C Tutorial - Chapter 3

PROGRAM CONTROL

THE WHILE LOOP

The C programming language has several structures for looping and conditional branching. We will cover them all in this chapter and we will begin with the while loop.

The while loop continues to loop while some condition is true. When the condition becomes false, the looping is discontinued. It therefore does just what it says it does, the name of the loop being very descriptive.

Example program ------> WHILE.C

Load the program WHILE.C and display it for an example of a while loop. We begin with a comment and the program entry point main(), then go on to define an integer variable named count within the body of the program. The variable is set to zero and we come to the while loop itself. The syntax of a while loop is just as shown here. The keyword while is followed by an expression of something in parentheses, followed by a compound statement bracketed by braces. As long as the expression in the parenthesis is true, all statements within the braces will be repeatedly executed. In this case, since the variable count is incremented by one every time the statements are executed, it will eventually reach 6. At that time the statement will not be executed because count is not less than 6, and the loop will be terminated. The program control will resume at the statement following the statements in braces.

We will cover the compare expression, the one in parentheses, in the next chapter. Until then, simply accept the expressions for what you think they should do and you will be correct for these simple cases.

Several things must be pointed out regarding the while loop. First, if the variable count were initially set to any number greater than 5, the statements within the loop would not be executed at all, so it is possible to have a while loop that never is executed. Secondly, if the variable were not incremented in the loop, then in this case, the loop would never terminate, and the program would never complete. Finally, if there is only one statement to be executed within the loop, it does not need delimiting braces but can stand alone.

Compile and run this program after you have studied it enough to assure yourself that you understand its operation completely. Note that the result of execution is given for this program, (and will be given for all of the remaining example programs in this tutorial) so you do not need to compile and execute every program to see the results. Be sure to compile and execute some of the programs however, to gain experience with your compiler.

You should make some modifications to any programs that are not completely clear to you and compile them until you understand them completely. The best way to learn is to try various modifications yourself.

We will continue to ignore the #include statement and the return statement in the example programs in this chapter. We will define them completely later in this tutorial.

THE DO-WHILE LOOP

Example program ------> DOWHILE.C

A variation of the while loop is illustrated in the program DOWHILE.C, which you should load and display. This program is nearly identical to the last one except that the loop begins with the keyword do, followed by a compound statement in braces, then the keyword while, and finally an expression in parentheses. The statements in the braces are executed repeatedly as long as the expression in the parentheses is true. When the expression in parentheses becomes false, execution is terminated, and control passes to the statements following this statement.

Several things must be pointed out regarding the do-while loop. Since the test is done at the end of the loop, the statements in the braces will always be executed at least once. Secondly, if the variable i were not changed within the loop, the loop would never terminate, and hence the program would never terminate.

It should come as no surprise to you that these loops can be nested. That is, one loop can be included within the compound statement of another loop, and the nesting level has no limit. This will be illustrated later.

Compile and run this program to see if it does what you think it should do.

THE FOR LOOP

Example program ------> FORLOOP.C

Load and display the file named FORLOOP.C on your monitor for an example of a program with a for loop. The for loop consists of the keyword for followed by a rather large expression in parentheses. This expression is really composed of three fields separated by semi-colons. The first field contains the expression "index = 0" and is an initializing field. Any expressions in this field are executed prior to the first pass through the loop. There is essentially no limit as to what can go here, but good programming practice would require it to be kept simple. Several initializing statements can be placed in this field, separated by commas.

The second field, in this case containing "index < 6", is the test which is done at the beginning of each pass through the loop. It can be any expression which will evaluate to a true or false. (More will be said about the actual value of true and false in the next chapter.)

The expression contained in the third field is executed each time the loop is exercised but it is not executed until after those statements in the main body of the loop are executed. This field, like the first, can also be composed of several operations separated by commas.

Following the for() expression is any single or compound statement which will be executed as the body of the loop. A compound statement is any group of valid C statements enclosed in braces. In nearly any context in C, a simple statement can be replaced by a compound statement that will be treated as if it were a single statement as far as program control goes. Compile and run this program.

