Collection of Hack-Phreak Scene Programs

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Chapter 2 - Getting started in C YOUR FIRST C PROGRAM The best way to get started with C is to actually look at a program, so load the file named TRIVIAL.C and display it on the monitor. You are looking at the simplest possible C program. There is no way to simplify this program or to leave anything out. Unfortunately, the program doesn't do anything. The word "main" is very important, and must appear once, and only once in every C program. This is the point where execution is begun when the program is run. We will see later that this does not have to be the first statement in the program but it must exist as the entry point. Following the "main" program name is a pair of parentheses which are an indication to the compiler that this is a function. We will cover exactly what a function is in due time. For now, I suggest that you simply include the pair of parentheses. The two curly brackets, properly called braces, are used to define the limits of the program itself. The actual program statements go between the two braces and in this case, there are no statements because the program does absolutely nothing. You can compile and run this program, but since it has no executable statements, it does nothing. Keep in mind however, that it is a valid C program. A PROGRAM THAT DOES SOMETHING For a much more interesting program, load the program named WRTSOME.C and display it on your monitor. It is the same as the previous program except that it has one executable statement between the braces. The executable statement is a call to a function supplied as a part of your C library. Once again, we will not worry about what a function is, but only how to use this one named "printf". In order to output text to the monitor, it is put within the function parentheses and bounded by quotation marks. The end result is that whatever is included between the quotation marks will be displayed on the monitor when the program is run. Notice the semi-colon at the end of the line. C uses a semi-colon as a statement terminator, so the semi-colon is required as a signal to the compiler that this line is complete. This program is also executable, so you can compile and run it to see if it does what you think it should. Page 6 Chapter 2 - Getting started in C ANOTHER PROGRAM WITH MORE OUTPUT Load the program WRTMORE.C and display it on your monitor for an example of more output and another small but important concept. You will see that there are four program statements in this program, each one being a call to the function "printf". The top line will be executed first, then the next, and so on, until the fourth line is complete. The statements are executed in order from top to bottom. Notice the funny character near the end of the first line, namely the backslash. The backslash is used in the printf statement to indicate that a special control character is following. In this case, the "n" indicates that a "newline" is requested. This is an indication to return the cursor to the left side of the monitor and move down one line. It is commonly referred to as a carriage return/line feed. Any place within text that you desire, you can put a newline character and start a new line. You could even put it in the middle of a word and split the word between two lines. The C compiler considers the combination of the backslash and letter n as one character. A complete description of this program is now possible. The first printf outputs a line of text and returns the carriage. The second printf outputs a line but does not return the carriage so that the third line is appended to the second, then followed by two carriage returns, resulting in a blank line. Finally the fourth "printf" outputs a line followed by a carriage return and the program is complete. Compile and run this program to see if it does what you expect it to do. It would be a good idea at this time for you to experiment by adding additional lines of printout to see if you understand how the statements really work. LETS PRINT SOME NUMBERS Load the file named ONEINT.C and display it on the monitor for our first example of how to work with data in a C program. The entry point "main" should be clear to you by now as well as the beginning brace. The first new thing we encounter is the line containing "int index;", which is used to define an integer variable named "index". The "int" is a reserved word in C, and can therefore not be used for anything else. It defines a variable that can have a value from -32768 to 32767 in most C compilers for microcomputers. The variable name, "index", can be any name that follows the rules for an identifier and is not one of the reserved words for C. The final character on the line, the semi-colon, is the statement terminator used in C. Page 7 Chapter 2 - Getting started in C Note that, even though we have defined a variable, we have not yet assigned a value to it. We will see in a later chapter that additional integers could also be defined on the same line, but we will not complicate the present situation. Observing the main body of the program, you will notice that there are three statements that assign a value to the variable "index", but only one at a time. The first one assigns the value of 13 to "index", and its value is printed out. (We will see how shortly.) Later, the value of 27 is assigned to "index", and finally 10 is assigned to it, each value being printed out. It should be intuitively clear that "index" is indeed a variable and can store many different values. Please note that many times the words "printed out" are used to mean "displayed on the monitor". You will find that in many cases experienced programmers take this liberty, probably due to the "printf" function being used for monitor display. HOW DO WE PRINT NUMBERS To keep our promise, let's return to the "printf" statements for a definition of how they work. Notice