copyright ©2004 pearson addison-wesley. all rights reserved. chapter 4 selection structures: if and...

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.

Chapter 4Selection Structures: if and switch StatementsDr. J.-Y. Pan Dept. Comm. Eng.Nat. Chung Cheng Univ.http://[email protected]

中正大學通訊工程系潘仁義老師 Advanced Network Technology Lab

Outline

Conditions and if statement Short-circuit evaluation DeMorgan’s Theorem

Case study: (Decision, Data flow) Water Bill problem

Case study: (Modified with changed spec)Water Bill with Conservation Requirement

Nested if statementsSwitch


Control Structures

Control the flow of execution in a program or function.

Combine individual instruction into a single logical unit with one entry point and one exit point.

Instructions are organized into three kinds of control structures to control execution flow: sequence, selection, and repetition.


Compound Statement

A group of statements bracketed by {and} that are executed sequentially.

{

statement 1;

statement 2; : :

statement n; }


Control Statement( 先偷偷看一下 )

Conditional/Selection statements the if statements the switch statements

Repetition statements the for statements the while statements the do-while statements


Conditions

A program chooses among alternative statements by testing the value of key variables.

An expression that is either false (represented by 0) or true (usually represented by 1)


Relational and Equality operators


Logical Operators

With the three logical operations-- &&(and), ||(or), !(not) —we can form more complicated conditions or logical expressions.

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Logical Operators

The logical operators perform logical operations

! Logical not

(TRUE if the operand is FALSE)

&& Logical and

(TRUE if both operand are TRUE)

|| Logical or

(TRUE if either or both operands are TRUE)


Operator Precedence

Not, plus sign, minus sign, and

address of


Figure 4.1 Evaluation Tree and Step-by-Step Evaluation for !flag || (y + z >= x - z)


Short-Circuit EvaluationThe evaluation of a Boolean expression

stops as soon as its answer is known. This style of evaluation is called short-circuit evaluation.

Stopping evaluation of a logical expression as soon as its value can be determined.

(0 && ?) …… True or false?

(1 || ?) …… True or false?Figure 4.1


Writing English Conditions in C

Figure 4.2 Range of True Values for min <= x && x <= max

x = 3.0, y = 4.0, z = 2.0

Figure 4.3 Range of True Values for z > x || x > y


Comparing Characters

Expression Value

‘9’ >= ‘0’ 1(true)

‘a’ < ‘e’ 1(true)

‘B’ <= ‘A’ 0(false)

‘Z’ == ‘z’ 0(false)

‘a’ <= ‘A’ system dependent

0(false, in ascii)

‘a’ <= ch && ch <= ‘z’ 1(true) if ch is a lowercase letter


Complementing a Condition

DeMorgan’s Theorem The complement of expr1 && expr2

comp1 || comp2

The complement of expr1 || expr2

comp1 && comp2

Example (Example 4.7 & 4.8)age > 25 && (status == ‘S’ || status == ‘D’)

age <= 25 || (status != ‘S’ && status != ‘D’)


The if statements

The if statement is used to express

alternations

if (bool-expr) stmt1 else stmt2

where the else clause is optionalThe bool-expr is evaluated.

If it is TRUE, stmt1 is executed.

If it is FALSE and if there is an else clause ,stmt2 is executed instead.


Figure 4.4 Flowcharts of if Statements with (a) Two Alternatives and (b) One Alternative


An Example

#include <stdio.h> main( ) { int score;

printf (“score =?”); scanf (“%d”, & score); If (score>=60) printf (“ pass!\n”); else printf (“ fail! \n ”); }


Figure 4.5 Example: if Statement to Order x and y


Table 4.9 Tracing an if Statement

Statement Part x y temp Effect

12.5 5.0 ?

if (x>y) { 12.5>5.0 is true.

temp = x ; 12.5 Store old x in temp.

x = y ; 5.0 Store y in x.

y = temp ; 12.5 Store old x in y.


Four if-statements forms

The Single-line if statements If (bool-expr) stmt;

The Multi-line if statements If (bool-expr) { stmt; }

The If-else statements If (bool-expr) { stmtT; } else { stmtF; }

The Cascading if statements If(bool-expr1) { stmt1; }

else if (bool-expri ) { stmtsi; } else { stmtsnone; }


Decision steps in algorithms

Algorithm steps that select from a choice of actions are called decision steps.

Case study: water bill problem This case contains decision steps to compute and display a customer’s water bill based on usage.


Case study water bill problem

基本費

使用費逾期未繳

滯納費


Case Study ： Water Bill Problems (cont)

Design (Figure 4.6) Initial algorithm

1. Display user instructions.

2. Get data: unpaid balance, previous and current meter

readings.

3. Compute use charge.

4. Determine applicable late charge.

5. Figure bill amount.

6. Display the bill amount and charges.


Figure 4.6 Structure Chart for Water Bill Problem

請由這開始看

所有費用相加

Data flow


Case Study ： Water Bill Problems (cont) Analysis and Design of COMP_USE_CHARGE

Input parameters int previous int current

Return value double use_charge

Program variable int used

Relevant formulas used = current meter reading – previous meter reading use charge = used x charge per thousand gallons


Case Study ： Water Bill Problems (cont) Analysis and Design of COMP_USE_CHARGE

Algorithm for COMP_USE_CHARGE

1. used is current – previous2. use charge = used x charge per thousand gallons


Case Study ： Water Bill Problems (cont) Analysis and Design of COMP_LATE_CHARGE

Input parameter double unpaid

Return value double late_charge

Algorithm for COMP_LATE_CHARGE1. if unpaid > 0

assess late charge else

assess no late charge pseudocodea combination of English and C constructs to describe algorithm steps