The while is convenient to use for a loop when you don't have any idea how many times the loop will be executed, and the for loop is usually used in those cases when you are doing a fixed number of iterations. The for loop is also convenient because it moves all of the control information for a loop into one place, between the parentheses, rather than at both ends of the code. It is your choice as to which you would rather use. Depending on how they are used, it is possible with each of these two loops to never execute the code within the loop at all. This is because the test is done at the beginning of the loop, and the test may fail during the first iteration. The do-while loop however, due to the fact that the code within the loop is executed prior to the test, will always execute the code at least once.

THE IF STATEMENT

Example program ------> IFELSE.C

Load and display the file IFELSE.C for an example of our first conditional branching statement, the if. Notice first, that there is a for loop with a compound statement as its executable part containing two if statements. This is an example of how statements can be nested. It should be clear to you that each of the if statements will be executed 10 times.

Consider the first if statement. It starts with the keyword if followed by an expression in parentheses. If the expression is evaluated and found to be true, the single statement following the if is executed, and if false, the following statement is skipped. Here too, the single statement can be replaced by a compound statement composed of several statements bounded by braces. The expression "data == 2" is simply asking if the value of data is equal to 2. This will be explained in detail in the next chapter. (Simply suffice for now that if "data = 2" were used in this context, it would mean a completely different thing. You must use the double equal sign for comparing values.)

NOW FOR THE IF-ELSE

The second if is similar to the first with the addition of a new keyword, the else in line 17. This simply says that if the expression in the parentheses evaluates as true, the first expression is executed, otherwise the expression following the else is executed. Thus, one of the two expressions will always be executed, whereas in the first example the single expression was either executed or skipped. Both will find many uses in your C programming efforts. Compile and run this program to see if it does what you expect.

THE BREAK AND CONTINUE

Example program ------> BREAKCON.C

Load the file named BREAKCON.C for an example of two new statements. Notice that in the first for loop, there is an if statement that calls a break if xx equals 8. The break will jump out of the loop you are in and begin executing statements immediately following the loop, effectively terminating the loop. This is a valuable statement when you need to jump out of a loop depending on the value of some results calculated in the loop. In this case, when xx reaches the value of 8, the loop is terminated and the last value printed will be the previous value, namely 7. The break always jumps out of the loop just past the terminating brace.

The next for loop starting in line 15, contains a continue statement which does not cause termination of the loop but jumps out of the present iteration. When the value of xx reaches 8 in this case, the program will jump to the end of the loop and continue executing the loop, effectively eliminating the printf() statement during the pass through the loop when xx is eight. The continue statement always jumps to the end of the loop just prior to the terminating brace. At that time the loop is terminated or continues based on the result of the loop test.

Be sure to compile and execute this program.

THE SWITCH STATEMENT

Example program ------> SWITCH.C

Load and display the file SWITCH.C for an example of the biggest construct yet in the C language, the switch. The switch is not difficult, so don't let it intimidate you. It begins with the keyword switch followed by a variable in parentheses which is the switching variable, in this case truck. As many cases as needed are then enclosed within a pair of braces. The reserved word case is used to begin each case, followed by the value of the variable for that case, then a colon, and the statements to be executed.

In this example, if the variable named truck contains the value 3 during this pass of the switch statement, the printf() in line 13 will cause "The value is three\n" to be displayed, and the break statement will cause us to jump out of the switch. The break statement here works in much the same manner as the loop, it jumps out just past the closing brace.

Once an entry point is found, statements will be executed until a break is found or until the program drops through the bottom of the switch braces. If the variable truck has the value 5, the statements will begin executing at line 17 where "case 5 :" is found, but the first statements found are where the case 8 statements are. These are executed and the break statement in line 21 will direct the execution out of the bottom of the switch just past the closing brace. The various case values can be in any order and if a value is not found, the default portion of the switch will be executed.

