ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 25 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 TURBO-LESSON 3. PROGRAM STRUCTURE OBJECTIVES - In this lesson you will learn about: 1. Program structure 2. How to compile a program 3. How to run a program 4. Write and WriteLn statements 5. Comments - at beginning of program 6. Comments - at end of statement line 7. Comments - deactivate a section of code temporarily 1. Program structure. PASCAL programs are written in a certain way to make them easier to write and easier to read. There are two parts to any PASCAL program: the DECLARATIONS part and the PROGRAM BODY. The PROGRAM BODY is where the processing statements occur, and any data item used there must first be defined in the DECLARATIONS section. The form of a PASCAL program is PROGRAM Demo; (Program name - not required) USES (Units used by the program are listed here) LABEL (Labels, if used, go here) CONST (Constants are defined here) TYPE (Define your own data types here) VAR (ALL variables must be defined here) BEGIN (Processing statements occur between END. BEGIN and END.) The simplest PASCAL program is: BEGIN END. In the next section you will learn to compile and run a program using this "smallest program". ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 26 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 2. How to Compile a Program Before you can execute a program, it has to be translated from Pascal to a form the computer can understand. This is the process called "compiling" the program. ##### DO: Type in the program: BEGIN END. ##### DO: Press Alt-C to pull down the Compile menu window. The highlight should be on the menu option: [Compile Alt-F9]. If it isn't, move the highlight to that line. Press Enter to start the compile process. The box you see on the screen now lets you know how your compile worked. If nothing was wrong, the word "Success" appears in the lower left corner and "Press any key" blinks for your attention. Let's see what happens with an incorrect program. A missing period after END in your program would cause a problem for Pascal since it looks for the period to mark the end of your program. ##### DO: Remove the period after END and compile the program again. What happened? The compile box appeared briefly, then an error message appeared at the top of the screen: Error 10: Unexpected end of file The Pascal compiler was still looking for the period to mark the end of your program when it bumped into the end of the editor work file where your program was stored. ##### DO: Put the period back in your program after "End". The error message disappears when you start editing. Compile the program again to be sure you have a correct program again. ##### DO: If you haven't tried the "Hot key", Alt-F9, try it now. This lets you bypass the compiler pull-down menu. Many of the menu options have hot keys listed to the right of the option in the menu. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 27 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 Next, you need to learn how to "Run" or "Execute" a program. 3. How to Run a Program In the last section you "compiled" your BEGIN END program. Now let's run it to see what it does. ##### DO: Press Alt-R to pull down the Run menu. The highlight should be on [Run Ctrl-F9]. Press to run the program. What did the program do? Maybe you better try that again. ##### DO: Run the program again several times. What did it do? Mine did the same thing--nothing! Since there are no statements between BEGIN and END nothing is exactly what it should do. Definitely not the most useful program, but it illustrates a point: the MAIN BODY is always required and as much of the DECLARATIONS section as needed to define the data used. Since this simple program uses no data, the DECLARATIONS section can be omitted. In the next section you start meeting the statements that are used to build a Pascal program. 4. Write and WriteLn statements. These processing statements are introduced first, because they can be used with messages which require no previous data declaration. This allows you to experiment with some programming in the MAIN BODY without any concern for the DECLARATIONS part. ##### DO: Insert the WriteLn statement as shown between BEGIN and END in your program and run the program. BEGIN WriteLn('HELLO'); END. To see the output from your program, you need to make "Output" the window at the bottom of the screen. The following DO's will help you find your way around Turbo's windows. Notice the Function key helps at the bottom of the screen: F6-Switch. ##### DO: ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 28 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 Press F6 several times. Notice the active window (double line and highlighted window name) switching between top and bottom window. ##### DO: Press F6 once more if necessary to leave the bottom window active. The bottom window is either "Watch" or "Output". For now, you