How to use new to declare 2d arrays in C + +?

How to use new to declare 2D arrays?

Like, for "normal" arrays, I'll:

int* ary = new int[Size]

but

int** ary = new int[sizeY][sizeX]

a) Unable to work / compile, b) unable to complete the following tasks:

int ary[sizeY][sizeX] 

Do it.

#1 building

In C ++ 11, it is possible to:

auto array = new double[M][N]; 

This way, memory is not initialized. To initialize it:

auto array = new double[M][N]();

Sample program (compiled with "g ++ -std = c ++ 11"):

#include <iostream>
#include <utility>
#include <type_traits>
#include <typeinfo>
#include <cxxabi.h>
using namespace std;

int main()
{
    const auto M = 2;
    const auto N = 2;

    // allocate (no initializatoin)
    auto array = new double[M][N];

    // pollute the memory
    array[0][0] = 2;
    array[1][0] = 3;
    array[0][1] = 4;
    array[1][1] = 5;

    // re-allocate, probably will fetch the same memory block (not portable)
    delete[] array;
    array = new double[M][N];

    // show that memory is not initialized
    for(int r = 0; r < M; r++)
    {
        for(int c = 0; c < N; c++)
            cout << array[r][c] << " ";
        cout << endl;
    }
    cout << endl;

    delete[] array;

    // the proper way to zero-initialize the array
    array = new double[M][N]();

    // show the memory is initialized
    for(int r = 0; r < M; r++)
    {
        for(int c = 0; c < N; c++)
            cout << array[r][c] << " ";
        cout << endl;
    }

    int info;
    cout << abi::__cxa_demangle(typeid(array).name(),0,0,&info) << endl;

    return 0;
}

Output:

2 4 
3 5 

0 0 
0 0 
double (*) [2]

#2 building

Dynamic declaration 2D array:

    #include<iostream>
    using namespace std;
    int main()
    {
        int x = 3, y = 3;

        int **ptr = new int *[x];

        for(int i = 0; i<y; i++)
        {
            ptr[i] = new int[y];
        }
        srand(time(0));

        for(int j = 0; j<x; j++)
        {
            for(int k = 0; k<y; k++)
            {
                int a = rand()%10;
                ptr[j][k] = a;
                cout<<ptr[j][k]<<" ";
            }
            cout<<endl;
        }
    }

Now, in the above code, we use a double pointer, allocate a dynamic memory for it, and give the value of the column. The memory allocated here is for columns only, and now for rows, we only need a for loop and allocate a dynamic memory for the values of each row. Now, we can use pointers just as we would with 2D arrays. In the above example, we then assign random numbers to 2D arrays (pointers), all of which are related to DMA of 2D arrays.

#3 building

In some cases, I've provided you with the most suitable solution for me. Especially if you know [? Size] one dimension of the array. It is very useful for char arrays, for example, if we need a variable size char [20] array.

int  size = 1492;
char (*array)[20];

array = new char[size][20];
...
strcpy(array[5], "hola!");
...
delete [] array;

The key is the parentheses in the array declaration.

#4 building

If your project is CLI (common language runtime support), then:

You can use an array class instead of the one you get when you write it:

#include <array>
using namespace std;

In other words, it is not the unmanaged array class obtained when using the std namespace and including the array header, it is not the unmanaged array class defined in the std namespace and array header, but the managed class array of CLI.

With this class, you can create arrays of any level.

The following code creates a new two-dimensional array with two rows and three columns of type int, which I name "arr":

array<int, 2>^ arr = gcnew array<int, 2>(2, 3);

Now, you can access the array of elements, by name, and write only a square bracket [] and in them, add rows and columns, and separate them from commas.

The following code accesses the elements in the second row and the first column of the array that I created in the previous code above:

arr[0, 1]

Only this row is written to read the value in the cell, that is, to get the value in the cell. However, if the = sign is added, the value will be written in the cell, that is, the value will be set in the cell. Of course, you can also use the + =, - =, * = and / = operators only for numbers (int, float, double, UU int16, UU int32, UU int64, etc.), but make sure you know.

If your project is not CLI, you can use the unmanaged array class of std namespace. Of course, if you include < array >, the problem is that this array class is different from CLI array. Creating an array of this type is the same as CLI, except that you must remove the ^ symbol and the gcnew keyword. Unfortunately, however, the second int parameter in parentheses specifies the length (size) of the array, not its rank!

You cannot specify a level in this array. The level is only the function of the CLI array.

The behavior of std array is similar to that of ordinary array in c + +, you can define it with pointer, for example, int * and then define: new int[size] or not: int arr[size], but unlike ordinary array in c + +, std array provides functions that can be used with array elements, such as fill, start, end, size, etc., but ordinary array provides nothing.

But std array is still a one-dimensional array, just like ordinary c + + array. However, because other people have put forward solutions on how to convert ordinary c + + one-dimensional arrays into two-dimensional arrays, we can apply the same idea to std arrays. For example, according to Mehrdad Afshari's idea, we can write the following code:

array<array<int, 3>, 2> array2d = array<array<int, 3>, 2>();

This line of code creates a "pseudo array", which is a one-dimensional array, each cell of which is or points to another one-dimensional array.

If the length / size of all one-dimensional arrays in one-dimensional array is equal, array2d variable can be regarded as a real two-dimensional array. In addition, special methods can be used to deal with rows or columns, depending on the viewing method, please remember that in 2D array, the std array supports.

You can also use Kevin Loney's solution:

int *ary = new int[sizeX*sizeY];

// ary[i][j] is then rewritten as
ary[i*sizeY+j]

But if you use an std array, the code must look different:

array<int, sizeX*sizeY> ary = array<int, sizeX*sizeY>();
ary.at(i*sizeY+j);

And it still has the unique function of std array.

Note that you can still use [] brackets to access the elements of the std array without having to call the at function. You can also define and assign a new int variable that calculates and retains the total number of elements in the std array and uses its value instead of repeating sizeX*sizeY

You can define your own two-dimensional array generic class, define the constructor of the two-dimensional array class to receive two integers to specify the number of rows and columns in the new two-dimensional array, and define the get function to receive two integer parameters to access the elements in the two-dimensional array and return their values, and set the function to receive three parameters, the first of which is an integer, which specifies the rows in the two-dimensional array And column, the third parameter is the component. Its type depends on the type you choose in the generic class.

You will be able to do all this by using common C + + arrays (pointers or none) or ideas, STD arrays proposed by others and using one, and can easily use 3D arrays like CLI arrays, or two that you can define, assign, and use in C.

#5 building

This problem has puzzled me for 15 years, and all the solutions provided are not satisfactory to me. How to create dynamic multidimensional array in memory? Today I finally found the answer. You can do this using the following code:

#include <iostream>

int main(int argc, char** argv)
{
    if (argc != 3)
    {
        std::cerr << "You have to specify the two array dimensions" << std::endl;
        return -1;
    }

    int sizeX, sizeY;

    sizeX = std::stoi(argv[1]);
    sizeY = std::stoi(argv[2]);

    if (sizeX <= 0)
    {
        std::cerr << "Invalid dimension x" << std::endl;
        return -1;
    }
    if (sizeY <= 0)
    {
        std::cerr << "Invalid dimension y" << std::endl;
        return -1;
    }

    /******** Create a two dimensional dynamic array in continuous memory ******
     *
     * - Define the pointer holding the array
     * - Allocate memory for the array (linear)
     * - Allocate memory for the pointers inside the array
     * - Assign the pointers inside the array the corresponding addresses
     *   in the linear array
     **************************************************************************/

    // The resulting array
    unsigned int** array2d;

    // Linear memory allocation
    unsigned int* temp = new unsigned int[sizeX * sizeY];

    // These are the important steps:
    // Allocate the pointers inside the array,
    // which will be used to index the linear memory
    array2d = new unsigned int*[sizeY];

    // Let the pointers inside the array point to the correct memory addresses
    for (int i = 0; i < sizeY; ++i)
    {
        array2d[i] = (temp + i * sizeX);
    }



    // Fill the array with ascending numbers
    for (int y = 0; y < sizeY; ++y)
    {
        for (int x = 0; x < sizeX; ++x)
        {
            array2d[y][x] = x + y * sizeX;
        }
    }



    // Code for testing
    // Print the addresses
    for (int y = 0; y < sizeY; ++y)
    {
        for (int x = 0; x < sizeX; ++x)
        {
            std::cout << std::hex << &(array2d[y][x]) << ' ';
        }
    }
    std::cout << "\n\n";

    // Print the array
    for (int y = 0; y < sizeY; ++y)
    {
        std::cout << std::hex << &(array2d[y][0]) << std::dec;
        std::cout << ": ";
        for (int x = 0; x < sizeX; ++x)
        {
            std::cout << array2d[y][x] << ' ';
        }
        std::cout << std::endl;
    }



    // Free memory
    delete[] array2d[0];
    delete[] array2d;
    array2d = nullptr;

    return 0;
}

When you call a program with sizeX = 20 and sizeY = 15, the output is as follows:

