Question

我正在尝试为包含double值的向量实现MergeSort算法。我有以下代码。但是，在我的输入中，两个值被视为零并且不合适。有人能告诉我哪里出错了吗？

P.S：我是数据结构的新手，所以我不知道在这里正确使用iterator。

代码：

#include <iostream>
#include <vector>
void sort(std::vector<double> vec);
std::vector<double> merge(std::vector<double> left, std::vector<double> right, int len);
std::vector<double> mergeSort(std::vector<double> vec, int len);
void sort(std::vector<double> vec) {
    std::vector<double> tmp = mergeSort(vec, vec.size());
    for (unsigned int i = 0; i < vec.size(); i++){
        vec[i] = tmp[i];
        std::cout << vec[i] << " ";
    }
}

std::vector<double> merge(std::vector<double> left, std::vector<double> right, int len){
    std::vector<double> ret(len + len);
    int left_position = 0;
    int right_position = 0;
    int ret_position = 0;
    while (left_position < len && right_position < len){
        double left_value = left[left_position];
        double right_value = right[right_position];
        if (left_value < right_value){
            ret.insert((ret.begin() + ret_position), left_value);
            //ret[ret_position] = left_value;
            ret_position++;
            left_position++;
        }
        else {
            ret.insert((ret.begin() + ret_position), right_value);
            ret_position++;
            right_position++;
        }
    }
    while (left_position < len){
        ret.insert((ret.begin() + ret_position), left[left_position]);
        ret_position++;
        left_position++;
    }
    while (right_position < len){
        ret.insert((ret.begin() + ret_position), right[right_position]);
        ret_position++;
        right_position++;
    }
    return ret;
}

std::vector<double> mergeSort(std::vector<double> vec, int len){
    if (len == 1){
        return vec;
    }
    int middle = len / 2;
    std::vector<double> left(middle);
    std::vector<double> right(middle);
    for (int i = 0; i < middle; i++){
        left.insert((left.begin() + i), vec[i]);
    }
    for (int j = 0; j < middle; j++){
        right.insert((right.begin() + j), vec[j + middle]);
    }
    left = mergeSort(left, middle);
    right = mergeSort(right, middle);
    std::vector<double> ret = merge(left, right, middle);
    return ret;
}

int main(){
    std::vector<double> vec;
    vec.push_back(7.04);
    vec.push_back(8.04);
    vec.push_back(6.04);
    vec.push_back(9.04);
    vec.push_back(1.04);
    vec.push_back(3.04);
    vec.push_back(5.04);
    vec.push_back(2.04);
    vec.push_back(10.04);
    vec.push_back(4.04);
    vec.push_back(12.04);
    vec.push_back(11.04);
    sort(vec);
    return 0;
}

输出为：1.04 2.04 4.04 5.04 0 7.04 8.04 0 9.04 0 12.04 0这很奇怪。（请注意，输入中的每个第三元素都被视为0）

直播：http://ideone.com/fHZzZf

Answer 1

我已更正您的代码。

第一个问题是，正如 UmNyobe 所说的那样，在计算时会失去元素
```
int middle = len / 2;  
```
使用构造函数时
```
std::vector<int>(size_type n);
```
构造一个包含n个元素0.0
的vecror 关于vector的构造函数的更多信息请阅读here。

使用时

vec.insert(iterator pos, const value_type& val);

你在位置pos之前插入一个新元素，并且增加向量的大小为1 在合并排序中，我建议您创建一个包含一些元素的向量，然后将它们分配给适当的值例如，您可以在下面的代码中看到：

std::vector<double> ret(left.size() + right.size(), -1); // construct vector with left.size() + right.size() elements assigned to -1
int left_position = 0;
int right_position = 0;
int ret_position = 0;
while (left_position < left.size() && right_position < right.size()) {
    double left_value = left[left_position];
    double right_value = right[right_position];
    if (left_value < right_value) {
        ret[ret_position++] =  left_value;
        left_position++;
    } else {
        ret[ret_position++] = right_value;
        right_position++;
    }
}

更正后的代码：

#include <iostream>
#include <vector>
std::vector<double> merge(std::vector<double> left, std::vector<double> right);
std::vector<double> mergeSort(std::vector<double> vec);

void sort(std::vector<double>& vec) { // using reference here std::vector<double>& - to be able to change vector inside function
    vec = mergeSort(vec);
    for (unsigned int i = 0; i < vec.size(); i++){
        std::cout << vec[i] << " ";
    }
}

std::vector<double> merge(std::vector<double> left, std::vector<double> right) {
    std::vector<double> ret(left.size() + right.size(), -1); // construct vector with left.size() + right.size() elements assigned to -1
    int left_position = 0;
    int right_position = 0;
    int ret_position = 0;
    while (left_position < left.size() && right_position < right.size()) {
        double left_value = left[left_position];
        double right_value = right[right_position];
        if (left_value < right_value) {
            ret[ret_position++] =  left_value;
            left_position++;
        } else {
            ret[ret_position++] = right_value;
            right_position++;
        }
    }
    while (left_position < left.size()) {
        ret[ret_position++] = left[left_position++];
    }
    while (right_position < right.size()) {
        ret[ret_position++] = right[right_position++];
    }
    return ret;
}

std::vector<double> mergeSort(std::vector<double> vec){
    if (vec.size() < 2){
        return vec;
    }
    int middle = vec.size() / 2;
    std::vector<double> left(vec.begin(), vec.begin() + middle);
    std::vector<double> right(vec.begin() + middle, vec.end());

    left = mergeSort(left);
    right = mergeSort(right);
    std::vector<double> ret = merge(left, right);
    return ret;
}

int main(){
    std::vector<double> vec;
    vec.push_back(7.04);
    vec.push_back(8.04);
    vec.push_back(6.04);
    vec.push_back(9.04);
    vec.push_back(1.04);
    vec.push_back(3.04);
    vec.push_back(5.04);
    vec.push_back(2.04);
    vec.push_back(10.04);
    vec.push_back(4.04);
    vec.push_back(12.04);
    vec.push_back(11.04);
    sort(vec);
    return 0;
}

现在输出

1.04 2.04 3.04 4.04 5.04 6.04 7.04 8.04 9.04 10.04 11.04 12.04

您应该仔细阅读 vector 类的方法这是一个关于c ++的好网站：http://www.cplusplus.com/reference/vector/vector/

Answer 2

将原始集分成left和right子集时，您忘记了元素。

int middle = len / 2;
for (int i = 0; i < middle; i++){
    ...
}
for (int j = 0; j < middle; j++){
   ...
}

如果len很奇怪，您将获得2*middle = len -1。您可以更改为

for (int j = 0; j < len - middle; j++){
   ...
}

修改您的整个程序基于左侧和右侧长度相等的事实。这意味着只有当输入向量的长度是2的幂时它才是正确的。 你需要重新考虑你的函数，假设左右大小不同

处理向量时，你真的需要传递一个长度参数吗？例如

std::vector<double> ret(len + len);
合并功能中的
可以是

std::vector<double> ret(left.size() + right.size());

MergeSort为双打矢量

2 个答案: