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Chapter 5 - Functions and variables OUR FIRST USER DEFINED FUNCTION Load and examine the file SUMSQRES.C for an example of a C program with functions. Actually this is not the first function we have encountered because the "main" program we have been using all along is technically a function, as is the "printf" function. The "printf" function is a library function that was supplied with your compiler. Notice the executable part of this program. It begins with a line that simply says "header()", which is the way to call any function. The parentheses are required because the C compiler uses them to determine that it is a function call and not simply a misplaced variable. When the program comes to this line of code, the function named "header" is called, its statements are executed, and control returns to the statement following this call. Continuing on we come to a "for" loop which will be executed 7 times and which calls another function named "square" each time through the loop, and finally a function named "ending" will be called and executed. For the moment ignore the "index" in the parentheses of the call to "square". We have seen that this program therefore calls a header, 7 square calls, and an ending. Now we need to define the functions. DEFINING THE FUNCTIONS Following the main program you will see another program that follows all of the rules set forth so far for a "main" program except that it is named "header()". This is the function which is called from within the main program. Each of these statements are executed, and when they are all complete, control returns to the main program. The first statement sets the variable "sum" equal to zero because we will use it to accumulate a sum of squares. Since the variable "sum" is defined as an integer type variable prior to the main program, it is available to be used in any of the following functions. It is called a "global" variable, and it's scope is the entire program and all functions. More will be said about the scope of variables at the end of this chapter. The next statement outputs a header message to the monitor. Program control then returns to the main program since there are no additional statements to execute in this function. It should be clear to you that the two executable lines from this function could be moved to the main program, replacing the header call, and the program would do exactly the same thing that it does as it is now written. This does not minimize the value of functions, it merely illustrates Page 29 Chapter 5 - Functions and variables the operation of this simple function in a simple way. You will find functions to be very valuable in C programming. PASSING A VALUE TO A FUNCTION Going back to the main program, and the "for" loop specifically, we find the new construct from the end of the last lesson used in the last part of the for loop, namely the "index++". You should get used to seeing this, as you will see it a lot in C programs. In the call to the function "square", we have an added feature, namely the variable "index" within the parentheses. This is an indication to the compiler that when you go to the function, you wish to take along the value of index to use in the execution of that function. Looking ahead at the function "square", we find that another variable name is enclosed in its parentheses, namely the variable "number". This is the name we prefer to call the variable passed to the function when we are in the function. We can call it anything we wish as long as it follows the rules of naming an identifier. Since the function must know what type the variable is, it is defined following the function name but before the opening brace of the function itself. Thus, the line containing "int number;" tells the function that the value passed to it will be an integer type variable. With all of that out of the way, we now have the value of index from the main program passed to the function "square", but renamed "number", and available for use within the function. Following the opening brace of the function, we define another variable "numsq" for use only within the function itself, (more about that later) and proceed with the required calculations. We set "numsq" equal to the square of number, then add numsq to the current total stored in "sum". Remember that "sum += numsq" is the same as "sum = sum + numsq" from the last lesson. We print the number and its square, and return to the main program. MORE ABOUT PASSING A VALUE TO A FUNCTION When we passed the value of "index" to the function, a little more happened than meets the eye. We did not actually pass the value of index to the function, we actually passed a copy of the value. In this way the original value is protected from accidental corruption by a called function. We could have modified the variable "number" in any way we wished in the function "square", and when we returned to the main program, "index" would not have been modified. We thus protect the value of a variable in the main program from being accidentally corrupted, but we Page 30 Chapter 5 - Functions and variables cannot return a value to the main program from a function using this technique. We will find a well defined method of returning values to the main program or to any calling function when we get to arrays and another method when we get to pointers. Until then the only way you will be able to communicate back to the calling function will be with global variables. We have already hinted at global variables above, and will discuss them in detail later in this chapter. Continuing in the main program, we come to the last function call, the call to "ending". This call simply calls the last function which has no local variables defined. It prints out a message with the value of "sum" contained in it to end the program. The program ends by returning to the main program and finding nothing else to do. Compile and run this program and observe the output. NOW TO CONFESS A LITTLE LIE I told you a short time ago that the only way to get a value back to the main program was through use of a global variable, but there is another way which we will discuss after you load and display the file named SQUARES.C. In this file we will see that it is simple to return a single value from a called function to the calling function. But once again, it is true that to return more than one value, we will need to study either arrays or pointers. In the main program, we define two integers and begin a "for" loop which will be executed 8 times. The first statement of the for loop is "y = squ(x);", which is a new and rather strange looking construct. From past experience, we should have no trouble understanding that the "squ(x)" portion of the statement is a call to the "squ" function taking along the value of "x" as a variable. Looking ahead to the function itself we find that the function prefers to call the variable "in" and it proceeds to square the value of "in" and call the result "square". Finally, a new kind of a statement appears, the "return" statement. The value within the parentheses is assigned to the function itself and is returned as a usable value in the main program. Thus, the function call "squ(x)" is assigned the value of the square and returned to