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î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 48 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 TURBO-LESSON 7. REPEAT STATEMENT OBJECTIVES - In lesson 7 you should learn about: 1. CHARacter variables 2. BOOLEAN variables 3. REPEAT statement 1. CHARacter Variables. The reserved word, CHAR, is used to declare a variable of character type. A variable of type CHAR can be used to refer to or store a single character. VAR Alpha : CHAR; Alpha is declared to be a variable which can be used to store any of the characters in the character set. This includes the upper and lower case alphabet, the digits, 0 to 9, special characters such as #, $, %, *, and the rest of the 256 characters in the PC's character set. ##### DO: Examine PROG7. A variable named Response, of type CHAR, is used to store the character entered in response to a multiple choice question. ##### DO: Set the lower window to Output and run the program several times, entering wrong responses, and the correct response, D. Also try lower case d. ##### DO: Study the first IF statement in PROG7. IF (Response = 'D') OR (Response = 'd') THEN . . . î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 49 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 The character 'D' must be enclosed in single quotes in the program. Note that you enter the character as input data without quotes when running the program. Notice the compound condition using OR to combine two simple conditions. This condition will be true if either or both of the simple conditions are true. The correct response, D, is checked in both upper and lower case to make responding easier. ##### DO: Modify the IF statement to accept A or B as the right response. Assume that the correct answer is A or B. (Ignore lower case responses to keep the statement shorter.) 2. BOOLEAN Variables. BOOLEAN variables can have only two values, TRUE or FALSE. The value of a condition, TRUE or FALSE, may be stored in a BOOLEAN variable for later use. VAR Correct_Response : Boolean; In PROG7, the Boolean variable, Correct_Response, is used to store the truth value (TRUE or FALSE) of the condition in the first IF statement. If the correct character, 'D' or 'd', is entered, TRUE is stored in Correct_Response. If anything else is entered, FALSE is stored. Actually, TRUE and FALSE are stored as 1 and 0 to take up less space, but you can always view a BOOLEAN variable as having a value of TRUE or FALSE. ##### DO: Identify the condition in the second IF statement in PROG7. Since BOOLEAN variables can only have two values, TRUE or FALSE, and conditions always evaluate to the same two values, a BOOLEAN variable may be substituted for a condition. If would be permissible, but unnecessary to write the IF statement: IF Correct_Response = TRUE THEN . . . ##### DO: Modify the IF statement as indicated above and run the program to verify that the IF still works exactly as before. î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 50 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 There is another way to assign the correct value to the variable, Correct_Response. ##### DO: Replace the FIRST IF statement in PROG7 with the following statement: Correct_Response := (Response = 'D') OR (Response = 'd'); Run the program. How does the program change when you run it? (If it doesn't do exactly as before, maybe you typed it wrong, or replaced the second IF instead of the first?) Since the condition on the right of the := must be evaluated by the computer and assigned a value of TRUE or FALSE, this value can be stored directly in a BOOLEAN variable without using the IF statement. ##### DO: Add Response and Correct_Response to the Watch window and trace the program. When does Correct_Response receive it's correct value? 3. REPEAT Statement. In a previous lesson, you learned that there are three ways to sequence the execution of statements in PASCAL: SIMPLE SEQUENCE, SELECTION STRUCTURES, and REPETITION STRUCTURES. One of the statements used for REPETITION is REPEAT . . UNTIL. The form of the REPEAT statement is: REPEAT Statement 1; Statement 2; . . . Statement n UNTIL condition; Statements 1, 2, . . . , n will be executed repeatedly until the condition becomes true. This implies that the condition is checking something that can be changed by the statements 1 to n. If this is not so, the statements will be repeated forever! In PROG7, you are prompted to respond to the multiple choice question. A REPEAT statement controls the block of statements which prompt for a response, and then check the response. The î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 51 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 block of statements will be repeated until a correct response makes the UNTIL condition true. ##### DO: Change the condition in the UNTIL in PROG7 to: UNTIL 'A' = 'B'; Run or Trace the program. Does the program correctly identify a correct response? What happens then. (Use ctrl-c or ctrl-Scroll-Lock to stop the program.) ##### DO: Change the condition again to: UNTIL 'A' = 'A'; How many times is the Repeat loop executed? Notice that the statements in a REPEAT structure are