3. { 4. Move wrench to lug nut. 5. While (lug nut attached to car) 6. { 7. 8. Turn wrench counterclockwise one turn. } 9. } 10. Move wrench to toolbox. 11. Release wrench. Here the program flows from Step 1 through Step 4 just as before. In Step 5, however, the processor must make a decision: Is the lug nut attached? On the first pass, we will assume that the answer is yes so that the processor will execute Step 7 and turn the wrench counterclockwise one turn.
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16 Part I: Let’s Get Started 6. Move wrench to lug nut. 7. While (lug nut attached to car) 8. { 9. Turn wrench counterclockwise one turn. 10. 11. } } 12. } 13. Move wrench to toolbox. 14. Release wrench. This program begins just as before with the grabbing of a wrench from the toolbox. Beginning with Step 2, however, the program loops through Step 12 for each lug nut bolt on the wheel. Notice how Steps 7 through 10 are still repeated for each wheel. This is known as a nested loop.
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Chapter 1: What Is a Program? The computer will not do anything that it hasn’t already been programmed for. The creation of a Tire Changing Language was not enough to replace my flat tire — someone had to write the program instructions to map out step by step what the computer will do. And writing a real-world program designed to handle all of the special conditions that can arise is not an easy task. Writing an industrial-strength program is probably the most challenging enterprise you can undertake.
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18 Part I: Let’s Get Started The programs that people write are known as source code because they are the source of all evil. The ones and zeros that the computer actually executes are called object code because they are the object of so much frustration. The following represents the first few assembler instructions from the Conversion program when compiled to run on an Intel processor executing Windows. This is the same information previously shown in binary form.
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Chapter 1: What Is a Program? INPUT CELSIUS_TEMP SET FAHRENHEIT_TEMP TO CELSIUS_TEMP * 9/5 + 32 WRITE FAHRENHEIT_TEMP The first line of this program reads a number from the keyboard or a file and stores it into the variable CELSIUS_TEMP. The next line multiplies this number by 9⁄5 and adds 32 to the result to calculate the equivalent temperature in Fahrenheit. The program stores this result into a variable called FAHRENHEIT_TEMP. The last line of the program writes this converted value to the display.
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20 Part I: Let’s Get Started In addition, C++ is efficient. The more things a high level language tries to do automatically to make your programming job easier, the less efficient the machine code generated tends to be. That doesn’t make much of a difference for a small program like most of those in this book, but it can make a big difference when manipulating large amounts of data, like moving pixels around on the screen, or when you want blazing real-time performance.