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PROGRAM 5-1
Program run
Turbo Pascal Version 6.0 Copyright (c) 1983,90 Borland International Please enter four scores separated by space 80 90 60 75 Average: 76.25 Please enter four scores separated by space 44 49 90 99 Average: 70.50 Please enter four scores separated by space ... program continues for total of 20 different students. This program will find the average of four scores for twenty students. To keep a count of how many we have done so far we use a counter called count. Count is incremented each time after the average of a set of scores has been calculated. We will begin with a one, after we calculate the first student's average the count will be incremented to two. Incrementing is done as follows: Count := Count + 1; This statement requires a special understanding. As we learned earlier, a variable is a name of a memory location. So there is a memory location called Count. With this instruction we are telling Pascal to add 1 to the number currently in the location called count, and to place the resulting number in the same memory location. Therefore, if count currently has 1, after carrying out this instruction, count will have a value of 2. Now that we know how to keep track of how many averages we have done, we need some way of telling Pascal to do the same set of instructions (to find the average) over and over again until we have calculated averages for all twenty students. This is accomplished by: while count <= 20 do begin . . end; Every instruction within the begin-end pair will be repeated as long as count is less than or equal to 20. If count is greater than 20 then the loop will terminate. In order for this loop to work properly, it is quite apparent that the count should have a value twenty or below for the loop to work. So we initialize the counter to 1. After the calculation is done, we increment the count by 1. count := 1; while count <= 20 do begin . . count := count + 1 end; Here is some more information about the while loop. The expression controlling the repetition (loop control variable - LCV) is evaluated BEFORE the loop is executed. Therefore, the controlling expression MUST be initialized first. In this example, Count is initialized to 1. At the BOTTOM OF THE LOOP the controlling expression must be changed. If not, an endless loop will occur. Suppose we forgot to increment the counter and the value of count remains as 1. Since the value of count is 1, the loop will continue to execute without ever going to the next statement after the loop. If this happens, you may even have to turn the computer off. So don't forget, initialize the LCV above the loop and change the value of LCV inside the loop. If we had initialized the LCV to 21 (instead of 1), the loop would not be executed at all. This is because when Pascal checks the condition, count <= 20, it will receive the value false.
Program run:
Please enter four scores separated by space 85 90 44 33 Average: 63.00 Please enter four scores separated by space 66 33 90 56 Average: 61.25 Please enter four scores separated by space Program run continues for 20 students... The statements between the symbols REPEAT and UNTIL are executed in sequence until the expression yields true. The condition is evaluated at the BOTTOM of the loop. Therefore, the loop is executed at least once. Notice in this example Count is initialized to 0 above the loop. As soon as the loop is entered the count is incremented to 1. Then at the bottom of the loop it is evaluated: Count :=0; repeat Count := Count +1; . . until count = 20; Among the other differences between the while loop and the repeat loop, the LCV will end up with different values (this is not always true, however). After termination of the while loop, the LCV would have the value of 21. After the repeat loop the LCV would have a value of 20. FOR DO STATEMENT If you know exactly how many time a sloop must be executed, then Pascal provides you with another version of the WHILE loop called the FOR DO loop. It works identically to the while loop, except Pascal takes care of initializing, incrementing, and checking the LCV. So why not use the FOR DO loop always? For Do loop is limited to doing a loop a particular number of times. Whereas the While loop and Repeat loops can be used with any kind of condition. You may want to say 'while not(end of file) do' or 'repeat..until end of file'. Here you do not know exactly how many times to do it, but to do it until the end of file has been reached. PROGRAM 5-3
Program run:
89 89 90 78 Average: 86.50 Please enter four scores separated by space 99 99 78 88 Average: 91.00 Please enter four scores separated by space Program run continues for 20 students... Pascal assigns the first and last value of the control variable based on the FOR statement. The control variable must be of an ordinal type and the value assigned should be compatible with that type. Ordinal type means that a type that has an order: something comes before it and something comes after it. For example, take a 10; we know 9 comes before ten and 11 comes after. Take the character 'C', we know 'B' comes before it and 'D' comes after it. You can count down using downto (instead of to in counting up). If these three loop concepts are difficult to follow, you need to study and compare the following few programs. Review these programs several times. Suppose we want to display 1 to 100 on the screen we could write some thing like this: Program CountUp (output); var
begin
writeln(count); {prints 1} count := count +1; {increment count by 1} writeln(count); {print 2} count := count +1; {increment count by 1} writeln(count); {print 3} . {continue these steps} . {for a total of 100 times} end. As you can see this can be a very tedious process. You will get very bored with writing the same lines over and over again. The following three programs do the same thing, writes 1 to 100 on the screen. PROGRAM 5-4
Program run:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 PROGRAM 5-5
Program run:
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 Type EXIT to return to Turbo Pascal... PROGRAM 5-6
All these three programs are self explanatory. No further explanation is given here. If you have any questions, please re-read this chapter. I had mentioned that you could count down as well. Here is a program that will count down from 100 to 1. PROGRAM 5-7
Program run:
100 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 83 82 81 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 Type EXIT to return to Turbo Pascal... Now let us write A..Z and then backwards (Z..A). Here also we will use the for loop: PROGRAM 5-8
Program run:
ZYXWVUTSRQPONMLKJIHGFEDCBA Type EXIT to return to Turbo Pascal... ASSIGNMENTS FOR CHAPTER 5 1. Write a program to write every even number from
0 to 100 using the while loop.
PROGRAM 5-9
Program run:
divide two numbers. It is ask for one number, then the operator (only enter +, -, *, or /) and finally for the second number. Enter a number 15 Operator * Enter second number (must be non-zero) 5 The result is: 75.00 Type EXIT to return to Turbo Pascal... Microsoft(R) MS-DOS(R) Version 5.00 (C)Copyright Microsoft Corp 1981-1991. C:\TP> Go to top of this chapter |