《C++ Primer》第五版课后习题解答_第三章（5）(37-45)

系统环境： windows 10 1703

编译环境：Visual studio 2017

3.37

程序想要将字符数组 ca 中的元素输出。因为 ca 的结尾没有空字符，所以会发生未定义行为（undefined behavior）。

3.38

  摘抄stack overflow 上的一个回答：

 
Why can't I add pointers?

  Pointer addition is forbidden in C++, you can only subtract two pointers.

  The reason for this is that subtracting two pointers gives a logically explainable result - the offset in memory between two pointers. Similarly, you can subtract or add an integral number to/from a pointer, which means "move the pointer up or down". Adding
a pointer to a pointer is something which is hard to explain. What would the resulting pointner represent?

  If by any chance you explicitly need a pointer to a place in memory whose address is the sum of some other two addresses, you can cast the two pointers to int, add ints, and cast back to a pointer. Remember though, that this solution needs huge care about
the pointer arithmetic and is something you really should never do.

3.39

#include <iostream>
#include <string>
using std::cout;
using std::endl;
using std::string;

// String
int main()
{
string s1 = "This is string 1";
string s2 = "This is string 2";
if (s1 != s2)
{
cout << "Not Equal!!!" << endl;
}
else
{
cout << "The two strings are equal" << endl;
}
return 0;
}

#include <iostream>
#include <string>
using std::cout;
using std::endl;
using std::string;

// C-style charater string
int main()
{
const char ch1[] = "This is C-style charater string 1";
const char ch2[] = "This is C-style charater string 2";
if (strcmp(ch1, ch2) < 0)
{
cout << "Not Equal!!!" << endl;
}
else
{
cout << "The two strings are equal" << endl;
}
return 0;
}

3.40
这道题搞了好久也没搞出来。去 github 上找了讨论帖，链接和答案如下：
https://github.com/Mooophy/Cpp-Primer/pull/241
#include <iostream>
#include <cstring>

using std::cout; using std::endl;

constexpr size_t constexpr_strlen(const char *s) {
return sizeof(s) / sizeof(*s);
}

constexpr size_t merge_size(const char* cs1, const char* cs2)
{
return constexpr_strlen(cs1) + constexpr_strlen(" ") + constexpr_strlen(cs2) + 1;

}

int main()
{
constexpr const char cs1[] = "Hi";
constexpr const char cs2[] = "frank67";
char cs3[merge_size(cs1, cs2)];
strcpy(cs3, cs1);
strcat(cs3, " ");
strcat(cs3, cs2);
cout << cs3 << endl;
return 0;
}
3.41

#include <iostream>
#include <string>
#include <cstring>
#include <vector>
using std::cout;
using std::endl;
using std::vector;
using std::string;

int main()
{
int ich[] = { 1, 2, 3, 4, 5, 6, 7 };
vector<int> ivec(std::begin(ich), std::end(ich));
for (auto a : ivec)
{
cout << a << endl;
}
return 0;
}

3.42
#include <iostream>
#include <string>
#include <cstring>
#include <vector>
using std::cout;
using std::endl;
using std::vector;
using std::string;

int main()
{
vector<int> ivec = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
int ich[10] = { };
for (int i = 0; i != ivec.size(); ++i)
{
ich[i] = ivec[i];
}
for (auto a : ich)
{
cout << a << endl;
}
return 0;
}
3.43

#include <iostream>
#include <string>
#include <cstring>
#include <vector>
using std::cout;
using std::endl;
using std::vector;
using std::string;
using std::begin;
using std::end;
int main()
{
int ia[2][3] = { {1, 2, 3}, {4, 5, 6} };

//1. 范围 for
for (const int(&i)[3] : ia)
{
for (int j : i)
{
cout << j << endl;
}
cout << endl;
}

//2. 普通 for 循环
for (size_t i = 0; i != 2; ++i)
{
for (size_t j = 0; j != 3; ++j)
{
cout << ia[i][j] << endl;
}
cout << endl;
}

//3. 指针
for (int (*i)[3] = begin(ia) ; i != end(ia); ++i)
{
for (int *j = begin(*i); j != end(*i); ++j)
{
cout << *j << endl;
}
cout << endl;
}
return 0;
}
3.44

#include <iostream>
#include <string>
#include <cstring>
#include <vector>
using std::cout;
using std::endl;
using std::vector;
using std::string;
using std::begin;
using std::end;
int main()
{
int ia[2][3] = { {1, 2, 3}, {4, 5, 6} };
using int_array = int[3];
//1. 范围 for
for (int_array &i : ia)
{
for (int j : i)
{
cout << j << endl;
}
cout << endl;
}

//2. 普通 for 循环
for (size_t i = 0; i != 2; ++i)
{
for (size_t j = 0; j != 3; ++j)
{
cout << ia[i][j] << endl;
}
cout << endl;
}

//3. 指针
for (int_array *i = begin(ia) ; i != end(ia); ++i)
{
for (int *j = begin(*i); j != end(*i); ++j)
{
cout << *j << endl;
}
cout << endl;
}
return 0;
}

3.45
#include <iostream>
#include <string>
#include <cstring>
#include <vector>
using std::cout;
using std::endl;
using std::vector;
using std::string;
using std::begin;
using std::end;
int main()
{
int ia[2][3] = { {1, 2, 3}, {4, 5, 6} };
using int_array = int[3];
//1. 范围 for
for (auto &i : ia)
{
for (auto j : i)
{
cout << j << endl;
}
cout << endl;
}

//2. 普通 for 循环
for (auto i = 0; i != 2; ++i)
{
for (auto j = 0; j != 3; ++j)
{
cout << ia[i][j] << endl;
}
cout << endl;
}

//3. 指针
for (auto *i = begin(ia) ; i != end(ia); ++i)
{
for (auto *j = begin(*i); j != end(*i); ++j)
{
cout << *j << endl;
}
cout << endl;
}
return 0;
}