使用数组作为映射值：无法看到错误

Question

我正在尝试创建一个映射，其中键是一个int，值是一个数组

int red[3]  = {1,0,0};
int green[3] = {0,1,0};
int blue[3]     = {0,0,1};

    std::map<int, int[3]> colours;

colours.insert(std::pair<int,int[3]>(GLUT_LEFT_BUTTON,red)); //THIS IS LINE 24 !
colours.insert(std::pair<int,int[3]>(GLUT_MIDDLE_BUTTON,blue));
colours.insert(std::pair<int,int[3]>(GLUT_RIGHT_BUTTON,green));

但是，当我尝试编译此代码时，出现以下错误。

g++ (Ubuntu 4.4.1-4ubuntu8) 4.4.1

 In file included from /usr/include/c++/4.4/bits/stl_algobase.h:66,
                 from /usr/include/c++/4.4/bits/stl_tree.h:62,
                 from /usr/include/c++/4.4/map:60,
                 from ../src/utils.cpp:9:
/usr/include/c++/4.4/bits/stl_pair.h: In constructor ‘std::pair<_T1, _T2>::pair(const _T1&, const _T2&) [with _T1 = int, _T2 = int [3]]’:
../src/utils.cpp:24:   instantiated from here
/usr/include/c++/4.4/bits/stl_pair.h:84: error: array used as initializer
/usr/include/c++/4.4/bits/stl_pair.h: In constructor ‘std::pair<_T1, _T2>::pair(const std::pair<_U1, _U2>&) [with _U1 = int, _U2 = int [3], _T1 = const int, _T2 = int [3]]’:
../src/utils.cpp:24:   instantiated from here
/usr/include/c++/4.4/bits/stl_pair.h:101: error: array used as initializer
In file included from /usr/include/c++/4.4/map:61,
                 from ../src/utils.cpp:9:
/usr/include/c++/4.4/bits/stl_map.h: In member function ‘_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = int, _Tp = int [3], _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, int [3]> >]’:
../src/utils.cpp:30:   instantiated from here
/usr/include/c++/4.4/bits/stl_map.h:450: error: conversion from ‘int’ to non-scalar type ‘int [3]’ requested
make: *** [src/utils.o] Error 1

我真的看不出错误在哪里。或者即使出现错误。

Answer 1

您不能像这样按值复制数组。

以下是几种解决方案，但我建议＃4满足您的需求：

1）使用std::vector代替数组

2）使用指向3个元素数组的指针映射。

int red[3]  = {1,0,0};
int green[3] = {0,1,0};
int blue[3]     = {0,0,1};
std::map<int,int(*)[3]> colours;
colours.insert(std::pair<int,int(*)[3]>(GLUT_LEFT_BUTTON,&red));
colours.insert(std::pair<int,int(*)[3]>(GLUT_MIDDLE_BUTTON,&blue));
colours.insert(std::pair<int,int(*)[3]>(GLUT_RIGHT_BUTTON,&green));
//Watch out for scope here, you may need to create the arrays on the heap.

3）使用boost tuples而不是3个元素的数组。

4）不使用数组而是创建一个带有3个元素的新结构。制作地图。或者将数组包装在一个结构中，这也可以工作。

struct Triple
{
    int color[3];
};

 //Later in code
Tripple red = {1, 0, 0}, green = {0, 1, 0}, blue = {0, 0, 1};
std::map<int,Triple> colours;
colours.insert(std::pair<int,Triple>(GLUT_LEFT_BUTTON,red));
colours.insert(std::pair<int,Triple>(GLUT_MIDDLE_BUTTON,blue));
colours.insert(std::pair<int,Triple>(GLUT_RIGHT_BUTTON,green));

Answer 2

数组不是C ++中的第一类构造。它们不是Copy Constructible也不是Assignable，它们是std::map值的要求。您可以使用boost::array或std::vector。

Answer 3

使用std::tr1::array。

typedef std::tr1::array<int, 3> Triple;
Triple red   = {1, 0, 0};
Triple green = {0, 1, 0};
Triple blue  = {0, 0, 1};
std::map<int, Triple> colours;
colours.insert(std::make_pair(GLUT_LEFT_BUTTON,   red));
colours.insert(std::make_pair(GLUT_MIDDLE_BUTTON, blue));
colours.insert(std::make_pair(GLUT_RIGHT_BUTTON,  green));

或C ++ 11及以上版本中的std::array

using  Triple = std::array<int, 3>; 
Triple red   = {1, 0, 0};
Triple green = {0, 1, 0};
Triple blue  = {0, 0, 1};
std::map<int, Triple> colours;
colours.insert(std::make_pair(GLUT_LEFT_BUTTON,   red));
colours.insert(std::make_pair(GLUT_MIDDLE_BUTTON, blue));
colours.insert(std::make_pair(GLUT_RIGHT_BUTTON,  green));