will want "Output" to occupy the bottom window. ##### DO: With the bottom window active, press Alt-F6 several times. Leave the "Output" window as the bottom window. ##### DO: Press F6 to switch to Edit as the active window. ##### DO: Run the program several times and watch the results in the bottom window. ##### DO: Change "HELLO" to "Hi" in your program and run the program again. ##### DO: If you haven't tried the Run "Hot Key", try it now: Ctrl-F9. You may have already discovered that the Turbo Environment "knows" when a new compile is needed. If you change your program and try to run it before compiling, it's OK. A compile will be done before running the program if it is needed. ##### DO: Change the "Hi" to "Lo" and run (Ctrl-F9) without first compiling. Did you see the compile box? ##### DO: Change the "Lo" back to "Hi" and run the program several times as you watch for the compile box. When did the compile happen? Only the first time you ran the program after changing it. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 29 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 Now let's explore the difference between the "Write" and WriteLn" statement. ##### DO: Remove the "Ln" from your WriteLn statement. The program should appear: BEGIN Write('Hi"); END. ##### DO: Run the program several times. What do you think the "Ln" part of the WriteLn statement does? If you're not sure, check out the next DO. ##### DO: Put the Ln back--change "Write" to "WriteLn". The statement should be: WriteLn('Hi'); Run the program several times. You're right! The Ln adds a linefeed--moves the cursor to the left side of the screen at the next line. NOTE: The way I write WriteLn is not required--you could use writeln or WRITELN or even wRiTeLn if you cared to. I prefer WriteLn because it provides a visual reminder of the two actions of this statement: Write (print whatever is in the parenthesis) and Ln, provide a Linefeed. ##### DO: Load PROG3 and use the editor to examine the program. A part of PROG3 is printed below: BEGIN WriteLn('* * * * * * * * * * * * * * * * *'); {Note that } WriteLn('* *'); {comments } WriteLn('* TURBO-LESSON 3 *'); {placed at the } WriteLn('* Edited, Compiled, Executed *'); {end of a } WriteLn('* by *'); {statement line} WriteLn('* (put your name here) *'); {must be } WriteLn('* *'); {terminated on } WriteLn('* * * * * * * * * * * * * * * * *'); {the same line.} In this section of PROG3, the WriteLn statement is used to display messages. ##### DO: ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 30 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 RUN the program to see the messages displayed on the screen. (Use Alt-F5 to see the full screen of output. Press Alt-F5 again to get back to the split screen so you can edit the program.) ##### DO: Use the editor to insert your name in the line indicated and run the program again. To illustrate the difference between WriteLn and Write statements, another portion of the program is included below: WriteLn; Write ('Check carefully when '); Write ('you run this program. '); Write ('How many lines '); WriteLn('are printed'); WriteLn('by this last set of Write and WriteLn statements?'); The "Ln" in WriteLn may be viewed as a linefeed or "return the cursor to the beginning of the next line". Any message, or other data item within the parentheses following WriteLn is printed first, the "Write" part, then the cursor is moved to the next line, the "Ln" part. Notice the first WriteLn in the program segment above. There is nothing in parentheses to print, so this statement is a way to print a blank line. The next three Write statements write on the same line. Then the following WriteLn statement writes it's message and causes the cursor to return to the beginning of the next line. The last WriteLn displays it's message on the line where the cursor is positioned, and returns the cursor to the next line. ##### DO: Add some WriteLn statements after the BEGIN to insert several blank lines at the top of the screen to push the boxed message down the screen. (Don't forget to put a semicolon after each WriteLn you add). Run the program. Did it work right? 5. Comments - at the beginning of program. There are two ways to indicate comments in a PASCAL program. The curly brackets, { }, may be used to enclose comments. The original comment delimiters, from standard PASCAL are (* *). ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 31 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 At the beginning of a program, or anywhere that multiple line comments are needed, it is convenient to mark the whole block of comments with the delimiters at the left margin. This makes it easy to insert or delete comments within the block. ##### DO: Add some comments at the beginning of the program. Try both types of comment delimiters, { }, and (* *). Be sure your comments are not "inside" any comments which are already in the program. 