ALWAYS executed at least once. Even if the UNTIL condition is TRUE before entering the REPEAT, the condition is not checked until the end of the statements in the REPEAT block. î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 52 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 TURBO-LESSON 8. CASE STATEMENT OBJECTIVES - In lesson 8 you will learn about: 1. Block statements 2. CASE statement 1. Block Statement. As noted in an earlier lesson, the form of the IF statement is: IF condition THEN statement_1 ELSE statement_2; IF the condition is true, statement_1 is executed, otherwise statement_2 is executed. However, a single statement may not always get the job done. The Block statement (also called a Compound statement) allows you to substitute a multiple statement block anywhere a simple statement is acceptable. The form of the Block statement is: BEGIN Statement_1; Statement_2 END; You can include as many statements as you like between the BEGIN and END. Notice that the main body of a Pascal program could be viewed as a single statement! That single statement is a block statement: BEGIN Statement_1; Statement_2; . . . Statement_n; END. To illustrate the use of a Block statement in an IF statement, consider the following problem: If the value of I is greater than the value of J, swap the two in memory (a common problem when sorting data). If you happen to have a single statement to swap the values of two memory locations, the IF statement might be: IF I > J THEN SWAP(I,J); Later, you will learn how to make up your own "statements" (by creating functions and procedures), but for now, the way to swap I and J is: î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 53 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 IF I > J THEN BEGIN Temp := I; I := J; J := Temp; END; Notice that the form of this IF statement is still correct: IF condition THEN statement; The statement, in this case, is a Block statement, rather than a simple statement. ##### DO: Take a look at the program called TEST1. This program is provided to make it quicker for you to test new statements and concepts. TEST1 has some integer variables and character variables declared and the main BEGIN END. All you need to do to test a statement, or group of statements, is edit them into the test program and run the program. ##### DO: Insert the following statements between the BEGIN and END of program TEST1: Write('Enter two numbers '); ReadLn(I, J); WriteLn('I=', I, ' J=', J); ##### DO: Set the lower window to Output and run the program. ##### DO: Between the ReadLn and WriteLn statements you just entered, add the IF statement to swap I and J if I is larger: IF I > J THEN BEGIN Temp := I; I := J; J := Temp; END; Run the program several times using several pairs of input numbers to test the program. Does the "swap" work right? î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 54 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 ##### DO: Switch the lower window to Watch. Add the following to the watch window: I, J, Temp and (I > J). Trace the program. It may not be clear why three statements are needed to "Swap" two numbers. Why not just use I := J; J := I; ##### DO: Remove the statement Temp:=I. Change the statement J:=Temp to J:=I; Trace the program several times with different values until you see why this doesn't work. Tracing while watching variables will often help you uncover weak logic and false assumptions in your programs. ##### DO: Restore the IF statement to it's original form with the 3-statement swap using Temp. Run it to make sure it works right before going on. What would happen if you forgot the BEGIN and END to enclose several statements that belong to an IF statement? ##### DO: Remove the BEGIN and END in the IF statement and run the program several times. Does the "swap" still work right? ##### DO: Run the program with 7 and 22 as input. (These don't need swapping.) Did the swap still work right? What happened? Without the BEGIN and END to make the three statements appear as one, the statements "appear" as follows to Pascal: IF I > J THEN Temp := I; I := J; J := Temp; î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 55 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 Only the statement, Temp := I, is controlled by the IF condition. The two statements, I:=J; and J:=Temp; are executed for all values of I and J. They are no longer under the control of the IF. When your IF statements don't perform quite right, check to be sure which statements are controlled by the IF. ##### DO: Trace the program. Watch the variables carefully as each statement is traced. 