the main program such that "y" is then set equal to that value. If "x" were therefore assigned the value 4 prior to this call, "y" would then be set to 16 as a result of this line of code. Another way to think of this is to consider the grouping of characters "squ(x)" as another variable with a value that is the square of "x", and this new variable can Page 31 Chapter 5 - Functions and variables be used any place it is legal to use a variable of its type. The values of "x" and "y" are then printed out. To illustrate that the grouping of "squ(x)" can be thought of as just another variable, another "for" loop is introduced in which the function call is placed in the print statement rather than assigning it to a new variable. One last point must be made, the type of variable returned must be defined in order to make sense of the data, but the compiler will default the type to integer if none is specified. If any other type is desired, it must be explicitly defined. How to do this will be demonstrated in the next example program. Compile and run this program. FLOATING POINT FUNCTIONS Load the program FLOATSQ.C for an example of a function with a floating point type of return. It begins by defining a global floating point variable we will use later. Then in the "main" part of the program, an integer is defined, followed by two floating point variables, and then by two strange looking definitions. The expressions "sqr()" and "glsqr()" look like function calls and they are. This is the proper way in C to define that a function will return a value that is not of the type "int", but of some other type, in this case "float". This tells the compiler that when a value is returned from either of these two functions, it will be of type "float". Now refer to the function "sqr" near the center of the listing and you will see that the function name is preceded by the name "float". This is an indication to the compiler that this function will return a value of type "float" to any program that calls it. The function is now compatible with the call to it. The line following the function name contains "float inval;", which indicates to the compiler that the variable passed to this function from the calling program will be of type "float". The next function, namely "glsqr", will also return a "float" type variable, but it uses a global variable for input. It also does the squaring right within the return statement and therefore has no need to define a separate variable to store the product. The overall structure of this program should pose no problem and will not be discussed in any further detail. As Page 32 Chapter 5 - Functions and variables is customary with all example programs, compile and run this program. There will be times that you will have a need for a function to return a pointer as a result of some calculation. There is a way to define a function so that it does just that. We haven't studied pointers yet, but we will soon. This is just a short preview of things to come. SCOPE OF VARIABLES Load the next program, SCOPE.C, and display it for a discussion of the scope of variables in a program. The first variable defined is a global variable "count" which is available to any function in the program since it is defined before any of the functions. In addition, it is always available because it does not come and go as the program is executed. (That will make sense shortly.) Farther down in the program, another global variable named "counter" is defined which is also global but is not available to the main program since it is defined following the main program. A global variable is any variable that is defined outside of any function. Note that both of these variables are sometimes referred to as external variables because they are external to any functions. Return to the main program and you will see the variable "index" defined as an integer. Ignore the word "register" for the moment. This variable is only available within the main program because that is where it is defined. In addition, it is an "automatic" variable, which means that it only comes into existence when the function in which it is contained is invoked, and ceases to exist when the function is finished. This really means nothing here because the main program is always in operation, even when it gives control to another function. Another integer is defined within the "for" braces, namely "stuff". Any pairing of braces can contain a variable definition which will be valid and available only while the program is executing statements within those braces. The variable will be an "automatic" variable and will cease to exist when execution leaves the braces. This is convenient to use for a loop counter or some other very localized variable. MORE ON "AUTOMATIC" VARIABLES Observe the function named "head1". It contains a variable named "index", which has nothing to do with the "index" of the main program, except that both are automatic variables. When the program is not actually executing Page 33 Chapter 5 - Functions and variables statements in this function, this variable named "index" does not even exist. When "head1" is called, the variable is generated, and when "head1" completes its task, the variable "index" is eliminated completely from existence. Keep in mind however that this does not affect the variable of the same name in the main program, since it is a completely separate entity. Automatic variables therefore, are automatically generated and disposed of when needed. The important thing to remember is that from one call to a function to the next call, the value of an automatic variable is not preserved and must therefore be reinitialized. WHAT ARE STATIC VARIABLES? An additional variable type must be mentioned at this point, the "static" variable. By putting the reserved word "static" in front of a variable declaration within a function, the variable or variables in that declaration are static variables and will stay in existence from call to call of the particular function. By putting the same reserved word in front of an external variable, one outside of any function, it makes the variable private and not accessible to use in any other file. This implies that it is possible to refer to external variables in other separately compiled files, and that is true. Examples of this usage will be given in chapter 14 of this tutorial. USING THE SAME NAME AGAIN Refer to the function named "head2". It contains another definition of the variable named "count". Even though "count" has already been defined as a global variable, it is perfectly all right to reuse the name in this function. It is a completely new variable that has nothing to do with the global variable of the same name, and causes the global variable to be unavailable in this function. This allows you to write programs using existing functions without worrying about what names were used for variables in the functions because there can be no conflict. You only need to worry about the variables that interface with the functions. WHAT IS A REGISTER VARIABLE? Now to fulfill a promise made earlier about what a register variable is. A computer can keep data in a register or in memory. A register is much faster in Page 34 Chapter 5 - Functions and variables operation than memory but there are very few