1. Repetition and Loop Statements
Chapter 5
Repetition and Loop Statements
chap5

2. 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
chap5

3. Flow Diagram of Loop Choice
chap5

4. Comparison of Loop Kinds
chap5

5. The while statement Counter-controlled loop (or counting loop)
A loop whose required number of iterations can be determined before loop execution begins.
The syntax
while (loop repetition condition)
statement;
Loop repetition condition: the condition that controls loop repetition.
Infinite loop: a loop that executes forever
chap5

6. Example count_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");
chap5

7. Flowchart for a while Loop
chap5

8. 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.
chap5

9. Example
chap5

10. Compound Assignment Operators
C provide special assignment operators variable op = expression;
chap5

11. 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 .
chap5

12. Counting Loop by for Statement
chap5

13. for Statement
chap5

14. 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.
chap5

15. Prefix and Postfix Increments
The value of the expression in which the ++/-- operator is used depends on the position of the operator.
chap5

16. Example: Factorial Function
chap5

17. Inc. and Dec. Other Than 1
chap5

18. 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)
chap5

19. chap5

20. chap5

21. 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
chap5

22. 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)
chap5

23. 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.
chap5

24. Example of Endfile-Controlled Loops
chap5

25. Infinite Loops on Faulty Data
Detect faulty data
chap5

26. Nested Loops
Consist of an outer loop with one or more inner loops. (Fig. 5.13)
chap5

27. The do-while Statement
When we know that a loop must execute at least one time.
chap5

28. Flag-Controlled Loops
A flag is a variable used to represent whether or not a certain event has occurred.
(Fig. 5.14)
chap5

29. chap5

30. 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.
* *******
*** *****
***** ***
******* *
chap5

31. Summary Repetition and Loop Statements while, for, and do-while
Counter-Controller Loop
Sentinel-Controller Loop
Endfile-Controller Loop
Input Validation Loop
General Conditional Loop
while, for, and do-while
Compound Assignment Operators
chap5