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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"><HTML><HEAD><!-- This document was created from RTF source by rtftohtml version 3.0.1 --> <META NAME="GENERATOR" Content="Symantec Visual Page 1.0"> <META HTTP-EQUIV="Content-Type" CONTENT="text/html;CHARSET=iso-8859-1"> <TITLE>Teach Yourself C++ in 21 Days</TITLE></HEAD><BODY TEXT="#000000" BGCOLOR="#FFFFFF"><H1 ALIGN="CENTER"><A HREF="ch03.htm"><IMG SRC="../buttons/BLANPREV.GIF"WIDTH="37" HEIGHT="37" ALIGN="BOTTOM" BORDER="0"></A><A HREF="http://www.mcp.com/sams"><IMGSRC="../buttons/BLANHOME.GIF" WIDTH="37" HEIGHT="37" ALIGN="BOTTOM"BORDER="0"></A><A HREF="../index.htm"><IMG SRC="../buttons/BLANTOC.GIF"WIDTH="37" HEIGHT="37" ALIGN="BOTTOM" BORDER="0"></A><A HREF="ch05.htm"><IMG SRC="../buttons/BLANNEXT.GIF"WIDTH="37" HEIGHT="37" ALIGN="BOTTOM" BORDER="0"></A></H1><H1></H1><UL> <LI><A HREF="#Heading1">Day 4</A> <UL> <LI><A HREF="#Heading2">Expressions and Statements</A> <UL> <LI><A HREF="#Heading3">Statements</A> <UL> <LI><A HREF="#Heading4">Whitespace</A> <LI><A HREF="#Heading5">Blocks and Compound Statements</A> </UL> <LI><A HREF="#Heading6">Expressions</A> <LI><A HREF="#Heading7">Listing 4.1. Evaluating complex expressions</A><A HREF="#Heading8">.</A> <LI><A HREF="#Heading9">Operators</A> <UL> <LI><A HREF="#Heading10">Assignment Operator</A> <LI><A HREF="#Heading11">Mathematical Operators</A> </UL> <LI><A HREF="#Heading12">Listing 4.2. A demonstration of subtraction and integer overflow</A><A HREF="#Heading13">.</A> <UL> <LI><A HREF="#Heading14">Integer Division and Modulus</A> </UL> <LI><A HREF="#Heading15">Combining the Assignment and Mathematical Operators</A> <LI><A HREF="#Heading16">Increment and Decrement</A> <UL> <LI><A HREF="#Heading17">Prefix and Postfix</A> </UL> <LI><A HREF="#Heading18">Listing 4.3. A demonstration of prefix and postfix operators</A><A HREF="#Heading19">.</A> <LI><A HREF="#Heading20">Precedence</A> <LI><A HREF="#Heading21">Nesting Parentheses</A> <LI><A HREF="#Heading22">The Nature of Truth</A> <UL> <LI><A HREF="#Heading23">Relational Operators</A> </UL> <LI><A HREF="#Heading24">The if Statement</A> <LI><A HREF="#Heading25">Listing 4.4. A demonstration of branching based on relational operators</A><A HREF="#Heading26">.</A> <UL> <LI><A HREF="#Heading27">Indentation Styles</A> <LI><A HREF="#Heading28">else</A> </UL> <LI><A HREF="#Heading29">Listing 4.5. Demonstrating the else keyword</A><A HREF="#Heading30">.</A> <LI><A HREF="#Heading31">The if Statement</A> <UL> <LI><A HREF="#Heading32">Advanced if Statements</A> </UL> <LI><A HREF="#Heading33">Listing 4.6. A complex, nested if statement</A><A HREF="#Heading34">.</A> <LI><A HREF="#Heading35">Using Braces in Nested if Statements</A> <LI><A HREF="#Heading36">Listing 4.7. A demonstration of why</A> <LI><A HREF="#Heading37">braces help clarify which else statement goes with which if statement.</A> <LI><A HREF="#Heading38">Listing 4.8. A demonstration of the proper use of braces with an if statement</A><A HREF="#Heading39">.</A> <LI><A HREF="#Heading40">Logical Operators</A> <UL> <LI><A HREF="#Heading41">Logical AND</A> <LI><A HREF="#Heading42">Logical OR</A> <LI><A HREF="#Heading43">Logical NOT</A> </UL> <LI><A HREF="#Heading44">Relational Precedence</A> <LI><A HREF="#Heading45">More About Truth and Falsehood</A> <LI><A HREF="#Heading46">Conditional (Ternary) Operator</A> <LI><A HREF="#Heading47">Listing 4.9. A demonstration of the conditional operator</A><A HREF="#Heading48">.</A> <LI><A HREF="#Heading49">Summary</A> <LI><A HREF="#Heading50">Q&A</A> <LI><A HREF="#Heading51">Workshop</A> <UL> <LI><A HREF="#Heading52">Quiz</A> <LI><A HREF="#Heading53">Exercises</A> </UL> </UL> </UL></UL><P><HR SIZE="4"><H2 ALIGN="CENTER"><A NAME="Heading1"></A><FONT COLOR="#000077">Day 4</FONT></H2><H2 ALIGN="CENTER"><A NAME="Heading2"></A><FONT COLOR="#000077">Expressions and Statements</FONT></H2><P>At its heart, a program is a set of commands executed in sequence. The power ina program comes from its capability to execute one or another set of commands, basedon whether a particular condition is true or false. Today you will learn<UL> <LI>What statements are. <P> <LI>What blocks are. <P> <LI>What expressions are. <P> <LI>How to branch your code based on conditions. <P> <LI>What truth is, and how to act on it.</UL><H3 ALIGN="CENTER"><A NAME="Heading3"></A><FONT COLOR="#000077">Statements</FONT></H3><P>In C++ a statement controls the sequence of execution, evaluates an expression,or does nothing (the null statement). All C++ statements end with a semicolon, eventhe null statement, which is just the semicolon and nothing else. One of the mostcommon statements is the following assignment statement:</P><PRE><FONT COLOR="#0066FF">x = a + b;</FONT></PRE><P>Unlike in algebra, this statement does not mean that <TT>x</TT> equals <TT>a+b</TT>.This is read, "Assign the value of the sum of <TT>a</TT> and <TT>b</TT> to <TT>x</TT>,"or "Assign to <TT>x</TT>, <TT>a+b</TT>." Even though this statement isdoing two things, it is one statement and thus has one semicolon. The assignmentoperator assigns whatever is on the right side of the equal sign to whatever is onthe left side.</P><DL> <DD><HR><FONT COLOR="#000077"><B>New Term:</B></FONT><B> </B>A <I>null statement</I> is a statement that does nothing. <HR></DL><H4 ALIGN="CENTER"><A NAME="Heading4"></A><FONT COLOR="#000077">Whitespace</FONT></H4><P>Whitespace (tabs, spaces, and newlines) is generally ignored in statements. Theassignment statement previously discussed could be written as</P><PRE><FONT COLOR="#0066FF">x=a+b;</FONT></PRE><P>or as</P><PRE><FONT COLOR="#0066FF">x =a+ b ;</FONT></PRE><P>Although this last variation is perfectly legal, it is also perfectly foolish.Whitespace can be used to make your programs more readable and easier to maintain,or it can be used to create horrific and indecipherable code. In this, as in allthings, C++ provides the power; you supply the judgment.</P><DL> <DD><HR><FONT COLOR="#000077"><B>New Term: </B></FONT><I>Whitespace characters</I> (spaces, tabs, and newlines) cannot be seen. If these characters are printed, you see only the white of the paper. <HR></DL><H4 ALIGN="CENTER"><A NAME="Heading5"></A><FONT COLOR="#000077">Blocks and CompoundStatements</FONT></H4><P>Any place you can put a single statement, you can put a compound statement, alsocalled a block. A block begins with an opening brace (<TT>{</TT>) and ends with aclosing brace (<TT>}</TT>). Although every statement in the block must end with asemicolon, the block itself does not end with a semicolon. For example</P><PRE><FONT COLOR="#0066FF">{ temp = a; a = b; b = temp;}</FONT></PRE><P>This block of code acts as one statement and swaps the values in the variables<TT>a</TT> and <TT>b</TT>.<BLOCKQUOTE> <P><HR><B>DO</B> use a closing brace any time you have an opening brace.<B> DO</B> end your statements with a semicolon. <B>DO</B> use whitespace judiciously to make your code clearer. <HR></BLOCKQUOTE><H3 ALIGN="CENTER"><A NAME="Heading6"></A><FONT COLOR="#000077">Expressions</FONT></H3><P>Anything that evaluates to a value is an expression in C++. An expression is saidto return a value. Thus, <TT>3+2;</TT> returns the value <TT>5</TT> and so is anexpression. All expressions are statements.</P><P>The myriad pieces of code that qualify as expressions might surprise you. Hereare three examples:</P><PRE><FONT COLOR="#0066FF">3.2 // returns the value 3.2PI // float const that returns the value 3.14SecondsPerMinute // int const that returns 60</FONT></PRE><P>Assuming that <TT>PI</TT> is a constant equal to <TT>3.14</TT> and <TT>SecondsPerMinute</TT>is a constant equal to <TT>60</TT>, all three of these statements are expressions.</P><P>The complicated expression</P><PRE><FONT COLOR="#0066FF">x = a + b;</FONT></PRE><P>not only adds <TT>a</TT> and <TT>b</TT> and assigns the result to <TT>x</TT>,but returns the value of that assignment (the value of <TT>x</TT>) as well. Thus,this statement is also an expression. Because it is an expression, it can be on theright side of an assignment operator:</P><PRE><FONT COLOR="#0066FF">y = x = a + b;</FONT></PRE><P>This line is evaluated in the following order: Add <TT>a</TT> to <TT>b</TT>.