It should be clear that any of the above constructs can be nested within each other or placed in succession, depending on the needs of the particular programming project at hand. Note that the switch is not used as frequently as the loop and the if statements. In fact, the switch is used infrequently but should be completely understood by the serious C programmer. Be sure to compile and run SWITCH.C and examine the results.

THE EVIL GOTO STATEMENT

Example program ------> GOTOEX.C

Load and display the file GOTOEX.C for an example of a file with some goto statements in it. To use a goto statement, you simply use the reserved word goto followed by the symbolic name to which you wish to jump. The name is then placed anywhere in the program followed by a colon. You can jump nearly anywhere within a function, but you are not permitted to jump into a loop, although you are allowed to jump out of a loop.

This particular program is really a mess but it is a good example of why software writers are trying to eliminate the use of the goto statement as much as possible. The only place in this program where it is reasonable to use the goto is the one in line 23 where the program jumps out of the three nested loops in one jump. In this case it would be rather messy to set up a variable and jump successively out of each of the three nested loops but one goto statement gets you out of all three in a very concise manner.

Some persons say the goto statement should never be used under any circumstances, but this is narrow minded thinking. If there is a place where a goto will clearly do a neater control flow than some other construct, feel free to use it. It should not be abused however, as it is in the rest of the program on your monitor.

Entire books are written on "gotoless" programming, better known as Structured Programming.

Compile and run GOTOEX.C and study its output. It would be a good exercise to rewrite it and see how much more readable it is when the statements are listed in order.

FINALLY, A MEANINGFUL PROGRAM

Example program ------> TEMPCONV.C

Load the file named TEMPCONV.C for an example of a useful, even though somewhat limited program. This is a program that generates a list of centigrade and fahrenheit temperatures and prints a message out at the freezing point of water and another at the boiling point of water.

Of particular importance is the formatting. The header is several lines of comments describing what the program does in a manner that catches the readers attention and is still pleasing to the eye. You will eventually develop your own formatting style, but this is a good way to start. Also if you observe the for loop, you will notice that all of the contents of the compound statement are indented 3 spaces to the right of the for keyword, and the opening and closing braces are lined up under the "f" in for. This makes debugging a bit easier because the construction becomes very obvious. (The next example program will illustrate two additional methods of formatting braces.) You will also notice that the printf() statements that are in the if statements within the big for loop are indented three additional spaces because they are part of yet another construct.

This is the first program in which we used more than one variable. The three variables are simply defined on three different lines and are used in the same manner as a single variable was used in previous programs. By defining them on different lines, we have an opportunity to define each with a comment. It would be possible to define them on one line, but to do so would remove the ability to include a comment on each line. This is illustrated in the next program. Be sure to compile and execute the current program.

ANOTHER POOR PROGRAMMING EXAMPLE

Example program ------> DUMBCONV.C

Recalling UGLYFORM.C from the last chapter, you saw a very poorly formatted program. If you load and display DUMBCONV.C you will have an example of poor formatting which is much closer to what you will find in practice. This is the same program as TEMPCONV.C with the comments removed and the variable names changed to remove the descriptive aspect of the names. Although this program does exactly the same as the last one, it is much more difficult to read and understand. You should begin to develop good programming practices now by studying this program to learn what not to do.

It would be beneficial for you to remove the indentation from the last two example programs to see how much more difficult it is to understand the structure of the program without the indentations.

OUR FIRST STYLE PROGRAM

Example program ------> STYLE1.C

This program does nothing practical except to illustrate various styles of formatting and how to combine some of the constructs introduced in this chapter. There is nothing in this program that we have not studied so far in this tutorial. The program is heavily commented and should be studied in detail by the diligent C student to begin learning proper C programming style. Like all other example programs, this one can be compiled and executed, and should be.

PROGRAMMING EXERCISES

  1. Write a program that writes your name on the monitor ten times. Write this program three times, once with each looping method.
  2. Write a program that counts from one to ten, prints the values on a separate line for each, and includes a message of your choice when the count is 3 and a different message when the count is 7.

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