第6章 函数

6.4

#include<iostream>

using namespace std;

int fact(int val)
{
    int ret = 1;
    while (val>=1)
    {
        ret *= val--;
    }
    return ret;
}


int main()
{
    int j ;
    cout << "请输入要求阶乘的那个整数：" << endl;
    cin >> j;
    cout <<j<<"的阶乘是："<< fact(j)<< endl;
    return 1;
}

6.5

#include<iostream>


using namespace std;

double abss(double val)
{
    if (val >= 0)
        return val;
    else
        return -val;
}

int main()
{
    double j ;
    cout << "请输入要求绝对值的那个数：" << endl;
    cin >> j;
    cout <<j<<"的绝对值是："<< abss(j)<< endl;
    return 1;
}

#include<iostream>


using namespace std;

double abss(double val)
{
    return val > 0 ? val : -val;
}


int main()
{
    double j ;
    cout << "请输入要求绝对值的那个数：" << endl;
    cin >> j;
    cout <<j<<"的绝对值是："<< abss(j)<< endl;
    return 1;
}

6.7

#include<iostream>
using namespace std;

int cnt_call()
{
    static int flag = 0;
    if (!flag)
    return flag++;
    else
    return 1;
}

int main()
{
    for (int i = 0; i != 4;i++)
    cout <<cnt_call()<< endl;
    return 1;
}

 局部静态变量的值等于上次调用函数退出时它的值。局部静态变量的生命周期从程序第一次执行开始，知道程序终止时结束。

 6.10

#include <iostream>
#include <string>
#include <vector>
using namespace std;

void sw(int *a, int *b)
{
    int tmp;
    tmp = *a;
    *a = *b;
    *b = tmp;
    cout << *a << *b << endl;
}

int main()
{
    int i = 4, j = 9;
    sw(&i, &j);
    cout << "i: " << i << endl;
    cout << "j: " << j << endl;

    return 1;
}

#include <iostream>
#include <string>
#include <vector>
using namespace std;

void sw(int &a, int &b)
{
    int tmp;
    tmp = a;
    a = b;
    b = tmp;
    cout << a << b << endl;
}

int main()
{
    int i = 4, j = 9;
    sw(i, j);
    cout << "i: " << i << endl;
    cout << "j: " << j << endl;

    return 1;
}

引用形参也会改变实参的值

6.17

#include <iostream>
#include <string>
#include <vector>

using namespace std;

bool isCapital(const string s)
{
    
    for (auto it = s.begin(); it != s.end(); ++it)
    {
        if (isupper(*it))
        {
            return true;
        }
    }
        return false;
}
void fun(string &s)
{
    for (auto &i : s)
    {
        i = isupper(i) ? tolower(i) : i;
    }
}

int main()
{
    string str = "ABCdehijklMNopqrstuvwxyz";
    cout << isCapital(str) << endl;
    fun(str);
    cout << str << endl;

    return 1;
}

不需要改变实参的值就用常量引用，否则不使用常量引用。尽量使用常量引用，防止编译出错。见p192

6.21

#include <iostream>
#include <string>
#include <vector>

using namespace std;

int fun(const int i, const int *p)
{
    return i > *p ? i : *p;
}

void main()
{
    int a = 5;
    int b[] = { 0, 1, 2, 3, 4, 5, 6 };
    int *c = b;
    c+=2;
    cout << fun(a, c) << endl;
}

6.22　　交换指针而非指针所指对象（交换cd，而不是ab）

#include <iostream>
#include <string>
#include <vector>

using namespace std;

void fun( int *&i,  int *&j)
{
    int *tmp=NULL;
    tmp = i;
    i = j;
    j = tmp;
}

void main()
{
    int a = 5;
    int b = 6;
    int *c = &a;
    int *d = &b;
    fun(c,d);
    cout << a<<"   "<<b<< endl;
    cout << *c << "   " << *d<< endl;
}

 6.25　　使用argv中的实参时，一定要记得可选参数从argv[1]开始，argv[0]保存的是程序的名字。

#include <iostream>
#include <string>
#include <vector>

using namespace std;

int main(int argc, char **argv)
{
    string ss;
         for (int i = 1; i != argc; ++i) {
                 ss += argv[i];
                ss += " ";
        
    }
         cout << ss << endl;
         return 0;
}

6.27

#include <iostream>
#include <vector>
#include <string>

using namespace std;

int fun(initializer_list<int> il)
{
    int sum = 0;
    for (auto &i : il)
        sum += i;
    return sum;
}

int main(int argc, char *argv[])    //形参argv是一个数组，它的元素是指向C风格字符串的指针  
{
    cout << fun({ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }) << endl;
    return 0;
}

6.33

#include <iostream>
#include <string>
#include <vector>

using namespace std;
vector<int> v1 = { 1, 2, 3, 4, 5 };

void fun(vector<int>::iterator it)
{
    if (it != v1.end())
    {
        cout << *it++ << endl;
        fun(it);
    }
    return;
}

int main()
{
    fun(v1.begin());
    return 0;
}

 6.42

#include <iostream>
#include <string>

using namespace std;

string make_plural(size_t ctr, const string &word, const string &ending="s")
{

    return (ctr > 1) ? word + ending : word;
}

int main()
{
    string str = "failure";

    cout<< make_plural(1, str)<<endl;
    cout << make_plural(2, str) << endl;

}

 6.55

#include <iostream>
#include<string>
#include<vector>
using namespace std;

int add(int a, int b)
{
    return a + b;
}
int sub(int a, int b)
{
    return a - b;
}
int mul(int a, int b)
{
    return a*b;
}
int divi(int a, int b)
{
    return decltype(1.0) (a) / b;
}

int main(int argc, char** argv)
{
    typedef int(*p)(int a, int b);    //声明函数指针,使用typedef的声明语句定义的
                                                    //不再是变量, 而是类型别名  
    vector<p> v1{ add, sub, mul, divi };
    for (auto f : v1)
        cout << f(3, 2) << endl;
    
    return 0;
}