c++ primer plus 第六版 11.15 randwalk 随机数问题

[zucchiniy]

请帮忙分析一下，下面这个程序，执行的steps > 100000了。对于main函数中，下面这个while循环里面，rand() % 360随机出来的是0~360之间的数，如果是90 < x < 270 的话会变成负值，是因为这个原因才导致steps大于100000吗？
输入的值是50，2的时候 ，请帮忙分析一下，谢谢~
while(result.magval() < target)
{
direction = rand() % 360;
// cout << "the direction is " << direction << endl;
step.reset(dstep,direction,vector::POL);
result = result + step;
steps++;
cout << "the steps is " << steps << endl;
}

#ifndef VECTOR_H
#define VECTOR_H
#include <iostream>
namespace VECTOR {

class vector
{
public:
    enum Mode {RECT, POL};
private :
    double x;
    double y;
    double mag;
    double ang;
    Mode mode;

    void set_mag();
    void set_ang();
    void set_x();
    void set_y();
public:
    vector();
    vector(double n1,double n2,Mode form = RECT);
    void reset(double n1,double n2,Mode form = RECT);
    ~vector();
    double xval() const {return x;}
    double yavl() const {return y;}
    double magval() const {return mag;}
    double angval() const {return ang;}
    void polar_mode();
    void rect_mode();

    vector operator+(const vector & b)const;
    vector operator-(const vector & b)const;
    vector operator-()const;
    vector operator*(double n)const;

    friend vector operator*(double n, vector & a);
    friend std::ostream & operator<<(std::ostream &os, const vector & v);
};


}
#endif // VECTOR_H

#include "vector.h"
#include <cmath>
using std::cout;
using std::endl;
using std::sqrt;
using std::sin;
using std::cos;
using std::atan;
using std::atan2;

namespace VECTOR {
const double Rad_to_deg = 45.0 / atan(1.0);
//private methods
void vector::set_mag()
{
    mag = sqrt(x * x + y * y);
}
void vector::set_ang()
{
    if(x == 0.0 && y == 0.0)
        ang = 0.0;
    else
        ang = atan2(y,x);
}
void vector::set_x()
{
    x = mag * cos(ang);
}
void vector::set_y()
{
    y = mag * sin(ang);
}
//pulice methods
vector::vector()
{
    x = y = mag = ang = 0.0;
    mode = RECT;
}
vector::vector(double n1, double n2, Mode form)
{
    mode = form;
    if(form = RECT)
    {
        x = n1;
        y = n2;
        set_mag();
        set_ang();
    }
    else if(form = POL)
    {
        mag = n1;
        ang = n2;
        set_x();
        set_y();
    }
    else
    {
        cout << "Incorrect 3rd argument to vector() --";
        cout << "vector set to 0\n";
        x = y = mag = ang = 0.0;
        mode = RECT;
    }
}
void vector::reset(double n1, double n2, Mode form)
{
    mode = form;
    if(form = RECT)
    {
        x = n1;
        y = n2;
        set_mag();
        set_ang();
    }
    else if(form = POL)
    {
        mag = n1;
        ang = n2;
        set_x();
        set_y();
    }
    else
    {
        cout << "Incorrect 3rd argument to vector() --";
        cout << "vector set to 0\n";
        x = y = mag = ang = 0.0;
        mode = RECT;
    }
}

vector::~vector()
{
    //nothing to ~vector
}

void vector::polar_mode()
{
    mode = POL;
}
void vector::rect_mode()
{
    mode = RECT;
}

vector vector::operator+(const vector & b)const
{
    return vector(x + b.x,y + b.y);
}
vector vector::operator-(const vector & b)const
{
    return vector(x - b.x,y - b.y);
}
vector vector::operator-()const
{
    return vector(-x,-y);
    //构造函数，只送入X,Y,但是ANG,MAG也同时进行了设置，如果不使用此方式，必须将代码加上MAG,ANG的计算
}
vector vector::operator*(double n)const
{
    return vector(n * x,n * y);
}
vector operator*(double n,const vector & v)
{
    return v * n;
}
std::ostream & operator<<(std::ostream & os,const vector & v)
{
    if(v.mode == vector::RECT)
        os << "(x,y) = (" << v.x << ", " << v.y << ")";
    else if(v.mode == vector::POL)
    {
        os << "(m,a) = (" << v.mag << ", " << v.ang * Rad_to_deg << ")";
    }
    else
        os << "vector object mode is invalid!";
    return os;
}

}

#include <iostream>
#include "vector.h"
#include <cstdlib>
#include <ctime>
int main()
{
    using namespace std;
    using VECTOR::vector;
    srand((unsigned)time(0));

    double direction;
    vector step;
    vector result(0.0,0.0);
    unsigned long steps = 0;
    double target;
    double dstep;
    cout << "Enter target distance (q to quit):";
    while(cin >> target)
    {
        cout << "Enter step length:";
        if(!(cin >> dstep))
            break;
        while(result.magval() < target)
        {
            direction = rand() % 360;
//            cout << "the direction is " << direction << endl;
            step.reset(dstep,direction,vector::POL);
            result = result + step;
            steps++;
            cout << "the steps is " << steps << endl;
        }
        cout << "After " << steps << " steps, the subject "
             "has the following lovation:\n";
        cout << result << endl;
        result.polar_mode();
        cout << " or\n" << result << endl;
        cout << "Average outward distance per step = "
             << result.magval()/steps << endl;
        steps = 0;
        result.reset(0.0,0.0);
        cout << "Enter target distance (q to quit): ";
    }
    cout << "Bye\n";
    cin.clear();
    while(cin.get() != '\n')
        continue;
    return 0;
}

[zucchiniy]

您好，强制转换成double有什么意义吗？rand返回一个int，对360取模后，一定是一个整型，强制转换成double没什么用处吧？ 另外我已经跟踪过程序了。是实在看不出来这步有什么问题。不过感谢您的答复，谢谢

引用 1 楼 zhao4zhong1 的回复:

direction = (double)(rand() % 360); 试试看。 代码功能归根结底不是别人帮自己看或讲解或注释出来的；而是被自己静下心来花足够长的时间和精力亲自动手单步或设断点或对执行到某步获得的中间结果显示或写到日志文件中一步一步分析出来的。 提醒：再牛×的老师也无法代替学生自己领悟和上厕所！ 单步调试和设断点调试（VS IDE中编译连接通过以后，按F10或F11键单步执行，按Shift+F11退出当前函数；在某行按F9设断点后按F5执行停在该断点处。）是程序员必须掌握的技能之一。

[赵4老师]

direction = (double)(rand() % 360); 试试看。 代码功能归根结底不是别人帮自己看或讲解或注释出来的；而是被自己静下心来花足够长的时间和精力亲自动手单步或设断点或对执行到某步获得的中间结果显示或写到日志文件中一步一步分析出来的。 提醒：再牛×的老师也无法代替学生自己领悟和上厕所！ 单步调试和设断点调试（VS IDE中编译连接通过以后，按F10或F11键单步执行，按Shift+F11退出当前函数；在某行按F9设断点后按F5执行停在该断点处。）是程序员必须掌握的技能之一。