Introduction. Pretest and Posttest Loops. Basic Loop Structures ว ทยาการคอมพ วเตอร เบ องต น Fundamentals of Computer Science

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1 ว ทยาการคอมพ วเตอร เบ องต น Fundamentals of Computer Science ภาคการศ กษาท ภาคการศกษาท 1 ป ปการศกษา การศ กษา การเข ยนโปรแกรมพ นฐาน 4.5 โครงสร างควบค มแบบท าซ า รวบรวมโดย อ. ดร. อาร ร ตน ตรงร ศม ทอง Introduction A section of code that is repeated is called a loop: because after the last statement in the code is executed, the program branches, or loops, back to the first statement and starts another repetition through the code Each repetition is also called an iteration ti or pass through h the loop A First Book of ANSI C, Fourth Edition 2 Basic Loop Structures Pretest and Posttest Loops Constructing a repeating section of code requires that four elements be present: Repetition statement while statement for statement do-while statement Condition A statement that initially sets the condition being tested A statement within the repeating section of code that alters the condition so that it eventually becomes false In a pretest loop, the condition is checked at the beginning of each iteration. In a post-test loop, the condition is checked at the end of each iteration. A First Book of ANSI C, Fourth Edition 3 A First Book of ANSI C, Fourth Edition 4 Counter Controlled Loop Counter controlled loop: the condition is used to keep track of the number of repetitions Also known as a fixed count loop int count=0; while (count < 5) printf("\n"); count = count + 1; printf("finished\n"); Finished Number of repetitions is known when entering the loop Condition Controlled Loop Condition controlled loop: the tested condition does not depend on a count being achieved, but rather on a specific value being encountered int mark; Enter a mark: -12 Invalid! Enter a new mark: -65 printf("enter a mark: "); Invalid! Enter a new mark: 1000 scanf("%d", &mark); Invalid! Enter a new mark: 200 Invalid! Enter a new mark: 56 Finished while (mark<0 mark>100) printf("invalid! Enter a new mark: "); scanf("%d", &mark); printf("finished\n"); Number of repetitions is not known when entering the loop A First Book of ANSI C, Fourth Edition 5 A First Book of ANSI C, Fourth Edition 6 1

2 Sentinel Controlled Loop Sentinel controlled loop: a special input value indicates that there is no more data int num; while (num!= -1) printf("enter a number: "); scanf("%d", &num); printf("finished\n"); Enter a number: 12 Enter a number: 23 Enter a number: 20 Enter a number: -1 Finished Number of repetitions is not known when entering the loop The while Statement The general form of the while statement is while (expression) statement; The transfer of control back to the start of a while statement to reevaluate ate the expression is known as a program loop The following is a valid but infinite loop: int count=1; while (count <= 10) printf("%d ",count); A First Book of ANSI C, Fourth Edition 7 A First Book of ANSI C, Fourth Edition 8 The while Statement (cont.) The while Statement (cont.) Output is: Output is: A First Book of ANSI C, Fourth Edition 9 A First Book of ANSI C, Fourth Edition 10 Computing Sums and Averages Using a while Loop Computing Sums and Averages Using a while Loop (cont.) A First Book of ANSI C, Fourth Edition 11 A First Book of ANSI C, Fourth Edition 12 2

3 Computing Sums and Averages Using a while Loop (cont.) Computing Sums and Averages Using a while Loop (cont.) Ensures that any previous value present in the storage locations assigned to the variable total is overwritten and the total starts at a correct value Accumulating statement Calculating an average A First Book of ANSI C, Fourth Edition 13 A First Book of ANSI C, Fourth Edition 14 Sentinels Sentinels (continued) A program, such as Program 5.7, can be made much more general by removing the restriction that exactly four numbers are to be entered The user enters a value for how many numbers will be averaged You can use a sentinel (a data value used to signal either the start or end of a data series) The sentinel values must be selected so as not to conflict with legitimate data values A First Book of ANSI C, Fourth Edition 15 A First Book of ANSI C, Fourth Edition 16 Sentinels (continued) Sentinels (continued) One useful sentinel in C is the named constant EOF (End Of File) The actual value of EOF is compiler dependent, but it is always assigned a code that is not used by any other character EOF is defined in stdio.h A First Book of ANSI C, Fourth Edition 17 A First Book of ANSI C, Fourth Edition 18 3

