文章目录
5.1
单独的分号就是一条空语句，在语法上需要一条语句而逻辑上不需要时，会用到空语句。
5.2
用花括号括起来的语句和声明序列等就是块，在语法上需要一条语句而逻辑上需要多条语句时，就会用到块。
5.3
#include <iostream>
using namespace std;
int main()
{
int sum=0,val=1;
while(val<=10)
sum+=val,++val;
cout<<"Sum of 1 to 10 inclusive is "<<sum<<endl;
return 0;
}
代码的可读性降低了。
5.4
iter=s.begin();while(string::iterator iter!=s.end()){/*......*/}原文中，iter未初始化。while(bool status=find(word)) {/*......*/}
if(!status){/*......*/}
status是while部分的局部变量，离开while语句，在if语句中已超出其作用域，所以其在if语句中是一个无效变量。
5.5
#include <iostream>
#include <vector>
#include <string>
using namespace std;
int main()
{
vector<string> svec{"F","D","C","B","A","A++"};
unsigned grade=0;
string gradelevel;
while(cin>>grade&&grade<=100)
{
if(grade<60)
gradelevel=svec[0];
else
gradelevel=svec[(grade-50)/10];
if(grade!=100)
{
if(grade%10>7)
gradelevel+="+";
else if(grade%10<4)
gradelevel+="-";
}
cout<<"grade is "<<gradelevel<<endl;
}
return 0;
}
5.6
#include <iostream>
#include <vector>
#include <string>
using namespace std;
int main()
{
vector<string> svec{"F","D","C","B","A","A++"};
unsigned grade=0;
string gradelevel;
while(cin>>grade&&grade<=100)
{
gradelevel=grade<60?svec[0]:grade<70?svec[1]:grade<80?svec[2]:grade<90?svec[3]:grade<100?svec[4]:svec[5];
gradelevel+=grade==100?"":grade%10>7?"+":grade%10<4?"-":"";
cout<<"grade is "<<gradelevel<<endl;
}
return 0;
}
5.7
if(ival1!=ival2)
ival1=ival2;
else
ival1=ival2=0;
if(ival<minval)
{
minval=ival;
occurs=1;
}
if(int ival=get_value())
{
cout<<"ival="<<ival<<endl;
if(!ival)
cout<<"ival=0\n";
}
if(ival==0)
ival=get_value();
5.8
悬垂else 描述的是，当if 分支多于else 分支时，else 和哪个if 匹配的问题。C++语言规定其与离它最近 的尚未匹配的if匹配。
5.9
#include <iostream>
#include <cstdlib>
using namespace std;
int main()
{
int cnt=0;
char ch;
while((ch=getchar())!=EOF)
{
if(ch=='a'||ch=='e'||ch=='i'||ch=='o'||ch=='u')
cnt++;
}
cout<<"cnt= "<<cnt<<"\n";
return 0;
}
5.10
#include <iostream>
using namespace std;
int main()
{
int acnt=0,ecnt=0,icnt=0,ocnt=0,ucnt=0;
char ch;
while(cin>>ch)
{
switch(ch)
{
case 'a':case'A':
acnt++;
break;
case 'e':case 'E':
ecnt++;
break;
case 'i':case 'I':
icnt++;
break;
case 'o':case 'O':
ocnt++;
break;
case 'u':case 'U':
ucnt++;
break;
default:
;
}
}
cout<<"Number of vowel a:\t"<<acnt<<endl;
cout<<"Number of vowel e:\t"<<ecnt<<endl;
cout<<"Number of vowel i:\t"<<icnt<<endl;
cout<<"Number of vowel o:\t"<<ocnt<<endl;
cout<<"Number of vowel u:\t"<<ucnt<<endl;
return 0;
}
5.11
#include <iostream>
using namespace std;
int main()
{
int acnt=0,ecnt=0,icnt=0,ocnt=0,ucnt=0,spacecnt=0,tcnt=0,ncnt=0;
char ch;
while(cin.get(ch))
{
switch(ch)
{
case 'a':case'A':
acnt++;
break;
case 'e':case 'E':
ecnt++;
break;
case 'i':case 'I':
icnt++;
break;
case 'o':case 'O':
ocnt++;
break;
case 'u':case 'U':
ucnt++;
break;
case ' ':
spacecnt++;
break;
case '\t':
tcnt++;
break;
case '\n':
ncnt++;
break;
default:
;
}
}
cout<<"Number of vowel a:\t"<<acnt<<endl;
cout<<"Number of vowel e:\t"<<ecnt<<endl;
cout<<"Number of vowel i:\t"<<icnt<<endl;
cout<<"Number of vowel o:\t"<<ocnt<<endl;
cout<<"Number of vowel u:\t"<<ucnt<<endl;
cout<<"Number of vowel space:\t"<<spacecnt<<endl;
cout<<"Number of vowel \\t:\t"<<tcnt<<endl;
cout<<"Number of vowel \\n:\t"<<ncnt<<endl;
return 0;
}
5.12
#include <iostream>
using namespace std;
int main()
{
int ffcnt=0,flcnt=0,ficnt=0;
char ch1,ch2;
cin>>ch1;
while(cin>>ch2)
{
if(ch1=='f')
{
switch(ch2)
{
case 'f':
ffcnt++;
break;
case 'l':
flcnt++;
break;
case 'i':
ficnt++;
break;
}
}
ch1=ch2;
}
cout<<"Number of ff:\t"<<ffcnt<<endl;
cout<<"Number of fl:\t"<<flcnt<<endl;
cout<<"Number of fi:\t"<<ficnt<<endl;
return 0;
}
5.13
unsigned aCnt=0,eCnt=0,iouCnt=0;
char ch=next_text();
switch(ch){
case `a`:aCnt++;break;
case `e`:eCnt++;break;
default:iouCnt++;
}
unsigned index=some_value();
switch(index){
case 1:{
int ix=get_value();
ivec[ix]=index;
break;
}
default:{
int ix=ivec.size()-1;
ivec[ix]=index;
}
}
unsigned evenCnt=0,oddCnt=0;
int digit=get_num()%10;
switch(digit){
case 1:
case 3:
case 5:
case 7:
case 9:
oddCnt++;
break;
case 0:
case 2:
case 4:
case 6:
case 8:
evenCnt++;
break;
}
constexpr unsigned ival=512,jval=1024,kval=4096;
unsigned bufsize;
unsigned swt=get_bufCnt();
switch(swt){
case ival:
bufsize=ival*sizeof(int);
break;
case jval:
bufsize=jval*sizeof(int);
break;
case kval:
bufsize=kval*sizeof(int);
break;
}
5.14
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main()
{
int maxcnt=1,markcnt=1;
string maxstr,curstr,markstr;
if(cin>>maxstr)
{
markstr=maxstr;
while(cin>>curstr)
{
if(curstr==markstr)
{
++markcnt;
if(markcnt>maxcnt)
{
maxcnt=markcnt;
maxstr=markstr;
}
}
else
{
markstr=curstr;
markcnt=1;
}
}
}
if(maxcnt>1)
cout<<maxstr<<" occurs "<<maxcnt<<" times."<<endl;
else
cout<<"no repeated words."<<endl;
return 0;
}
5.15
原文中ix超出其作用域，在if语句中是无效的变量
int ix=0;
for(;ix!=sz;++ix){/*......*/}
if(ix!=sz)
//...
