Introduction to Computer Programming

School of Computer and Information Science Southwest Forestry University 2012.9 Introduction to Computer Programming

Chapter 3 Conditional Statements Danju Lv Autumn semester , 2012

Chap3 Conditional Statements • REVIEW • Definition of Conditional Statements • Logical judgment and conditional expression • if statement • else statement • elif statement • Mix of conditionals • 条件语句的定义 • 逻辑判断与条件表达式 • 单分支语句 • 双分支语句 • 多分支语句 • 条件嵌套语句

Chap3 conditional statements 条件语句是根据条件表达式的值是True/not zero还是False/zero做出决策，控制执行的代码块。由表达式的值控制执行的代码块 • 4 key points • Expression • Value of the Expression • Control • Block of Code/suit • 4个要点： • 表达式 • 值 • 控制 • 代码块

False 表达式 True true语句块 if 单分支语句 ifexpression: expr_true_suite The suite of the if clause, expr_true_suite, will be executed only if the above conditional expression results in a Boolean true value. Otherwise, execution resumes at the next statement following the suite. If 语句：若表达式为真/非零则执行冒号后的语句块，若表达式为假则跳过该语句块的执行。

【例3-2】 Input the radius of the circle from the keyboard, if the radius is larger than or equal to zero , then calculate the area of circle. Exp of if statement import math r=eval(input(‘请输入圆的半径：’)) if r>=.0: s=math.pi*r**2 print("s=pi*r*r=",s) 开始输入半径 False 半径不小于0 True 计算圆面积输出圆面积结果验证：请输入圆的半径：4 s=pi*r*r= 50.26548245743669 结束

Think • When input r is negative number, what’s the result? What we will see on the screen? • 2. The output of the program could be improved? • Such as when the input r is negative, give a prompt input error message, when the input r is non-negative, calculate the area of the circle.

Syntax of else statement: if 表达式 : Ture语句块 else : False语句块 The else statement identifies a block of code（expre false suite） to be executed if the conditional expression of the if statement resolves to a false Boolean value. 表达式 False True Ture语句块 False语句块图3-4 双分支语句的执行方式 Else statement 双语句分支 ifexpression: expr_true_suite else: expr_false_suite

请画出以下程序的框图结构，该程序的执行路线如何，结果是？若程序的x=2替换为x=-2,程序的执行路线又如何，结果？请画出以下程序的框图结构，该程序的执行路线如何，结果是？若程序的x=2替换为x=-2,程序的执行路线又如何，结果？ Understanding of else statement X=2 x = 2 if x > 0: y = 1+2*x else: y = 0 print('y=', y) x>0 False True y=1+2*x y=0 print(‘y=’,y)

Eg. Of Else Statements • Eg3_3. input an integer from the keyboard, if it is even, then print “even” ; if it is odd, then print "odd". [Analyze:] An integer is either an even number or an odd number, so it is an else statement case • The variable settings 变量的设置 • The conditional expression 条件表达式的表示 • Choose the right conditional statements 条件结构的确定

Block diagram 1. 使用 input(): 获取一个整数 2. 使用“=“ 将从键盘获取的数存于变量X start 1. Use input(): get a integer 2. Use “=“ save the num. in x variable from the keyboard Get num. from the keyboard False x被2整除? X%2==0 or x%2 is 0 True 输出“偶数” 输出“奇数” Use print() show the result end

start Get num. from the keyboard False x被2整除? True end program ifexpression: expr_true_suite else: expr_false_suite #Exp3_3.py x=eval(input('请输入一个整数：')) if x%2==0: #judge an even print(‘偶数') else: print('奇数') 输出“偶数” 输出“奇数” 程序运行结果：请输入一个整数：2 偶数

Eg. Of Else Statements • 【eg3-4】Design a program for a telecommunications company interview job seekers. The program is to tell the job seekers whether meet the required educational conditions. If Job seekers meet one of the following conditions ,they will receive an interview notice, if not , they will receive a rejection notice. The educational conditions are the following: 为某电信公司面试求职者设计一程序。该程序是给满足某些教育条件的求职者提供面试机会。满足如下条件的求职者会接到面试通知：

Educational conditions • (1) at least 25 years of age , the electronic information engineering graduates. • (2) Key Universities , Electronic Information Engineering graduates. • (3) Under 28-year-old , computer science graduate • Meet One of the above conditions can gain an interview （1）25岁以上，电子信息工程专业毕业生。（2）重点大学电子信息工程专业毕业生。（3）28岁以下，计算机专业毕业满足以上条件之一即可获得面试机会

【分析】：变量的设置，选择表达式， 分支语句分析条件可知公司面试条件涉及3个方面：年龄、所学专业和毕业学校。为此设定3个变量（age，subject和college）分别表示；选择条件设置：3个条件只需满足其中一个即可，其逻辑关系应为:或者or；而在每一个条件里涉及的方面又应为“与and”的逻辑关系。分支判断：若满足如上复合条件，则显示“恭喜，你已获得我公司的面试机会”，否则显示“抱歉，你未达到面试要求”。 Eg of else statement