6. Comments - at the end of a statement line. Look at the comments following the WriteLn statements in PROG3. Short comments may be inserted to clarify the action of one or more statements. NOTICE CAREFULLY that each of these comments has to be completed on the same line, if the next line is another active processing statement. ##### DO: Add a short comment, {Blank Line}, after one of the WriteLn statements which prints a blank line. 7. Comments - deactivate a section of code. Having two sets of comment delimiters makes it possible to use one set for most comments, while reserving the second set for "deactivating" sections of statements when debugging a program. I prefer to use the { } for most comments, and (* *) for deactivating code. You should keep the delimiters used for commenting out code in the left margin where they are very visible. ##### DO: Deactivate the three WriteLn statements following the one which prints TURBO-LESSON 3. Since { } are in use at the end of these lines you should use the other set of comment symbols, (* and *) to deactivate the statements. Run the program to see the results. DON'T BE AFRAID TO EXPERIMENT WITH THE SAMPLE PROGRAMS. YOU CAN ALWAYS GET ANOTHER COPY, FROM YOUR BACKUP DISK (you did make one, didn't you), FROM THE .BAK FILE PROVIDED BY TURBO, OR FROM THE ORIGINAL FILE IF YOU HAVEN'T SAVED AN EDITED COPY. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 32 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 TURBO-LESSON 4. DECLARATIONS, INPUT OBJECTIVES - In Lesson 4 you will learn about: 1. The DECLARATIONS part of a program 2. VAR declaration 3. Input using the ReadLn statement 4. DEBUG: Tracing Statements 5. DEBUG: Watching Variables 6. Integer variables 1. The DECLARATIONS Part of a Program. You learned in the previous lesson that there are two main parts to any PASCAL program: DECLARATIONS, and MAIN BODY. The various entries in the DECLARATIONS section define the data items used in the processing in the MAIN BODY. Not all declaration entries will occur in every program, but only the ones needed to support the processing. Standard PASCAL requires that declarations occur in the order specified and each type of declaration may occur only once. TURBO is a little more forgiving in some respects than standard PASCAL (declarations don't have to occur in the specified order, and the same type of declaration may occur more than once). The various types of declaration entries will be introduced as needed in the sample programs. Only the VAR entry will be used in this program. 2. VAR Declaration. All variables, (spelled A-L-L, no exceptions), must be defined before they are referenced by processing statements. The VAR entry is used to define variables. The form of the entry is: variable-name : type; The variable-name may be one or several variable-names separated by commas. The type may be a predefined type, such as Integer, or a type you have constructed using the predefined types. The colon must occur between the variable-name(s) and the type. Extra spaces are acceptable to allow more readable format. Below are some VAR entries: VAR i,j,k : Integer; Inkey : Char; ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 33 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 Rate : Real; Count : Integer; (The example above includes types not yet discussed, to illustrate the form of the VAR entry.) ##### DO: Look at PROG4. (You know from earlier lessons how to load the program and use the editor to look at the program.) A segment of PROG4 is: VAR Number : Integer; The VAR entry above defines a variable called "Number" to be of type "Integer". This means the computer must set up a memory location large enough to store an integer, which can be accessed by referring to the variable name "Number". Notice the variable, Number, is later referenced in the processing statements. ##### DO: Add an integer variable called "Age" to Prog4. This will be used later in this lesson. You can either add the new variable to the declaration of Number Number, Age : Integer; or add another line with the Age declaration Number : Integer; Age : Integer; Compile the program to be sure you haven't made an error. 3. Input Using the ReadLn Statement. ReadLn is the statement used to input variables. The form of the statement is: ReadLn(var_1, var_2, . . . ,var_n); When the statement is executed, the computer will wait for you to type values for the variables, separated by one or more spaces, followed by depressing the enter key. Part of PROG4 follows: ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 34 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 BEGIN Write('Enter a number (no decimals please): '); ReadLn(Number); WriteLn; { Display one blank line } WriteLn('Number: ',Number); { Display the number entered } END. The Write statement provides a prompt before the ReadLn accepts Number. Notice that ReadLn does not provide a "?" or any other prompt. The programmer must provide any prompting required. The use of the Write statement