Answer 4

不要映射到int []，而是映射到int *，如下所示：

#include <iostream>
#include <map>
using namespace std;
int main(){
    std::map<int,int*> colors;
    int red[] = {3,7,9};
    colors[52] = red;
    cout << colors[52][1];  //prints 7
    colors[52][1] = 11;
    cout << colors[52][1];  //prints 11
    return 0;
}

Answer 5

另一种方法是将数组放在包装结构中：

    struct Wrapper { int value[3]; };

    // ...
    Wrapper red = {{1,0,0}};    
    std::map<int,Wrapper> colours;    
    colours.insert(std::pair<int,Wrapper>(1, red));

Answer 6

数组不能是标准容器（std::pair）

中存储的数据

Answer 7

在C ++中使用结构的方法

int MAX_DATA_PER_INSTR = 8;
//struct to hold the values. remember to write the constructor
struct InstrChar
{
  InstrChar(int in[MAX_DATA_PER_INSTR]) { 
    //c alternative is memcopy
    std::copy(in, in+MAX_DATA_PER_INSTR, data);
  }
  int data[MAX_DATA_PER_INSTR];
};

// create a key value pair 
std::map <int, InstrChar> address_instructions;
std::map <int, InstrChar>::iterator it;

// sample array, 8 elements
int xp[MAX_DATA_PER_INSTR ] = {31,4,3,4,4,3,1,2};
address_instructions.insert(std::pair<int, InstrChar>(PC, xp));
it = address_instructions.find(PC);
InstrChar buf1 = it->second;
//integer pointer to the array, can be dereferenced as *p, *(p+1), ....    //*(p+7)
int *p = buf1.data;

//in case you need to print these values out. They can also be referred to as buf1.data[0], buf1.data[1], buf1.data[2]
printf("%d\n", (*p));
printf("%d\n", *(p+1));
printf("%d\n", *(p+2));
printf("%d\n", *(p+3));
printf("%d\n", *(p+4));
printf("%d\n", *(p+5));
printf("%d\n", *(p+6));
printf("%d\n", *(p+7));

Answer 8

我想在Brian R. Bondy's answer的第三项上进行扩展：自C++11以来，类模板std::tuple可用。因此，您不再需要Boost来处理元组。

元组是一个固定大小的集合，可以容纳多个元素。相较于std::vector的优点是可以存储异构类型。例如，如果要存储颜色名称及其RGB值，则可以向元组添加类型std::string的第四个元素作为颜色名称。但是对于您的特定用例，代码可以编写如下：

int main() {
    using col_t = std::tuple<int, int, int>;
    col_t red   = { 1, 0, 0 };
    col_t green = { 0, 1, 0 };
    col_t blue  = { 0, 0, 1 };

    std::map<int, col_t> colours;

    colours.emplace(GLUT_LEFT_BUTTON, red);
    colours.emplace(GLUT_MIDDLE_BUTTON, blue);
    colours.emplace(GLUT_RIGHT_BUTTON, green);

    for (auto const &kv : colours)
        std::cout << kv.first << " => { " << std::get<0>(kv.second) << ", "
                                          << std::get<1>(kv.second) << ", "
                                          << std::get<2>(kv.second) << " }" << std::endl;
    return 0;
}

输出：

0 => {1，0，0}
1 => {0，0，1}
2 => {0，1，0}

注意：使用C++17可以更轻松地处理元组，尤其是当您要同时访问多个元素时。 例如，如果您使用structured binding，则可以如下打印元组：

for (auto const &[k, v] : colours) {
    auto [r, g, b] = v;
    std::cout << k << " => { " << r << ", " << g << ", " << b << " }" << std::endl;
}

Code on Coliru

Answer 9

如果你想在 C++ 中将一个数组传递给 Map 函数。 此代码可能对您有所帮助。 这将数组元素作为输入并将其插入到具有出现次数的 map 函数中。 对于数组 {1, 2, 1, 2, 3, 4, 1} 映射将是 >> 元素：出现次数 1 3, 2 2。 3 1, 4 1

#include<bits/stdc++.h>

using namespace std;

int main()
{
    int size;
    cin>>size;
    int arr[size];
    for(int i = 0 ; i < size ; i++)
    {
        cin >> arr[i];
    }
    
    map <int,int> mp;
    for(int i = 0 ; i < size ;  i++)
        mp[arr[i]]++;
        
        for (auto i = mp.begin(); i != mp.end(); i++) 
        cout << i->first << "      " << i->second << endl; 
        
        return 0;
        //@rsMayank
}

希望能帮到你~Mayank Srivastava