Block of Diagram

Program （1）25岁以上，电子信息工程专业毕业生。（2）重点大学电子信息工程专业毕业生。（3）28岁以下，计算机专业毕业满足以上条件之一即可获得面试机会 #程序： #Exp3-6.py age=24 subject="计算机" college="非重点" if (age > 25 and subject=="电子信息工程")\ or (college=="重点" and subject=="电子信息工程" )\ or (age<=28 and subject=="计算机"): print(“恭喜，你已获得我公司的面试机会!") else: print(“抱歉，你未达到面试要求“) 程序运行结果：恭喜，你已获得我公司的面试机会!

例题中，作为面试者的条件是在源程序中进行设定的，如何获取求职者三方面的信息呢？可以在程序中设置3个问题：哪所学校毕业？学的专业是什么？求职者的年龄？例题中，作为面试者的条件是在源程序中进行设定的，如何获取求职者三方面的信息呢？可以在程序中设置3个问题：哪所学校毕业？学的专业是什么？求职者的年龄？ 1. 毕业的学校是重点院校吗？（1/2）： 1.重点 2. 非重点 2.学的专业是什么？（1/2/3）： 1.电子信息工程；2.计算 3.其它 3. 你的年龄？此时程序应如何改动呢？ Think

“Dangling else” avoidance • Python's design of using indentation rather than braces for code block delimitation not only helps to enforce code correctness, but it even aids implicitly in avoiding potential problems in code that is syntactically correct. One of those such problems is the (in)famous "dangling else" problem, a semantic optical illusion.

Diagram • Please draw diagram for those programs. And tell the results of those programs under the balance is negative or positive.

elif statement • Syntax of elif statement • ifexpression1: • expr1_true_suite • elif expression2: • expr2_true_suite • …. • elifexpressionN: • exprN_true_suite • else: • none_of_the_above_suite It allows one to check multiple expressions for truth value and execute a block of code as soon as one of the conditions evaluates to true. Like the else, the elif statement is optional. However, unlike else, for which there can be at most one statement, there can be an arbitrary number of elif statements following an if.

Block of diagram of elif statement arbitrary number of elif ifexpression1: expr1_true_suite elif expression2: expr2_true_suite …. elifexpressionN: exprN_true_suite else: none_of_the_above_suite

Eg. Of elif Statement • [3-5]Grade student achievement by computer (Fail, Pass, intermediate, Good, Excellent). Its criteria for the classification : • Fail is less than 60; • 60 to 70 are classified as Pass, • 70 to 80 are divided into Intermediate • 80 to 90 divided into Benign • 90 to 100-Excellent. • Finally, display its class information

Analyze • 1. Variable setting • Where to get date, how to save date • 2. Expression • Comparison expr. • 3. Conditions • Elif statement

Diagram of Example

Program 程序 score=eval(input('请输入你的成绩（0～100）：')) if score<0 or score>100: print('无效的成绩') elif score<60: print('不及格') elif score<70: print('及格') elif score<80: print('中等') elif score<90: print('良好') else: print('优秀') 程序运行结果：请输入你的成绩（0～100）：88 良好

在某一个分支的语句块中，需要进行新的分支。这种结构你为选择结构的嵌套。嵌套的形式如下：在某一个分支的语句块中，需要进行新的分支。这种结构你为选择结构的嵌套。嵌套的形式如下： if 表达式1: #语句块1 … if 表达式11: 语句块11… else: 语句块12 … else: 语句块2 选择结构的嵌套

【例3-6】选择结构的嵌套问题。购买地铁车票的规定如下：【例3-6】选择结构的嵌套问题。购买地铁车票的规定如下：乘1-4站，3元/位；乘5-9站，4元/位；乘9站以上，5元/位。输入人数、站数，输出应付款。分析：需要进行两次分支。根据“人数<=4”分支一次，表达式为假时，还需要根据“人数<=9”分支一次。选择程序举例

开始输入人数n、站数m False m<=4 True m<=9 False pay=5*n True pay=4*n 输出应付款结束图3-9 计算乘地铁应付款程序框图 pay=3*n

程序 #Exp3_5.py n, m=eval(input('请输入数，站数:')) if m<=4: pay=3*n else: if m<=9: pay=4*n else: pay=5*n print('应付款：', pay) 输入及程序运行结果：请输入数，站数:3,5 应付款： 12

Chapter Summarizes • Conditional statements • If statement • Else statement • Elif statement • Mix/nested of conditional statement • Deeply understand control structure • Core: the value of the expr. control the execution of suits/blocks of code.

Introduction to Computer Programming