for prompting, instead of the WriteLn, keeps the cursor on the same line so the input will occur right after the prompt message. The ReadLn accepts the number you type and stores it in the memory location which the computer has set aside for the Integer variable, Number. ##### DO: Compile and run PROG4. When prompted, type 12 and enter. Run it again and enter -34. ##### DO: Using the statements of PROG4 as an example, expand the program to do the following: (1) Write a prompt to 'Enter your age'. (2) Read a value for age to be stored in the integer variable, Age, which you added to the VAR declarations in the previous section. (3) Write a message which prints the age entered, in a manner similar to the way Number was printed. Run the program. 4. DEBUG - Tracing Statements Turbo Pascal 5.0 has a new DEBUG feature to help you find problems in programs. It is also an excellent way to learn about Pascal programming. You can execute your program one statement at a time with TRACE to see what each statement does. ##### DO: Press F7 to begin TRACEing your program. Note the highlighted line in your program. The highlighted statement will be executed when you press F7 again. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 35 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 As you press F7 to trace each statement, first look at the statement and predict what you think will happen. ##### DO: Continue pressing F7, watching the output in the bottom window. ##### DO: Trace through the program again to be sure you understand what the TRACE feature is doing for you. 5. DEBUG - Watching Variables Another of the DEBUG features allows you to WATCH the values stored in the computer's memory. TRACE and WATCH used together form a very effective debugging tool. ##### DO: Press F6 to make the bottom window active. Press Alt-F6 to switch from Output window to Watch window. Press F6 again to make the top (Edit) window active again. Now let's add a variable to the Watch window. ##### DO: Press Alt-B to pull down the Break/Watch window. Notice the "Hot Key" for Adding a Watch is Ctrl-F7. ##### DO: With the option "Add Watch Ctrl-F7" highlighted, press Enter. The Add Watch box appears on the screen. This is where you enter the name of the variable you want to watch. ##### DO: Type Number in the box and press Enter. The variable, Number, should now appear in the Watch window at the bottom of the screen. ##### DO: Trace the program. Watch the variable Number as you Trace the statement, ReadLn(Number). There is another variable in the program. You added the variable, Age, in an earlier exercise. Let's add Age to the watch window. Notice there is a shortcut. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 36 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 ##### DO: Place the cursor on the word Age in your program. Press Ctrl-F7. The word with the cursor appears in the Watch box. You don't have to type it, just press enter to accept it. ##### DO: Trace the program again, watching both variables, Age and Number. 6. Integer Variables. Integers are counting numbers, with no decimal points. They may be positive or negative. In TURBO the range of Integers permitted is: -32768 to +32767 Integers are used for subscripts, indexes, counting, input and output of such things as counts, limits, menu choices. Decimal numbers (Real type, discussed later), are needed for such things as dollar amounts and calculations. When decimal numbers are not actually needed, Integers should be used since they are easier to use and take less memory. ##### DO: Run or Trace PROG4 again, this time entering data which will cause errors. When prompted to enter a number, enter the letter A instead. What happened? A message appears to indicate the nature of the problem. Note that the cursor is positioned at the statement which caused the error. (In this case, it is not the statement which caused the problem, but the type of data entered.) ##### DO: Run or trace the program again, entering too large a number, 88888. Note that no error message appears but the result is wrong. What number appears in the watch window? Not 88888. Run it again with too small a number, -55555. When you use Integer variables, you have to be sure they will be within the acceptable range or errors will occur. NOTE: IN THE LESSONS THAT FOLLOW, BE SURE TO MAKE USE OF TRACE AND WATCH. WHEN INSTRUCTED TO "RUN" A PROGRAM, YOU COULD TRACE IT. TRACING TAKES A LITTLE LONGER, BUT YOU WILL PROBABLY LEARN MORE THAT WAY. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 37 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 TURBO-LESSON 5. INTEGER EXPRESSIONS OBJECTIVES - In lesson 5 you will learn about: 1. Assignment statements 2. DEBUG: Removing Watch Variables 3. Integer expressions 4. Problems with expressions 1. Assignment Statements. A large part of computer processing is accomplished by storing numbers, strings, and other data objects in memory locations in the computer. There are several ways to store a value in memory in Pascal: (1) use an Input statement (ReadLn), (2) use an Assignment statement, (3) use a Procedure or Function (in later lessons). The Assignment statement has the following form: Variable := Expression; An example: A := B + C; The way it works: The computer evaluates the expression on the right side of the replacement operator, :=, and stores the resulting value in the memory location named by the variable on the left. In the example above, the computer would obtain whatever value is currently stored in the memory location called B, add that value to the value it finds in the memory location called C, and store the sum in the memory location, A. If B holds the value 3, and C holds the value 4, then 7 would be stored in the memory location called A. ##### DO: Use the editor (Alt-F, New) to enter the following short program (omit the comments, if you like): PROGRAM ABC; VAR A, B, C : Integer; BEGIN A := 6; {Assign the value 6 to the memory location, A} B :=7; {Assign 7 to B } ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 38 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 C := A + B; {Add the values in A and B, Store in C } WriteLn ('A=',A, ' B=',B, ' C=',C); END. 2. DEBUG - Removing Watch Variables There are some "Leftover" variables in the watch window. You will need to get rid of them and add this program's variables. ##### DO: Press Alt-B to get the Break/Watch menu. Press R to "Remove all watches". ##### DO: Use Ctrl-F7 to add A, B, and C to the watch window. ##### DO: Press F7 once to start tracing. What values do the variables A, B, and C have? Pascal doesn't preset variables to zero as some languages do. ##### DO: Trace the program, watching the values of A, B, and C. ##### DO: Trace the program, watching the variables and expressions. ##### DO: Change the statement A:=6 to A:=3 and run the program again. Note that you have just written a complete PASCAL program - from here on, you will just be adding new features in each lesson to enable you to write more complicated programs. (It didn't run? Use the error message to help correct any errors. If the error message doesn't make sense, try looking for misplaced or omitted semicolons. Also note that the assignment statement uses the compound symbol, ":=" and not "=".). 3. Integer Expressions. Integer expressions are composed of integer variables, constants, and operators. The operators used with integer expressions are: +, -, *, div, mod. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 39 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 The + and - have their usual meaning, addition and subtraction. The * indicates multiplication. Division of integer numbers is done with div and mod. ##### DO: Examine PROG5. Two numbers, which you enter, are added, subtracted, and one is cubed using the multiply operator, "*". WriteLn is used to print out the results. Notice that expressions may be calculated in an assignment statement, I_Cubed := I * I * I; or calculated in a WriteLn statement, WriteLn('I - J ', I - J); ##### DO: Remove the variables from the watch window (Alt-B, R). ##### DO: Add the following to the Watch window. (Add them in reverse order if you prefer to see them as listed below.) I J I + J I - J I_Cubed ##### DO: Trace the program. Note that the ReadLn statement will attempt to read 2 values. The values should be typed with a space between them, not a comma. Enter the values 2 and 5. Check all the results. Are they all correct? ##### DO: Run the program again, this time entering -3 and 5. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 40 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 Is everything correct again? Notice the negative cube of -3 is as expected, -27. (Some incorrect negative cubes will appear a little later in this lesson.) The division operators, div and mod are not used in PROG5. To see how they work, ##### DO: Just before the END of PROG5, edit in the following statements: WriteLn('I div J = ', I div J); WriteLn('I mod J = ', I mod J); Run the program, entering the values 5 and 3. Did you get the results expected? Is 5 divided by 3 really 1 and not 1.666? ##### DO: Add these two statements at the end of the program: WriteLn(I,' divided by ', J, ' = ', I div J); WriteLn(' with a remainder of ', I mod J); Run the program with the values 5 and 3. Often, when working with integers, it is useful to know one or both of these components of the division. If you really want the decimal result of the division, the slash, /, could be used with integers. ##### DO: Add I/J to the watch window. Run the program with the values 5 and 3. Note the result of division using the slash is the usual result. Observe that you can watch expressions that are not in the program. I and J have values in the program so you can look at I/J even though the expression I/J does not occur in the program. ##### DO: Before going on, try to improve the "readability" of the output by adding some blank lines between the different parts of the output. Use Alt-F5 to see the full Output screen. For example, you might add a blank line just before printing the cube of the number. Remember, whenever you want to print a blank line, use WriteLn without any parentheses. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 41 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 4. Problems with Expressions. You should be aware of the possibilities for various types of errors involving expressions. If you find this section a little heavy going, come back to it later. It is not essential for understanding the lessons that follow. However, it will help you later when you are debugging your own programs if you understand the ideas presented here. First, an easy to detect error. ##### DO: Declare K to be an integer variable. (Add K to the list of integers in the VAR section.) Add this statement at the end of the program: K := I / J; Run the program. How did it go? This error, "Type mismatch", is easy to find, since the compiler finds it. The reason for the type mismatch is that the result of the division using the "/" is a real number (covered in a later lesson). The variable, K, is an integer. A real number can't be stored in an integer memory location. (Why not? One reason: a real number takes more memory space than an integer.) A second type of error, is illustrated in the following: ##### DO: First, remove the statement that caused the error (K := I/J), then run the program using the values 31 and 5. Check the results of the cube of I. Is it correct? O.K., but notice the cube of I, 29791, is approaching the upper limit of integer variables, 32767. What will happen if you enter 32 and 5? (The correct cube of 32 is 32768, just 1 too large to fit as an integer.) Try it! ##### DO: Run the program using the values 32 and 5. The computer, known for its reliability, informs you: The cube of I = -32768 ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 42 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 Now, you know the sign is wrong - positive numbers do not produce negative cubes. But look at the number, -32768. Correct number with the wrong sign? (The cube of 32 is 32768.) ##### DO: Run the program again with the values 33 and 5. (The correct cube of 33 is 35937.) The cube of I = -29599 Wrong number! Wrong sign! So why does the computer go merrily on its way - giving you these wrong answers? The computer is very good at detecting errors in the format of program statements, missing declarations, wrong punctuation. There are other types of errors that are more difficult to detect. It is up to you, the programmer, to find ways to keep these errors from going unnoticed. For now, you need to be aware that these problems can occur. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 43 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 TURBO-LESSON 6. CONDITIONAL PROCESSING OBJECTIVES - You will learn, in this lesson, about: 1. Selection structures used for conditional processing 2. DEBUG: Goto Cursor 3. IF statement (two-way selection) 4. IF statement (one-way selection) 1. Selection Structures for Conditional Processing. There are three types of statement sequencing used in PASCAL: 1. SIMPLE SEQUENCE. One statement follows another with no branching. 2. SELECTION STRUCTURES. Based on a condition, supplied by the programmer, the next statement to execute is chosen from two alternatives (IF statement) or chosen from many alternatives (CASE statement). 3. REPETITION STRUCTURES. A group of program statements may be repeated more than once, dependent on a condition supplied by the programmer. The repetition statements are WHILE, REPEAT, and FOR, included in later lessons. The Selection statement, IF .. THEN .. ELSE, is illustrated in this lesson, the CASE statement appears in a later lesson. 2. DEBUG - Goto Cursor In previous lessons you traced through each line of the programs. As programs grow larger you probably won't want to trace the entire program. You will often want to run a program without tracing up to a certain point (where you suspect a problem exists), then have the program pause to let you start tracing at that point. The Goto Cursor debug feature let's you do just that. First, let's see what the first five statements of PROG6A do. ##### DO: Set the bottom screen to Output and trace the first 5 statements. Then use Ctrl-F2 to reset the program so you can start the program from the beginning again. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 44 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 To see how the Goto Cursor works, we'll assume you want to run the first five statements without tracing, then stop at the IF statement to begin tracing. ##### DO: Move the cursor to the first line of the IF statement. (Anywhere in the line is OK.) This marks the place you want the program to pause so you can take control. ##### DO: Press F4 to run the program up to the statement with the cursor. Input values at the prompts. Notice the program stops at the statement where you positioned the cursor. This statement will be highlighted. Study the next section to find out what to expect of the IF statement. The DO activities will let you see the IF in action. 