2. CASE Statement. First, a bit of review from an earlier lesson: Program sequencing is done in Pascal with (1) Simple Sequence, one statement follows another, (2) Selection Structures, IF statement for one-way and two-way selection, CASE statement for many-way selection, (3) Repetition Structures, REPEAT statement, WHILE statement, FOR statement. The CASE statement is useful when there are more than two actions or statement sequences needed. The form of the CASE statement: CASE variable OF value_1 : Statement_1; value_2 : Statement_2; . . . value_n : Statement_n; ELSE Statement; END; {CASE} Note the following: The variable must be a simple type such as Integer, CHAR, BOOLEAN. (REAL is not allowed). The values used to determine which Statement to execute must be of the same type as the variable. The values may be a constant, a list of constants, or a subrange such as 1..10 (all integers from 1 to 10). î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 56 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 How it works : (Refer to the "form of the CASE statement" above.) If the variable has a value of "value_1" then Statement_1 is executed. If "value_2" then Statement_2, . . . If the value of the variable matches none of the values, the statement following the ELSE is executed. There must be an END to mark the end of the CASE statement. (It's a good idea to add the comment {CASE} after the END). ##### DO: Use the editor to examine PROG8. This is the same problem as in the previous lesson, with a bit more programming flexibility derived from the CASE statement and the block statements. Notice that the list of variables in the CASE statement allow appropriate responses for acceptable responses: A, a, B, b, C, c correct responses: D, d unacceptable responses: anything else. ##### DO: Run the program with the Output window to see how it works. ##### DO: Switch the lower window to Watch. Remove any watch variables left from earlier programs (Alt-B, R). Add Response and Correct_Response as watch variables. ##### DO: Trace the program. Watch the action of the CASE statement. (You may need to move the Cursor down in the program to force the CASE statement up in the Edit window. This will allow you a better view of the action of the CASE statement.) ##### DO: Modify the CASE statement in PROG8 to accept C as the best answer. Run the program. Did it work? î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 57 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 TURBO-LESSON 9. FOR STATEMENT OBJECTIVES - In lesson 9, you will learn about: 1. Controlling field width in Write statements 2. FOR statement 1. Controlling Field Width in Write Statements. Spacing of output in previous lessons has been done using spaces in string constants, such as, 'I= '. The space after the = will be printed as part of the string. There is another way to add spaces. When listing the items to write in a Write or WriteLn statement, ":n" can be added to specify how many characters the item is to occupy in the output. For example: WriteLn(I:4); The value of I would be printed in a space 4 characters wide. ##### DO: Withe the bottom window set to Output, run Program TEST1 several times with the following statements in the main part of the program: Write('Enter a Number: '); ReadLn(I); WriteLn('[', I:4, ']'); Look closely at the output. Are the numbers "left-justified" or "right-justified"? If I has a value of 23, and 23 prints in columns 1 and 2 of the 4 column field, it is left-justified. If the 23 appears in columns 3 and 4 of the 4 column field, it is right-justified. What happens if you enter a number that is too large to fit the field specified? ##### DO: Run the program several times using the following values for I: 123, 1234, -1234, 12345 î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 58 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 2. FOR Statement. In the previous lesson, you were reminded of the various ways to control the sequencing of program actions. One of the REPETITION structures is the FOR statement. The forms of the statement are: FOR variable := lowvalue TO highvalue DO Statement; {May be a block statement with BEGIN END} FOR variable := highvalue DOWNTO lowvalue DO Statement; ##### DO: Run PROG9 several times to see how it works. Experiment with both positive and negative numbers. What happens if High is smaller than Low? ##### DO: Run PROG9 using 5 for Low (first number entered), 3 for High? What was the result? Also try 5 for both values. How many times is the loop repeated if Low = High? What results would you expect if you entered 3.4 for Low and 5.3 for High? ##### DO: Run the program with the values 3.4 and 5.3. Were you correct in predicting the outcome? Can constants be used instead the variables Low and High? ##### DO: Edit the first FOR statement in PROG9 as follows: FOR Index := 1 to 5 DO Run the Program. Is it possible to use expressions for Low and High? ##### DO: Change the FOR statement to: î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 59 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 FOR Index := Low + 1 TO High DO Run the Program. ##### DO: Trace the program with High, Low, and Index in the watch window. î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 60 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 TURBO-LESSON 10. WHILE STATEMENT OBJECTIVES - In lesson 10 you will learn about: 1. CONSTant declaration 2. WHILE statement 3. Delay timing loop 1. CONSTant Declaration. In the declaration section of the Pascal programs in previous lessons, only VAR and PROGRAM have appeared. CONST is used to declare constants and assign values to them. The form is: CONST Constant_Name_1 = Value_1; Constant_Name_2 = Value_2; . . . Constant_Name_n = Value_n; Some examples: CONST PI = 3.14159; Fahr_Freeze = 32; Cels_Freeze = 0; Message_1 = 'This is a string constant'; ##### DO: Run PROG10 a time or two with the Output window to see what it does. ##### DO: Edit the constant declaration, Wobble_Size = 5; to Wobble_Size = 10; Run the program. What effect did you see from the change? ##### DO: Change the value of the constant Left_Edge to 20 and run the program. To understand how Left_Edge and Wobble_Size work, look at the WriteLn statement: î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 61 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 WriteLn(': ':Left_Edge, First:Index, Last:2, Message); 1. ': ' is printed in the right 2 columns of a field whose width is determined by the value of the constant, Left_Edge. 2. First initial is printed in the rightmost column of a field of width, Index. Note that Index is changing in the FOR loop, so the width of this field changes. 3. Last initial is printed next in the rightmost column of a 2-character wide field. 4. The string constant, Message, is printed immediately to the right of Last initial. ##### DO: ##### DO: In the WriteLn statement described above, change Last:2 to Last:Index. The statement should read: WriteLn(': ':Left_Edge, First:Index, Last:Index, Message); ##### DO: Run the program to see the effect of the change you just made. ##### DO: Run the program several times with the values of the constants reset to various values. (LIVE DANGEROUSLY - try some wild values: 50 for Wobble_Size or 70 for Left_Edge. Your CRT won't explode! At least mine hasn't - yet!) 2. WHILE Statement. The three Pascal REPETITION Structures are: (1) WHILE (2) REPEAT (3) FOR The form of the WHILE statement is: WHILE condition DO statement; Note that the statement is often a block statement and appears: WHILE condition DO BEGIN Statement_1; Statement_2; . î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 62 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 . . Statement_n; END; The WHILE is executed "while" the condition is true. Contrast the REPEAT which is executed while the condition is false, UNTIL the condition becomes true. ##### DO: Run PROG10 again, using 0 as the number "less than 8". (You may want to load a fresh copy of PROG10 if you have made a lot of changes to the program.) How many times was the loop executed? You could substitute REPEAT for WHILE in PROG10. ##### DO: Replace: WHILE Count > 0 DO with: REPEAT add: UNTIL Count < 1; after: END; {WHILE} (Notice that you don't have to remove the BEGIN END when you change the loop from WHILE to REPEAT. They are not needed in the REPEAT statement, but a block statement is always acceptable wherever a simple statement can be used.) Run the program several times to verify that it still works the same as with the WHILE loop. ##### DO: Run the program with 0 for the "number less than 8". How many times was the loop executed? Notice that the WHILE loop may be executed 0 times, but the REPEAT loop is always executed at least once, since the condition is not checked until the end of the loop. NOTE: YOU SHOULD BE ON THE LOOKOUT FOR SUCH CONTRASTS BETWEEN "INTERCHANGEABLE" STATEMENTS. CHOOSING THE BEST STATEMENT TO USE IN EACH CASE IS BASED ON THIS KIND OF AWARENESS. ##### DO: Change the REPEAT loop back to the original WHILE loop.(An easy way would be to load a fresh copy of the original PROG10.) î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 63 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 Change the WHILE statement to accept only Last initials less than 'G'. WHILE Last < 'G' DO Run the program. Try it once with H (uppercase) for a last initial. Then try it with A (uppercase). How did it work? The WHILE loop didn't terminate at all when you used an A for the last initial - you had to abort the program, right? (If it is still executing, use Ctrl-C to stop it.) One of the things needed in controlling loops: something in the loop must be related to the condition. In the original WHILE loop, the Count was being decreased in the loop. When the condition was changed to (Last < 'G'), the loop no longer has any effect on the condition. Either the loop will not be executed at all, if Last is = or > 'G', or it will execute indefinitely if Last is < 'G'. ##### DO: Change the WHILE statement to: WHILE (Last < 'G') AND (Count > 0) DO Run the program several times using last initials before and after G in the alphabet. ##### DO: Run the program with last initial of 'a' (lower-case a). How many times did the loop execute? Why? The ASCII value of 'G' is 71. The ASCII value of 'a' is 91. So 'a' is > 'G'. 3. Delay Timing Loop. You may have noticed the "wobble" proceeds faster in one direction than the other. This is due to a timing delay in the first WHILE loop. (You may need to use a number larger than 5000 in the delay statement for faster computers. Notice that the variable, Delay, is declared to be a "longint" type. This is an integer variable which can store larger numbers.) î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 64 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 FOR Delay := 1 to 5000 DO; {Do nothing, except loop} This looks a little strange? Shouldn't there be a statement after the DO? Pascal has a "null" statement for just such times as this. When a statement is required, but none is present, Pascal just substitutes its "null" or "do-nothing" statement. ##### DO: Change the delay in the loop from 5000 to 20000 and run the program. ##### DO: Add a timing loop to the other WHILE loop and run the program. How did it work? One last little twist on timing loops: ##### DO: Change the Wobble_Size to 10. Change both delay statements to: FOR Delay := 1 to 5000 * Index DO; Run the program. Note that delays don't have to be constant. This delay varies as the FOR loop progresses! î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 65 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 TURBO-LESSON 11. INTRODUCTION TO FUNCTIONS OBJECTIVES - In this lesson, you will learn about: 1. Pascal Subprograms 2. FUNCTION declaration 3. How a FUNCTION Works 4. Providing Input to the FUNCTION 1. Pascal Subprograms. Why bother with subprograms? Doesn't that just complicate programs? You will find that subprograms are used to simplify, rather than complicate programs. The example programs in earlier lessons have been rather small. As programs grow in size and complexity, it is essential that you utilize the appropriate tools to simplify the programming effort. The subprogram is one of the most powerful tools for simplifying a complex program. Subprograms may be viewed as "building blocks" for constructing programs. As an illustration of the building block approach, consider a program to enter and sort names for a phone directory, and then print the sorted directory. The main program might be: BEGIN EnterNames; SortDirectory; PrintDirectory; END. This shows the "big picture", what the 3 main steps are. The subprograms, EnterNames, SortDirectory, and PrintDirectory may each contain up to a page of program statements. If all of this processing were done in the main program, it might be difficult to understand the program as a whole. As you learn to write Pascal programs, you will find Pascal subprograms rather easy to write - they are almost identical to programs! There are two types of subprograms in Pascal: FUNCTIONS and PROCEDURES. Both types may be either user-defined (written by you) or pre-defined as a part of Pascal. In this lesson you will work with user-defined FUNCTIONS. î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 66 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 Most of what you learn about FUNCTIONS also applies to PROCEDURES. 2. FUNCTION Declaration. Below is a simplified structure of a Program, Function, and Procedure: PROGRAM FUNCTION PROCEDURE LABEL LABEL LABEL CONST CONST CONST TYPE TYPE TYPE VAR VAR VAR FUNCTION(s) FUNCTION(s) FUNCTION(s) (0 or more FUNCTIONS) PROCEDURE(s) PROCEDURE(s) PROCEDURE(s) ( and PROCEDURES ) BEGIN BEGIN BEGIN (processing (processing (processing statements) statements) statements) END. END; END; Notice that they look very similar. One difference, visible here, is the END of the processing block: A period follows the END of a PROGRAM block, but a semi-colon follows the END of a FUNCTION or PROCEDURE block. 3. User-defined Function. We start with a very simple function to illustrate several key concepts. ##### DO: Look at PROG11. The Function declaration is: FUNCTION GiveMe5:Integer; The significance of each part of this declaration is: FUNCTION Type of subprogram, a FUNCTION, not a PROCEDURE. The two don't work in quite the same way. GiveMe5 The NAME of the FUNCTION. This gives you a way to refer to this function. The statement, WriteLn(GiveMe5), in the main program uses the name, GiveMe5, to use this function. INTEGER The name, GiveMe5, is associated with a memory location which can store an Integer. Somewhere î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 67 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 within the BEGIN END block of the FUNCTION, GiveMe5 must be given an Integer value to store. First, observe how the program sequences the execution of statements. ##### DO: Trace PROG11, watching the highlight progress through the main program, to the FUNCTION, and back to the main program. The function name, GiveMe5 must be given a value in the function. What do you think would happen if no value were given to the name, GiveMe5? ##### DO: Remove the statement, GiveMe5:=5; in the FUNCTION. The function BEGIN END block now has no active statements. ##### DO: Run the program. What was printed? Mine printed 22005. Yours might print 0 or something else. This is just whatever happens to be in the memory location allocated for GiveMe5. 