Do jagged arrays exist in C/C++?

Question

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

Answer 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.

Answer 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 };

Answer 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.

Answer 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.

Answer 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 ;)

Answer 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 *).

Answer 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

Answer 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.

Answer 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;
}

Answer 10

By using dynamic allocation in cpp we can create jagged arrays.

For example:

#include<iostream>
using namespace std;
    
int main(){
    int n;
    cout<<"Enter n:";
    cin>>n;
    
    cout<<"Enter elements:"; 
    int **p = new int *[n];
    
    for(int i=0;i<n;i++){
        p[i] = new int[i+1];
        for(int j=0;j<(i+1);j++){
            cin>>p[i][j];
        }
    } 
    
    cout<<"Jagged Array:"<<endl; 
    for(int i=0;i<n;i++){
        for(int j=0;j<(i+1);j++){
            cout<<p[i][j]<<" ";           
        }
        cout<<endl;
    }
    
    for(int i=0;i<n;i++){
        delete []p[i];
    }
    delete []p;
}
    

For n=3, we have created a jagged array in the following look:

• Enter n: 3

Enter elements:
1
1 2
1 2 3
Jagged Array:
1
1 2
1 2 3

Answer 11

The jagged arrays do exist in c++/c but the syntax is quite complex and you have to handle many things.

There are two types of jagged arrays in c++.
1) STATIC JAGGED ARRAY(A 2d array in which the size will be a constant number and there will be different number of columns in each row).
2) DYNAMIC JAGGED ARRAY(A 2d array in which the size will be any number taken from user and there will be different number of columns in each row)

1)STEPS OF IMPLEMENTING STATIC JAGGED ARRAY

Using array and a pointer

• 1) Declare 1-D arrays with the number of rows you will need
• 2) The size of each array(array for the elements in the row) will be the number of columns (or elements) in the row
• 3) Declare a 1-D array of pointers that will hold the addresses of the arrows
• 4) The size of the 1-D array is the number of rows you want in the jagged array

#include<iostream>
#include<string>
using namespace std;
int main()
{
    int row0[4] = { 1,2,3,4 };
    int row1[2] = { 5,6 };
    int* jagged[2] = { row0,row1 };
    int Size[2] = { 4,2 }, k = 0;
    for (int i = 0; i < 2; i++)
    {
        int* ptr = jagged[i];
        for (int j = 0; j < Size[k]; j++)
        {
            cout << *ptr << "";
            ptr++;
        }
        cout << endl;
        k++;
        jagged[i]++;
    }
    return 0;
}

The output is as follows

---

1234
56

---

1)STEPS OF IMPLEMENTING DYNAMIC JAGGED ARRAY

Using an array of pointer

• 1) Declare an array of pointers(jagged array)
• 2) The size of this array will be the number of rows required in the jagged array
• 3) For each pointer in the array allocate memory for the number of elements you want in this row.

#include<iostream>
#include<string>
using namespace std;
int main()
{
    //2 rows
    int* jagged[2];

    //Allocate memeory for the elements in the row 0
    jagged[0] = new int[1];

    //Allocate memory for the elements in row 1
    jagged[1] = new int[5];

    //Array to hold the size of each row
    int Size[2] = { 1,5 },k = 0, number = 100;

    //User enters the numbers
    for (int i = 0; i < 2; i++)
    {
        int* p = jagged[i];

        for (int j = 0; j < Size[k]; j++)
        {
            *p = number++;

            //move the pointer
            p++;
        }
        k++;
    }
    k = 0;

    //Display elements in Jagged array
    for (int i = 0; i < 2; i++)
    {
        int* q = jagged[i];
        for (int j = 0; j < Size[k]; j++)
        {
            cout << *q << "";
            //move the pointer to the next element
            q++;
        }
        cout << endl;
        k++;
        //move the pointer to the next row
        jagged[i]++;
    }
    delete[] jagged[0];
    delete[] jagged[1];
    return 0;
}

The output is as follows

---

100
101102103104105

---

Answer 12

No, there are no jagged multidimensional arrays in C nor C++. You can create various constructs that perform similar function at some memory cost (like array of pointers to arrays), but not an actual C-style multidimensional array.

The reason is that C-style arrays, no matter how many dimensions, occupy contiguous memory area with no metadata. So, memory-wise, they're all single-dimensional. It's only the cleverness of pointer arithmetic (striding the pointer by the size of a row) that gives you the functionality of extra dimensions. A jagged array laid out serially has different row sizes, so it cannot be strode by a constant value, so it requires additional storage depending on data size, thus is impossible to express in C type system.

It becomes clearer when you consider to what pointer multidimensional array decay to: Array to pointer decay and passing multidimensional arrays to functions

And that's why you see the error message must have bounds for all dimensions except the first, because all dimensions except the first are necessary to stride the array.