Do jagged arrays exist in C/C++?

问题

Is there such a thing as a jagged array in C or C++?

When I compile this:

int jagged[][] = { {0,1}, {1,2,3} };

I get this error:

error: declaration of `jagged\' as multidimensional array must have bounds for all dimensions except the first

回答1:

In C I would use an array of pointers.

For instance:

int *jagged[5];

jagged[0] = malloc(sizeof(int) * 10);
jagged[1] = malloc(sizeof(int) * 3);

etc etc.

回答2:

There's a bunch of ways to do it. Here's another way:

int jagged_row0[] = {0,1};
int jagged_row1[] = {1,2,3};
int *jagged[] = { jagged_row0, jagged_row1 };

回答3:

If you just want to initialise it, you can say:

int jagged[][3] = { {0,1}, {1,2,3} };

but the array will still have the shape [2][3]. If you want a true jagged array, you will have to create it dynamically. And if you do that, and are using C++, you should use a std::vector, as friol suggests.

回答4:

In C++ (not compiled, and probably there's a more compact syntax):

std::vector<std::vector<int> > myArray;

myArray.push_back(std::vector<int>());
myArray.push_back(std::vector<int>());

myArray[0].push_back(0);
myArray[0].push_back(1);

myArray[1].push_back(1);
myArray[1].push_back(2);
myArray[1].push_back(3);

So now you can access the elements with, for example, myArray[0][0], etc.

回答5:

In C99 you can do the following:

int jagged_row0[] = {0,1};
int jagged_row1[] = {1,2,3};

int (*jagged[])[] = { &jagged_row0, &jagged_row1 }; // note the ampersand

// also since compound literals are lvalues ...
int (*jagged2[])[] = { &(int[]){0,1}, &(int[]){1,2,3} };

The only difference here (as compared to rampion's answer) is that the arrays don't decay to pointers and one has to access the individual arrays via another level of indirection - (e.g. *jagged[0] - and the size of each row has to be recorded - i.e. sizeof(*jagged[0]) will not compile) - but they're jagged-appearing to the bone ;)

回答6:

The reason you got the error is that you must specify the bounds for at least the outer dimension; i.e.

int jagged[][3] = {{0,1},{1,2,3}};

You cannot have jagged[0] be a 2-element array of int and jagged[1] be a 3-element array of int; an N-element array is a different type from an M-element array (where N != M), and all elements of an array must be the same type.

What you can do is what the others have suggested above and create jagged as an array of pointers to int; that way each element can point to integer arrays of different sizes:

int row0[] = {0,1};
int row1[] = {1,2,3};
int *jagged[] = {row0, row1};

Even though row0 and row1 are different types (2-element vs. 3-element arrays of int), in the context of the initializer they are both implicitly converted to the same type (int *).

回答7:

With C++11 initializer lists this can be written more compactly:

#include <vector>
#include <iostream>

int main() {
    // declare and initialize array
    std::vector<std::vector<int>> arr = {{1,2,3}, {4,5}};
    // print content of array
    for (auto row : arr) {
        for (auto col : row)
            std::cout << col << " ";
        std::cout << "\n";
    }
}

The output is:

$ g++ test.cc -std=c++11 && ./a.out
1 2 3 
4 5

For reference:

http://en.cppreference.com/w/cpp/utility/initializer_list

回答8:

You can also use the compound literals in c to initialize a truly jagged array which is contiguous in memory as follows:

int (*arr[]) = { (int []) {0, 1}, (int []){ 2, 3, 4}, (int []){5, 6, 7, 8} }

This will be laid out contiguously in memory.

回答9:

//
//jaggedArrays.cpp
//
//program to implement jagged arrays in c++
//
#include<iostream>
#include<iomanip>
using namespace std;

int main()
{
    int rows, i, j;
    cout << endl << "Enter no of rows : ";
    cin >> rows;

    int columnsSizeOfEachRow[rows];

    cout << endl;
    for( i = 0 ; i < rows ; i++ )
    {
        cout << "Enter column size for row no " << i + 1 << " : ";
        cin >> columnsSizeOfEachRow[i];
    }

    int *jaggedArray[rows];
    for (i = 0 ; i < rows ; i++)
        jaggedArray[i] = new int[columnsSizeOfEachRow[i]];

    cout << endl;
    for(i = 0 ; i < rows ; i++)
    {
        for ( j = 0 ; j < columnsSizeOfEachRow[i] ;j++)
        {
            cout << "Array[" << i + 1 << "][" << j + 1 << "] << ";
            cin >> jaggedArray[i][j];
        }
        cout << endl;
    }

    cout << endl << endl << "Jagged array is as follows : " << endl;
    for( i = 0 ; i < rows ; i++)
    {
        for ( j = 0 ; j < columnsSizeOfEachRow[i] ;j++)
            cout << setw(3) <<jaggedArray[i][j] << " ";
        cout << endl;
    }    

    return 0;
}

来源：https://stackoverflow.com/questions/1083658/do-jagged-arrays-exist-in-c-c

标签

c++

jagged-arrays