that they are all identical and that they all begin just like the "printf" statements we have seen before. The first difference occurs when we come to the % character. This is a special character that signals the output routine to stop copying characters to the output and do something different, namely output a variable. The % sign is used to signal the output of many different types of variables, but we will restrict ourselves to only one for this example. The character following the % sign is a "d", which signals the output routine to get a decimal value and output it. Where the decimal value comes from will be covered shortly. After the "d", we find the familiar \n, which is a signal to return the video "carriage", and the closing quotation mark. All of the characters between the quotation marks define the pattern of data to be output by this statement, and after the pattern, there is a comma followed by the variable name "index". This is where the "printf" statement gets the decimal value which it will output because of the "%d" we saw earlier. We could add more "%d" output field descriptors within the brackets and more variables following the description to cause more data to be printed with one statement. Keep in mind however, that it is important that the number of field descriptors and the number of variable definitions must be the same or the runtime system will get confused and probably quit with a runtime error. Page 8 Chapter 2 - Getting started in C Much more will be covered at a later time on all aspects of input and output formatting. A reasonably good grasp of these fundamentals are necessary in order to understand the following lessons. It is not necessary to understand everything about output formatting at this time, only a fair understanding of the basics. Compile and run ONEINT.C and observe the output. HOW DO WE ADD COMMENTS IN C Load the file COMMENTS.C and observe it on your monitor for an example of how comments can be added to a C program. Comments are added to make a program more readable to you but the compiler must ignore the comments. The slash star combination is used in C for comment delimiters. They are illustrated in the program at hand. Please note that the program does not illustrate good commenting practice, but is intended to illustrate where comments can go in a program. It is a very sloppy looking program. The first slash star combination introduces the first comment and the star slash at the end of the first line terminates this comment. Note that this comment is prior to the beginning of the program illustrating that a comment can precede the program itself. Good programming practice would include a comment prior to the program with a short introductory description of the program. The next comment is after the "main()" program entry point and prior to the opening brace for the program code itself. The third comment starts after the first executable statement and continues for four lines. This is perfectly legal because a comment can continue for as many lines as desired until it is terminated. Note carefully that if anything were included in the blank spaces to the left of the three continuation lines of the comment, it would be part of the comment and would not be compiled. The last comment is located following the completion of the program, illustrating that comments can go nearly anywhere in a C program. Experiment with this program by adding comments in other places to see what will happen. Comment out one of the printf statements by putting comment delimiters both before and after it and see that it does not get printed out. Comments are very important in any programming language because you will soon forget what you did and why you did it. It will be much easier to modify or fix a well Page 9 Chapter 2 - Getting started in C commented program a year from now than one with few or no comments. You will very quickly develop your own personal style of commenting. Some C compilers will allow you to "nest" comments which can be very handy if you need to "comment out" a section of code during debugging. Since nested comments are not a part of the proposed ANSI standard, none will be used in this tutorial. Check the documentation for your compiler to see if they are permitted with your implementation of C. GOOD FORMATTING STYLE Load the file GOODFORM.C and observe it on your monitor. It is an example of a well formatted program. Even though it is very short and therefore does very little, it is very easy to see at a glance what it does. With the experience you have already gained in this tutorial, you should be able to very quickly grasp the meaning of the program in it's entirety. Your C compiler ignores all extra spaces and all carriage returns giving you considerable freedom concerning how you format your program. Indenting and adding spaces is entirely up to you and is a matter of personal taste. Compile and run the program to see if it does what you expect it to do. Now load and display the program UGLYFORM.C and observe it. How long will it take you to figure out what this program will do? It doesn't matter to the compiler which format style you use, but it will matter to you when you try to debug your program. Compile this program and run it. You may be surprised to find that it is the same program as the last one, except for the formatting. Don't get too worried about formatting style yet. You will have plenty of time to develop a style of your own as you learn the language. Be observant of styles as you see C programs in magazines, books, and other publications. This should pretty well cover the basic concepts of programming in C, but as there are many other things to learn, we will forge ahead to additional program structure. PROGRAMMING EXERCISES 1. Write a program to display your name on the monitor. 2. Modify the program to display your address and phone number on separate lines by adding two additional "printf" statements. Page 10