3. IF Statement (Two-way Selection). The two-way IF causes one of two alternative statements to be executed, based on whether the condition is TRUE or FALSE. The form of the statement is: IF condition THEN Statement_1 ELSE Statement_2; If the condition is TRUE, the statement following the THEN is executed. If the condition is FALSE, the statement following the ELSE is executed. The condition is an expression or comparison which the computer can evaluate as TRUE or FALSE. Examples of a condition: 7 < 10 TRUE I < 10 TRUE, if the memory location named I holds a value less than 10, otherwise FALSE. NOT(7 < 10) FALSE (7 < 10 is TRUE, but NOT reverses the value to FALSE). ##### DO: Look at PROG6A. The IF statement in PROG6A is a two-way IF. A congratulations message is printed if the condition is true, condolences if false. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 45 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 The condition is (Have >= Want). This condition is true if the number you enter for computers owned is greater than or equal to the number you enter for computers you would like to have. ##### DO: Run the program several times, experimenting with various input values. ##### DO: Trace the IF statement action several times. Try to predict which statement will be highlighted next based on your input values. ##### DO: Switch the bottom window to Watch and add the two variables, Have, and Want to the watch window. ##### DO: Watch the values of Have and Want as you trace the program again. You could also watch the CONDITION part of the IF statement. This should convince you that the only values a condition can have are TRUE and FALSE. ##### DO: Add (Have >= Want) to the watch window. Trace the program several times with input values chosen to make the condition sometimes TRUE, sometimes FALSE. 4. IF Statement (One-way Selection). The one-way IF is really a special case of the two-way IF, where the second alternative is to do nothing, just go on to the next statement. The form of the IF statement is: IF condition THEN statement; ##### DO: Observe PROG6B. The second IF statement is a one-way selection. If the condition is true, the WriteLn statement will be executed. If the condition is false, the WriteLn will be ignored. ##### DO: Run (or Trace) the program using 0 for the number of computers owned. ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 46 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 Run it again with 1 for the number of computers owned. The "No Computer!" message should print for 0 computers owned, but not print for 1 computer owned. Remember you can use Alt-F5 to see your output screen when the bottom window is the Watch window. NOTE: At this point you know what the TRACE feature and Goto Cursor feature can do for you. In the sections and lessons to follow I will usually tell you to "Run" a program. Remember you can always stop to Trace parts of the program. Tracing a step at a time while observing variables or expressions in the Watch window will usually provide extra learning opportunities for you. ##### DO: Examine the third IF statement, added in PROG6C. This is a one-way IF with a slightly more complicated condition. The condition contains an integer expression, (Want - Have). The computer first evaluates the integer expression to get a number to compare with the 2 on the right side of the ">". For example, if you want 3 more computers than you have, the "Greedy" message should print. ##### DO: Add the expression, (Want - Have) to the Watch window. ##### DO: Run the program several times with different input to see the effect of this IF. Also note the misspelled "Aren''t" in the message. The double apostrophe is used in the message to represent a single apostrophe. If a single apostrophe were used, it would appear to be the end of the message. ##### DO: Try your hand at writing an IF statement to do the following: If the number of computers owned is more than the number of computers wanted, print a message 'Send extra computers to (put your name here?) '. *********************** SHAREWARE REMINDER *********************** Congratulations! You have finished the first 6 of 16 lessons. You should be able to decide now whether these lessons are helping you get started with Pascal. If you decide to continue, remember that payment for the lessons is expected. This is SHAREWARE, not ΠTURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 47 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 freeware. If you missed the "Newspaper" concept of primary value in "first use", please go back and read the SHAREWARE notice at the beginning of the lessons. Thank you for your support for TURBO-LESSONS and SHAREWARE.