</P><P>Assign the result of the expression <TT>a + b</TT> to <TT>x</TT>.<BR><BR>Assign the result of the assignment expression <TT>x = a + b</TT> to <TT>y</TT>.<BR><BR>If <TT>a</TT>, <TT>b</TT>, <TT>x</TT>, and <TT>y</TT> are all integers, and if <TT>a</TT>has the value <TT>2</TT> and <TT>b</TT> has the value <TT>5</TT>, both <TT>x</TT>and <TT>y</TT> will be assigned the value <TT>7</TT>.</P><P><A NAME="Heading7"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 4.1. Evaluatingcomplex expressions.</B></FONT><FONT COLOR="#0066FF"></FONT><PRE><FONT COLOR="#0066FF">1: #include <iostream.h>2: int main()3: {4: int a=0, b=0, x=0, y=35;5: cout << "a: " << a << " b: " << b;6: cout << " x: " << x << " y: " << y << endl;7: a = 9;8: b = 7;9: y = x = a+b;10: cout << "a: " << a << " b: " << b;11: cout << " x: " << x << " y: " << y << endl;12: return 0;<TT>13: }</TT>Output: a: 0 b: 0 x: 0 y: 35a: 9 b: 7 x: 16 y: 16</FONT></PRE><P><FONT COLOR="#000077"><B>Analysis: </B></FONT>On line 4, the four variables aredeclared and initialized. Their values are printed on lines 5 and 6. On line 7, <TT>a</TT>is assigned the value <TT>9</TT>. One line 8, <TT>b</TT> is assigned the value <TT>7</TT>.On line 9, the values of <TT>a</TT> and <TT>b</TT> are summed and the result is assignedto <TT>x</TT>. This expression (<TT>x = a+b</TT>) evaluates to a value (the sum of<TT>a + b</TT>), and that value is in turn assigned to <TT>y</TT>.<H3 ALIGN="CENTER"><A NAME="Heading9"></A><FONT COLOR="#000077">Operators</FONT></H3><P>An operator is a symbol that causes the compiler to take an action. Operatorsact on operands, and in C++ all operands are expressions. In C++ there are severaldifferent categories of operators. Two of these categories are<UL> <LI>Assignment operators. <P> <LI>Mathematical operators.</UL><H4 ALIGN="CENTER"><A NAME="Heading10"></A><FONT COLOR="#000077">Assignment Operator</FONT></H4><P>The assignment operator (<TT>=</TT>) causes the operand on the left side of theassignment operator to have its value changed to the value on the right side of theassignment operator. The expression</P><PRE><FONT COLOR="#0066FF">x = a + b;</FONT></PRE><P>assigns the value that is the result of adding <TT>a</TT> and <TT>b</TT> to theoperand <TT>x</TT>.</P><P>An operand that legally can be on the left side of an assignment operator is calledan lvalue. That which can be on the right side is called (you guessed it) an rvalue.</P><P>Constants are r-values. They cannot be l-values. Thus, you can write</P><PRE><FONT COLOR="#0066FF">x = 35; // ok</FONT></PRE><P>but you can't legally write</P><PRE><FONT COLOR="#0066FF">35 = x; // error, not an lvalue!</FONT></PRE><DL> <DD><HR><FONT COLOR="#000077"><B>New Term:</B></FONT><B> </B>An <I>lvalue</I> is an operand that can be on the left side of an expression. An rvalue is an operand that can be on the right side of an expression. Note that all l-values are r-values, but not all r-values are l-values. An example of an rvalue that is not an lvalue is a literal. Thus, you can write <TT>x = 5;</TT>, but you cannot write <TT>5 = x;</TT>. <HR></DL><H4 ALIGN="CENTER"><A NAME="Heading11"></A><FONT COLOR="#000077">Mathematical Operators</FONT></H4><P>There are five mathematical operators: addition (<TT>+</TT>), subtraction (<TT>-</TT>),multiplication (<TT>*</TT>), division (<TT>/</TT>), and modulus (<TT>%</TT>).</P><P>Addition and subtraction work as you would expect, although subtraction with <TT>unsigned</TT>integers can lead to surprising results, if the result is a negative number. Yousaw something much like this yesterday, when variable overflow was described. Listing4.2 shows what happens when you subtract a large <TT>unsigned</TT> number from asmall <TT>unsigned</TT> number.</P>⌨️ 快捷键说明
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