原文中，for循环缺少初始化部分。
for(int ix=0;ix!=sz;++ix){/*......*/}
ix 与sz同时自增，无法跳出循环
for(int ix=0;ix!=sz;++ix){/*......*/}
5.16
待完善
5.17
#include <iostream>
#include <vector>
using namespace std;
int main()
{
bool flag(true);
vector<int> ivec1{1,2,3,4,5,6,7,8};
vector<int> ivec2{1,3,4};
decltype(ivec1.size()) index=0,max_length;
max_length=ivec1.size()>ivec2.size()?ivec2.size():ivec1.size();
for(;index<max_length;++index)
{
if(ivec1[index]!=ivec2[index])
{
flag=false;
break;
}
}
if(flag)
cout<<"Yes, it's the prefix."<<endl;
else
cout<<"No, it's not the prefix."<<endl;
return 0;
}
5.18
读入两个整形变量并输出它们的和
do
{
int v1,v2;
cout<<"Please enter two numbers to sum:";
if(cin>>v1>>v2)
cout<<"Sum is: "<<v1+v2<<endl;
}
while(cin);
条件部分使用的变量需要定义在循环体外。
int ival=get_response();
do
{
//...
}
while(ival);
条件部分使用的变量需要定义在循环体外。
int ival=get_response();
do
{
//...
}
while(ival);
5.19
#include <iostream>
#include <string>
using namespace std;
int main()
{
string str1,str2;
do
{
cout<<"Enter two string:";
if(cin>>str1>>str2)
{
if(str1.size()>str2.size())
cout<<str2<<endl;
else
cout<<str1<<endl;
}
else
break;
}
while(cin);
return 0;
}
5.20
#include <iostream>
#include <string>
using namespace std;
int main()
{
bool flag(false);
string str1,str2;
if(cin>>str1)
{
while(cin>>str2)
{
if(str1==str2)
{
flag=true;
break;
}
else
str1=str2;
}
}
if(flag)
cout<<str1<<" is repeated."<<endl;
else
cout<<"No words repeated."<<endl;
return 0;
}
5.21
5-21.cpp
#include <iostream>
#include <string>
#include <cctype>
using namespace std;
int main()
{
bool flag(false);
string str1,str2;
if(cin>>str1)
{
while(cin>>str2)
{
if(str1==str2&&isupper(str1[0]))
{
flag=true;
break;
}
else
str1=str2;
}
}
if(flag)
cout<<str1<<" is repeated."<<endl;
else
cout<<"No words repeated."<<endl;
return 0;
}
5-21-1.cpp
#include <iostream>
#include <string>
#include <cctype>
using namespace std;
int main()
{
bool flag(false);
string str1,str2;
if(cin>>str1)
{
while(cin>>str2)
{
if(!isupper(str1[0]))
{
str1=str2;
continue;
}
else if(str1==str2)
{
flag=true;
break;
}
str1=str2;
}
}
if(flag)
cout<<str1<<" is repeated."<<endl;
else
cout<<"No words repeated."<<endl;
return 0;
}
5.22
int sz;
while((sz=getsize())<=0)
;
5.23
#include <iostream>
using namespace std;
int main()
{
int i_1,i_2;
cout<<"Enter two integer:";
cin>>i_1>>i_2;
cout<<i_1/i_2<<endl;
return 0;
}
5.24
#include <iostream>
using namespace std;
int main()
{
int i_1,i_2;
cout<<"Enter two integer:";
cin>>i_1>>i_2;
if(i_2==0)
throw runtime_error("除数不能为0");
cout<<i_1/i_2<<endl;
return 0;
}
5.25
#include <iostream>
using namespace std;
int main()
{
int i_1,i_2;
cout<<"Enter two integer:";
while(cin>>i_1>>i_2)
{
try
{
if(i_2==0)
throw runtime_error("除数不能为0");
cout<<"The result is "<<i_1/i_2<<endl;
}
catch(runtime_error err)
{
cout<<err.what()<<"\nTry again?Enter y or n:";
char ch;
cin>>ch;
if(!cin||ch=='n')
break;
}
}
return 0;
}