4 The break and continue Statements A break forces an immediate exit from while, switch, for, and do-while statements only int count=1; float num; while(count <= 10) printf("enter a number: "); scanf("%f", &num); if (num > 76) printf("you lose!"); break; /* break out of the loop */ else printf("keep on truckin!"); count++; /* break jumps to here */ Sample output 1: Enter a number: 1 Keep on truckin!enter a number: 2 Keep on truckin!enter a number: 20 Keep on truckin!enter a number: 100 You lose! Sample output 2: Enter a number: 1 Keep on truckin!enter a number: 2 Keep on truckin!enter a number: 3 Keep on truckin!enter a number: 4 Keep on truckin!enter a number: 5 Keep on truckin!enter a number: 6 Keep on truckin!enter a number: 7 Keep on truckin!enter a number: 8 Keep on truckin!enter a number: 9 Keep on truckin!enter a number: 10 Keep on truckin! A First Book of ANSI C, Fourth Edition 19 The break and continue Statements The continue applies to loops only; when a continue statement is encountered in a loop, the next iteration of the loop begins immediately int count=0; float grade, total=0; while (count < 5) printf("enter a grade: "); scanf("%f", &grade); if(grade < 0 grade > 100) continue; total = total + grade; count = count + 1; printf("total is %.2f\n", total); Sample output 1: Enter a grade: 20 Enter a grade: 30 Enter a grade: 40 Enter a grade: 50 Total is Sample output 2: Enter a grade: 30 Enter a grade: Enter a grade: 20 Total is A First Book of ANSI C, Fourth Edition 20 The Null Statement A semicolon with nothing preceding it is also a valid statement, called the null statement ; Use the null statement where a statement is syntactically required, but no action is needed Null statements typically are used either with while or for statements The for Statement The for statement combines all four elements required to easily produce a loop on the same line for (initializing list; tested expression; altering list) statement; This statement does notreq require thatan any ofthe items in parentheses be present or that they actually be used for initializing or altering the values in the expression statements However, the two semicolons must be present for ( ; count <= 20;) is valid Omitting tested expression results in infinite loop A First Book of ANSI C, Fourth Edition 21 A First Book of ANSI C, Fourth Edition 22 Output is: A First Book of ANSI C, Fourth Edition 23 A First Book of ANSI C, Fourth Edition 24 4

5 A First Book of ANSI C, Fourth Edition 25 A First Book of ANSI C, Fourth Edition 26 Computing Sums and Averages Using a for Loop Comma-separated list A First Book of ANSI C, Fourth Edition 27 A First Book of ANSI C, Fourth Edition 28 Case Studies: Loop Programming Techniques Technique 1: Selection within a loop Technique 2: Input data validation Technique 3: Interactive loop control Technique 4: Evaluating equations Technique 1: Selection within a Loop A First Book of ANSI C, Fourth Edition 29 A First Book of ANSI C, Fourth Edition 30 5

6 Technique 2: Input Data Validation Technique 2: Input Data Validation (cont.) Same code used in lines 6-7! A First Book of ANSI C, Fourth Edition 31 A First Book of ANSI C, Fourth Edition 32 Technique 3: Interactive Loop Control Technique 4: Evaluating Equations A First Book of ANSI C, Fourth Edition 33 A First Book of ANSI C, Fourth Edition 34 Technique 4: Evaluating Equations (cont.) Nested Loops A First Book of ANSI C, Fourth Edition 35 A First Book of ANSI C, Fourth Edition 36 6

7 Nested Loops (continued) Nested Loops (continued) Sample run: i is now 1 i is now 2 i is now 3 i is now 4 i is now 5 A First Book of ANSI C, Fourth Edition 37 A First Book of ANSI C, Fourth Edition 38 The do-while Statement The do-while Statement (cont.) The general form of the do statement is do statement; while (expression); do-while is a posttest loop One type of application is ideally suited for a posttest loop: input data validation application do printf("\nenter an ID number: "); scanf("%f", &idnum); while (idnum < 1000 idnum > 1999); A First Book of ANSI C, Fourth Edition 39 A First Book of ANSI C, Fourth Edition 40 7