Chapter 5

20061018 chap5

Chapter 5

Repetition and Loop Statements

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Repetition in Programs In most commercial software, you can

repeat a process many times. When using an editor program, you can

move the cursor to a program line and perform as many edit operations as you need to.

Loop is a control structure that repeats a group of steps in a program.

Three C loop control statements while for do-while

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Flow Diagram of Loop Choice

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Comparison of Loop Kinds

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The while statement Counter-controlled loop (or counting

loop) A loop whose required number of iterations

can be determined before loop execution begins.

The syntaxwhile (loop repetition condition)

statement; Loop repetition condition: the condition

that controls loop repetition. Infinite loop: a loop that executes forever

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Examplecount_emp = 0; /* no employees processed yet

*/while (count_emp < 7) { /* test value of

count_emp */ printf("Hours> "); scanf("%d", &hours); printf("Rate> "); scanf("%lf", &rate); pay = hours * rate; printf("Pay is $%6.2f\n", pay); count_emp = count_emp + 1; /* increment

count_emp */}printf("\nAll employees processed\n");

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Flowchart for a while Loop

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Computing a Sum or a Product in a Loop Loops often accumulate a sum

or a product by repeating an addition or multiplication operation.

Accumulator A variable used to store a value

being computed in increments during the execution of a loop.

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Example

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Compound Assignment Operators

• C provide special assignment operators variable op = expression;

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The for Statement Three loop control components with

the loop body. Initialization of the loop control variable, Test of the loop repetition condition,

and Change (update) of the loop control

variable. The for statement in C supplies a

designed place for each of these three components .

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Counting Loop by for Statement

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for Statement

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Increment and Decrement Operators

The increment (i.e., ++) or decrement (i.e., --) operators are the frequently used operators which take only one operand.

for(int i=0; i<100; i++) {…}

for(int i=100; i>0; i--) {…}

Side effect: the value of its operand is incremented/decremented by one.

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Prefix and Postfix Increments

The value of the expression in which the ++/-- operator is used depends on the position of the operator.

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Example: Factorial Function

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Inc. and Dec. Other Than 1

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Conditional Loops In many programming situations,

you will not be able to determine the exact number of loop repetitions before loop execution begins.

Example: Monitor Gasoline Storage Tank

(Fig. 5.9)

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Loop Design Problem-Solving Questions for Loop Design

What are the inputs? What are the outputs? Is there any repetition? Do I know in advance how many time steps will

be repeated? How do I know how long to keep repeating the

steps? Sentinel-Controlled Loops Endfile-Controlled Loops Infinite Loops on Faulty Data

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Sentinel-Controlled Loops

One way to do this is to instruct the user to enter a unique data value, called a sentinel value, after the last data item.(Fig. 5.10)

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Endfile-Controlled Loops

A data file is always terminated by an endfile character that can be detected by the scanf and fscanf functions.

EOF stands for the endfile character.

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Example of Endfile-Controlled Loops

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Infinite Loops on Faulty Data 70 7o

Detect faulty data

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Nested Loops Consist of an outer loop with one

or more inner loops. (Fig. 5.13)

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The do-while Statement When we know that a loop

must execute at least one time.

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Flag-Controlled Loops A flag is a variable used to represent

whether or not a certain event has occurred.(Fig. 5.14)

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Homework #5 Due: 2006/10/25 利用迴圈和 '*' 可以畫出許多圖形

1. 定義一個 CENTER 常數 (#define CENTER 20) 表示所有圖形的中心位置

2. 實作出一個 function prototype 如下 : void triangle(int n); 其作用是繪出 n 排星號的等腰三角形 若 n 為正時，則畫出三角形是正立的，如 a. (n=4) 若 n 為負時，則畫出三角形是倒立的，如 b. (n=-4)

3. 主程式將輸入多個整數 ( 最後以 0 結尾 ) ，印出其對應的圖案 並判斷每個整數若不在 -9 與 9 的範圍內則不予理會之

a. b. * ******* *** ***** ***** *** ******* *

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Summary Repetition and Loop Statements

Counter-Controller Loop Sentinel-Controller Loop Endfile-Controller Loop Input Validation Loop General Conditional Loop

while, for, and do-while Compound Assignment Operators