Case Study ： Water Bill Problems (cont) Analysis and Design of DISPLAY_BILL

Input parameters double late_charge double bill double unpaid

Algorithm for DISPLAY_BILL 1. if late_charge > 0

display late charge and unpaid balance

2. Display the bill amount.


Figure 4.7 Program for Water Bill Problem

Constant macros


Figure 4.7 Program for Water Bill Problem (cont’d)


Figure 4.7 Program for Water Bill Problem (cont’d)

Cohesive Function:Performs a single operation


Figure 4.7 Program for Water Bill Problem and Sample Run


Figure 4.8 Sample Run of Water Bill Program


Program Style

Consistent use of names in functions Avoid the confusion from using different names

to reference the same information Ex. late_charge

Cohesive( 有結合力的 ) functions a function that performs a single operation easier to read, write, debug, maintain, reusable

Using constant macros to enhance readability and ease maintenance Ex.DEMAND_CHG, PER_1000_CHG, LATE_C

HG


More problem solvingAn example of maintenance and update

Often what appears to be a new problem will turn out to be a variation of one that you have already solved .

An important skill: ability to recognize one problem is similar to another solved earlier

If the original program is well designed and modular, you can accommodate changing spec with minimum effort


Case Study ： Water Bill with Conservation Requirement

<Step 1> Problem We need to modify the water bill program so

that customers who fail to meet conservation requirements are charged for all their water use at twice the rate of customers who meet the guidelines.

Residents of this water district are required to use no more than 95% of the amount of water they used in the same quarter last year in order to qualify for the lower use rate of $1.10 per thousand gallons.


Case Study ： Water Bill with Conservation Requirement (cont)

<Step 2> Analysis (Additions to data requirements) Problem constants

OVER_CHG_RATE 2.0CONSERV_RATE 95

Problem inputsint use_last_year


Case Study ： Water Bill with Conservation Requirement (cont)

<Step 3> Algorithm for COMP_USE_CHANGE 1. used is current – previous 2. if guidelines are met

use_charge is used*PER_1000_CHANGE

else

notify customer of overuse

use_charge is used*overuse_chg_rate *

PER_1000_CHANGE

(Figure 4.9)


Figure 4.9( CASE STUDY ) Function comp_use_charge Revised


4.7 Nested if statements and multiple-alternative decisions


4.7.2 Multiple-Alternative Decision Form of Nested if

Syntax:if (condition1)

statement1

else if (condition2)

statement2

.

.else if (conditio

nn)statementn

elsestatemente

Example:

if (x>0) num_pos = num_pos +1else if (x<0) num_neg = num_neg +1else num_zero = num_zero+1


Example 4.17Computing tax

Use a multiple-alternative if statement to implement a decision table that describes several alternatives. (Figure 4.10)

Salary Range($) Base Tax($) Percentage of Excess

0.00- 1,4999.99 0.00 15

15,000.00- 29,999.99 2,250.00 18

30,000.00- 49,999.99 5,400.00 22

50,000.00- 79,999.99 11,000.00 27

80,000.00- 150,000.00 21,600.00 33


Figure 4.10 Function comp_tax

Salary = $25000


Example 4.19 Warning signal controller(Figure 4.11)

Control the warning signs at the exists of major tunnels.

If roads are slick, you want to advise drivers that stopping times are doubled or quadrupled, depending on whether the roads are wet or icy.

Access to current temperature for checking whether the temperature is below or above freezing.


Figure 4.11 Flowchart of Road Sign Decision Process


Example 4.19 (cont)

if (road_status == ‘S’ )if (temp >0){

printf(“Wet roads ahead\n”);printf(“Stopping time doubled\n

”);}else {

printf(“Icy roads ahead\n”);printf(“Stopping time quardrupl

ed\n”);}

elseprintf(“Drive carefully!\n”);

假如 else 沒了


The switch statement

The switch statement is especially useful when the selection is base in the value of a single variable or of a simple expression (called the controlling expression)

The value of this expression may be of type int or char ,but not of type double or string.


An Example#include <stdio.h>int main(void) { int cardRank; printf (“cardRank=?) ;

scanf(“%d”,&cardRank); switch (cardRank) { case1:printf(“Ace\n”);break; case11:printf(“Jack\n”);break; case12:printf(“Queen\n”);break; case13:printf(King\n”);break; default:printf(“%d\n”,cardRank);break; }}


Figure 4.12 Example of a switch Statement with Type char Case Labels


Common programming errors(1/4)

When testing for equality , be sure to use the“ == “operator instead of the“ =“ operator .

This error is extremely common

if (x==0) {…}

if (x=0) {…} /*always FALSE */else 跟最近的 if 成對，如果不是，記得用 { }Multiple-alternative, 舉例 : leap year



Be careful when using logical operators because human languages can be somewhat funny in expressing logic

“ x is not equal to either 2 or 3 ”

if (x!=2 || x!=3) {…}

if (!(x==2 || x==3)) {…}

if (x!=2&&x!=3) {..}



To test whether a number is in a particular range ,it is not sufficient to combine relational operators , as is conventional in mathematics

0 < x < 1 0The two part of the conditon must be written

explicity using &&

( 0 < x ) && ( x < 10 )



In switch statement, control expression and case labels are of the same permitted type (int or char but not double)

“default” if requiredEach alternative is ended by a break

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