registers available for the programmer to use. If there are certain variables that are used extensively in a program, you can designate that those variables are to be stored in a register if possible in order to speed up the execution of the program. Depending on the computer and the compiler, a small number of register variables may be allowed and are designated by putting the word "register" in front of the desired variable. Check your compiler documentation for the availability of this feature and the number of register variables. Most compilers that do not have any register variables available, will simply ignore the word "register" and run normally, keeping all variables in memory. Register variables are only available for use with integer and character type variables. This may or may not include some of the other integer-like variables such as unsigned, long, or short. Check the documentation for your compiler. WHERE DO I DEFINE VARIABLES? Now for a refinement on a general rule stated earlier. When you have variables brought to a function as arguments to the function, they are defined immediately after the function name and prior to the opening brace for the program. Other variables used in the function are defined at the beginning of the function, immediately following the opening brace of the function, and before any executable statements. STANDARD FUNCTION LIBRARIES Every compiler comes with some standard predefined functions which are available for your use. These are mostly input/output functions, character and string manipulation functions, and math functions. We will cover most of these in subsequent chapters. In addition, most compilers have additional functions predefined that are not standard but allow the programmer to get the most out of his particular computer. In the case of the IBM-PC and compatibles, most of these functions allow the programmer to use the BIOS services available in the operating system, or to write directly to the video monitor or to any place in memory. These will not be covered in any detail as you will be able to study the unique aspects of your compiler on your own. Many of these kinds of functions are used in the example programs in chapter 14. Page 35 Chapter 5 - Functions and variables WHAT IS RECURSION? Recursion is another of those programming techniques that seem very intimidating the first time you come across it, but if you will load and display the example program named RECURSON.C, we will take all of the mystery out of it. This is probably the simplest recursive program that it is possible to write and it is therefore a stupid program in actual practice, but for purposes of illustration, it is excellent. Recursion is nothing more than a function that calls itself. It is therefore in a loop which must have a way of terminating. In the program on your monitor, the variable "index" is set to 8, and is used as the argument to the function "count_dn". The function simply decrements the variable, prints it out in a message, and if the variable is not zero, it calls itself, where it decrements it again, prints it, etc. etc. etc. Finally, the variable will reach zero, and the function will not call itself again. Instead, it will return to the prior time it called itself, and return again, until finally it will return to the main program and will return to DOS. For purposes of understanding you can think of it as having 8 copies of the function "count_dn" available and it simply called all of them one at a time, keeping track of which copy it was in at any given time. That is not what actually happened, but it is a reasonable illustration for you to begin understanding what it was really doing. WHAT DID IT DO? A better explanation of what actually happened is in order. When you called the function from itself, it stored all of the variables and all of the internal flags it needs to complete the function in a block somewhere. The next time it called itself, it did the same thing, creating and storing another block of everything it needed to complete that function call. It continued making these blocks and storing them away until it reached the last function when it started retrieving the blocks of data, and using them to complete each function call. The blocks were stored on an internal part of the computer called the "stack". This is a part of memory carefully organized to store data just as described above. It is beyond the scope of this tutorial to describe the stack in detail, but it would be good for your programming experience to read some material describing the stack. A stack is used in nearly all modern computers for internal housekeeping chores. Page 36 Chapter 5 - Functions and variables In using recursion, you may desire to write a program with indirect recursion as opposed to the direct recursion described above. Indirect recursion would be when a function "A" calls the function "B", which in turn calls "A", etc. This is entirely permissible, the system will take care of putting the necessary things on the stack and retrieving them when needed again. There is no reason why you could not have three functions calling each other in a circle, or four, or five, etc. The C compiler will take care of all of the details for you. The thing you must remember about recursion is that at some point, something must go to zero, or reach some predefined point to terminate the loop. If not, you will have an infinite loop, and the stack will fill up and overflow, giving you an error and stopping the program rather abruptly. ANOTHER EXAMPLE OF RECURSION The program named BACKWARD.C is another example of recursion, so load it and display it on your screen. This program is similar to the last one except that it uses a character array. Each successive call to the function named "forward_and_backward" causes one character of the message to be printed. Additionally, each time the function ends, one of the characters is printed again, this time backwards as the string of recursive function calls is retraced. Don't worry about the character array defined in line 3 or the other new material presented here. After you complete chapter 7 of this tutorial, this program will make sense. It was felt that introducing a second example of recursion was important so this file is included here. One additional feature is built into this program. If you observe the two calls to the function, and the function itself, you will see that the function name is spelled three different ways in the last few characters. The compiler doesn't care how they are spelled because it only uses the first 8 characters of the function name so as far as it is concerned, the function is named "forward_". The remaining characters are simply ignored. If your compiler uses more that 8 characters as being significant, you will need to change two of the names so that all three names are identical. Compile and run this program and observe the results. Page 37 Chapter 5 - Functions and variables PROGRAMMING EXERCISES 1. Rewrite TEMPCONV.C, from an earlier chapter, and move the temperature calculation to a function. 2. Write a program that writes your name on the monitor 10 times by calling a function to do the writing. Move the called function ahead of the "main" function to see if your compiler will allow it. Page 38