0x603010 0x603014 0x603018 0x60301c 0x603020 0x603024 0x603028 0x60302c 0x603030 0x603034 0x603038 0x60303c 0x603040 0x603044 0x603048 0x60304c 0x603050 0x603054 0x603058 0x60305c 0x603060 0x603064 0x603068 0x60306c 0x603070 0x603074 0x603078 0x60307c 0x603080 0x603084 0x603088 0x60308c 0x603090 0x603094 0x603098 0x60309c 0x6030a0 0x6030a4 0x6030a8 0x6030ac 0x6030b0 0x6030b4 0x6030b8 0x6030bc 0x6030c0 0x6030c4 0x6030c8 0x6030cc 0x6030d0 0x6030d4 0x6030d8 0x6030dc 0x6030e0 0x6030e4 0x6030e8 0x6030ec 0x6030f0 0x6030f4 0x6030f8 0x6030fc 0x603100 0x603104 0x603108 0x60310c 0x603110 0x603114 0x603118 0x60311c 0x603120 0x603124 0x603128 0x60312c 0x603130 0x603134 0x603138 0x60313c 0x603140 0x603144 0x603148 0x60314c 0x603150 0x603154 0x603158 0x60315c 0x603160 0x603164 0x603168 0x60316c 0x603170 0x603174 0x603178 0x60317c 0x603180 0x603184 0x603188 0x60318c 0x603190 0x603194 0x603198 0x60319c 0x6031a0 0x6031a4 0x6031a8 0x6031ac 0x6031b0 0x6031b4 0x6031b8 0x6031bc 0x6031c0 0x6031c4 0x6031c8 0x6031cc 0x6031d0 0x6031d4 0x6031d8 0x6031dc 0x6031e0 0x6031e4 0x6031e8 0x6031ec 0x6031f0 0x6031f4 0x6031f8 0x6031fc 0x603200 0x603204 0x603208 0x60320c 0x603210 0x603214 0x603218 0x60321c 0x603220 0x603224 0x603228 0x60322c 0x603230 0x603234 0x603238 0x60323c 0x603240 0x603244 0x603248 0x60324c 0x603250 0x603254 0x603258 0x60325c 0x603260 0x603264 0x603268 0x60326c 0x603270 0x603274 0x603278 0x60327c 0x603280 0x603284 0x603288 0x60328c 0x603290 0x603294 0x603298 0x60329c 0x6032a0 0x6032a4 0x6032a8 0x6032ac 0x6032b0 0x6032b4 0x6032b8 0x6032bc 0x6032c0 0x6032c4 0x6032c8 0x6032cc 0x6032d0 0x6032d4 0x6032d8 0x6032dc 0x6032e0 0x6032e4 0x6032e8 0x6032ec 0x6032f0 0x6032f4 0x6032f8 0x6032fc 0x603300 0x603304 0x603308 0x60330c 0x603310 0x603314 0x603318 0x60331c 0x603320 0x603324 0x603328 0x60332c 0x603330 0x603334 0x603338 0x60333c 0x603340 0x603344 0x603348 0x60334c 0x603350 0x603354 0x603358 0x60335c 0x603360 0x603364 0x603368 0x60336c 0x603370 0x603374 0x603378 0x60337c 0x603380 0x603384 0x603388 0x60338c 0x603390 0x603394 0x603398 0x60339c 0x6033a0 0x6033a4 0x6033a8 0x6033ac 0x6033b0 0x6033b4 0x6033b8 0x6033bc 0x6033c0 0x6033c4 0x6033c8 0x6033cc 0x6033d0 0x6033d4 0x6033d8 0x6033dc 0x6033e0 0x6033e4 0x6033e8 0x6033ec 0x6033f0 0x6033f4 0x6033f8 0x6033fc 0x603400 0x603404 0x603408 0x60340c 0x603410 0x603414 0x603418 0x60341c 0x603420 0x603424 0x603428 0x60342c 0x603430 0x603434 0x603438 0x60343c 0x603440 0x603444 0x603448 0x60344c 0x603450 0x603454 0x603458 0x60345c 0x603460 0x603464 0x603468 0x60346c 0x603470 0x603474 0x603478 0x60347c 0x603480 0x603484 0x603488 0x60348c 0x603490 0x603494 0x603498 0x60349c 0x6034a0 0x6034a4 0x6034a8 0x6034ac 0x6034b0 0x6034b4 0x6034b8 0x6034bc 

0x603010: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 
0x603060: 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 
0x6030b0: 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 
0x603100: 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 
0x603150: 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 
0x6031a0: 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 
0x6031f0: 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 
0x603240: 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 
0x603290: 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 
0x6032e0: 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 
0x603330: 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 
0x603380: 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 
0x6033d0: 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 
0x603420: 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 
0x603470: 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299

As you can see, multidimensional arrays are continuously in memory, and there is no overlap of two memory addresses. Even routines that release arrays are simpler than the standard way to dynamically allocate memory for each column (or row, depending on how you view the array). Since arrays are basically composed of two linear arrays, only those arrays must (can be freed).

Methods with the same concept can be extended to more than two dimensions. I won't do it here, but when you have an idea of it, it's a simple task.

I hope this code can help you.

Posted on Thu, 16 Jan 2020 11:59:54 -0500 by zoozoo