Chapter Four2

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if/else Selection Structure
• Nested if/else structures – One inside another, test for multiple cases – Once condition met, other statements skipped
false
34 is true
Author: J. de Wet
10
relational operator
Operator == != < > <= >= Meaning equivalent to not equal to less than greater than less than or equal to greater than or equal to
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Logical Operators
• Used as conditions in loops, if statements • && (logical AND)
– true if both conditions are true if ( gender == 1 && age >= 65 ) ++seniorFemales;
4
例 从键盘输入大写字母，用小写字母输出
#include <iostream> using namespace std; int main() { char c1,c2; cin>>c1; cout<<c1<<endl; c2=c1+32; cout<<c2<<endl; return 0; }
输入：A 输出：A a
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１．
ＩＦ（条件表达式）
语句；
表达式 1 语句 0
或
ＩＦ（条件表达式） ｛ ｝ 语句；
或
ＩＦ（条件表达式）； 语句;
空语句作ＩＦ中的语句
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２． if．．．else 语句
if（条件表达式） 语句１； else 语句２；Y 判断语句1N
语句2
例 判闰年 #include <iostream.h> void main() { int year; cout << "Please input a year..."; cin >> year; // think about the sequence of the logical expression if (year %4 == 0 && year % 100 != 0 || year % 400 == 0) cout << "The year is leap year "; else cout << "The year isn't leap year "; }
if student’s grade is greater than or equal to 90 Print “A” else if student’s grade is greater than or equal to 80 Print “B” else if student’s grade is greater than or equal to 70 Print “C” else if student’s grade is greater than or equal to 60 Print “D” else Print “F”
——嵌套
一般形式
if ( ) if ( else else if ( else ) 语句 1 语句 2 语句 3 语句 4
)

注意
语句 1、2、3、4 可以是复合语句，每层的 if 与 else 配对，或用 { } 来确定层次关系。
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– if ~ else 配对原则：缺省{ }时，else总是和它 上面离它最近的未配对的if配对 if(……) if(……) if(……) else…... else…... else…...
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if 语句
——三种形式
if (表达式) 语句
例：if (x>y) cout<<x;
if (表达式) 语句1 else 语句2
例：if (x>y) cout<<x; else cout<<y;
if (表达式1) 语句1 else if (表达式2) 语句2 else if (表达式3) 语句3 … else 语句 n
2
•
Control Structures
control structures – Sequence structure • Programs executed sequentially by default – Selection structures •if, if/else, switch – Repetition structures（Iteration） • while, do/while, for
6
顺序结构
思考：鸡兔同笼，已知鸡兔总头数为h,总 脚数为f,求鸡兔各多少只？
分析：设鸡x只，兔y只 则x+y=h,2*x+4*y=f从而可解得x=2h-1/2f y=f/2-h
7
if Selection Structure
• Selection structure – Choose among alternative courses of action – Pseudocode example: If student’s grade is greater than or equal to 60 Print “Passed” – If the condition is true • Print statement executed, program continues to next statement – If the condition is false • Print statement ignored, program continues
5
#include <iostream> #include <cmath> using namespace std; main() { float a,b,c,disc,x1,x2,p,q; 输入：1 3 2 cin>>a>>b>>c; 输出：x1=-1.00 disc=b*b-4*a*c; x2=-2.00 p=-b/(2*a); q=sqrt(disc)/(2*a); x1=p+q; x2=p-q; cout<<"x1=“<<x1<<endl; cout<<"x2="<<x2<<endl; }
• Block
– Set of statements within braces
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if/else Selection Structure
• Example
if ( grade >= 90 ) cout << "A"; else if ( grade >= 80 ) cout << "B"; else if ( grade >= 70 ) cout << "C"; else if ( grade >= 60 ) cout << "D"; else cout << "F"; // 90 and above // 80-89 // 70-79 // 60-69 // less than 60
Learn C++ through English and Chinese
Lecture 4: Selection and Iteration （选择与循环）
Chapter Four: Selection and Iteration （选择与循环）
• Selection（选择） The if statement The switch statement The conditional operator ?: Iteration（循环） The while statement The do-while loop The for statement
11
relational operator
• 关系运算是比较简单的一种逻辑运算， 优先次序为： < <= > >= == !=
优先级相同（高） 优先级相同（低）
• 关系表达式是一种最简单的逻辑表达 式
其结果类型为 bool，值只能为 true 或 false。
• 例如：a>b，c<=a+b，x+y==3
8
if Selection Structure
• Translation into C++ If student’s grade is greater than or equal to 60 Print “Passed” if ( grade >= 60 ) cout << "Passed"; • if structure – Single-entry/single-exit
9
if Selection Structure
• Flowchart of pseudocode statement
A decision can be made on any expression.
grade >= 60 true
zero - false