4. Providing Input to the FUNCTION You would no doubt agree that the function, GiveMe5, in the previous section is pretty useless. You could use the statement WriteLn(5); in the main program to do the same thing as WriteLn(GiveMe5); Most useful functions operate on data which is provided as input to the function. PROG11A uses a function with input. A number is input to the Function named Cube. The function calculates the cube of the number and stores the result in a memory location associated with the name, Cube. ##### DO: Examine FUNCTION Cube in PROG11A using the editor. FUNCTION Cube(Number:Integer) : Integer; The FUNCTION declaration above means: FUNCTION Type of subprogram, a FUNCTION. Cube The name of the FUNCTION. î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 68 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 (Number:Integer) An integer "parameter" called Number is used within the FUNCTION. This is the number to be cubed. : Integer; Type of value to be assigned to Cube. ##### DO: Study the connection between the following two statements: In FUNCTION Cube: FUNCTION Cube(Number:Integer) : Integer; In PROG11A: WriteLn('The cube is: ', Cube(No) ); Cube(No) in the statement above invokes (calls) the FUNCTION Cube to operate on the number called No. PROG11A, No -----> Number, in FUNCTION Cube. The value of the variable called No in PROG11A is provided to the FUNCTION to use as a value of its variable called Number. ##### DO: Put the variables Number and No in the watch window. Trace the Write and ReadLn in the main program. Enter 3. Stop when the WriteLn statement is highlighted. Notice that No now has the value 3. Number (in the Function) is an "Unknown identifier". ##### DO: Trace the WriteLn statement--press F7 once. Observe that 2 things happened: 1. The program execution transferred from the WriteLn statement in the main program to the beginning of the Function Cube. 2. Number now has the value 3, which it received from No in the main program. ##### DO: Trace the rest of the program. Watch the program execution return to the main program and end there. î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 69 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 ##### DO: Set the lower window to Output and run PROG11A several times using the following values for input: 3, -3, 0 31 (cube should be 29791) 32 (cube should be 32768) Is it? Remember this problem from an earlier lesson? You will explore some techniques for detecting this problem in the next lesson. Now, let's see what happens if you change some of the things in the FUNCTION and the reference to it. What would happen if you used a constant instead of No in the reference, Cube(No)? ##### DO: Change the WriteLn statement to: WriteLn('The cube is: ', Cube(3) ); Run the program several times with different input values. Do you get the same result no matter what you input? Notice that the variable, No, which you input, is no longer used in the FUNCTION reference, Cube(3). Can you use expressions in the reference to the function? ##### DO: Change the FUNCTION reference to Cube(No - 2). Run the program several times with input values: 5 (No - 2) is 3, so cube should be 27. 1 (No - 2) is -1, so cube should be -1. -1 (No - 2) is -3, so cube should be -27. Do integer expressions work o.k? What if you tried to use a non-integer value in the Cube(No) reference? ##### DO: Modify the WriteLn statement to: WriteLn('The cube is: ', Cube('This is a string') ) î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 70 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 Run the program. Did the compiler complain? Error 26: Type mismatch. Note that the cursor is positioned at or near the problem. The problem: 'This is a string' was given to the function to use as an integer value, but it is a string, not an integer. A good way to learn to recognize errors, is to introduce one error at a time and check the results. Let's look at some errors which you might make in setting up the FUNCTION itself. ##### DO: Change the type of Number to Char. The FUNCTION declaration should appear: FUNCTION Cube(Number:Char) : Integer; What happens when you attempt to run or compile the program? Error 41: Operand types do not match operator. Notice that the cursor stops, not at the line you modified, but at the line: Cube := Number * Number * Number; Why? There is nothing wrong with the line you changed. It may not do what you want it to, but to the computer, it is just an Integer FUNCTION with a character for input. The problem appears (to the compiler) when an attempt is made to multiply a character times a character in the line marked by the cursor. At this point the compiler alerts you to the problem. DEBUGGING NOTE: DON'T FORGET, THE CURSOR POINTS TO THE PLACE WHERE THE ERROR WAS DETECTED BY THE COMPILER. THE CAUSE OF THE ERROR MAY BE ELSEWHERE! What would happen if you inadvertently designated the function as something other than Integer? ##### DO: Change the FUNCTION declaration to: î TURBO-LESSONS - A Pascal Tutorial Version 5.0 Page 71 Copyright 1989 Lyle M. Faurot Lessons 1-16: $10.00 FUNCTION Cube(Number:Integer) : Char; Run the program. What results did you get? Error 26 again - Type mismatch. Again, the cursor points to the calculation. This time though, the calculation itself is o.k. The problem occurs when an attempt is made to assign the integer result to the Character FUNCTION, Cube. ********************** SHAREWARE REMINDER ************************ Are you enjoying the lessons? Have you sent your payment yet? If you are like me, no matter how good your intentions, you may neglect to send your check if you wait until you have finished the lessons and set them aside! (Just a friendly reminder--I would like to write more tutorials. Your check makes that possible.)