How to create dynamic 2D array of different size strings

Question

I want to read a .txt file.

.txt file will have N-rows and M-cols.

Each word present in the txt file will have varying length.

Sample txt file:

Suppose N = 4 rows

Suppose M = 5 cols

content of txt file:

aa bbb cc dddddddd eeee

aa bbbbbbbbbbbb cc ddddddddddd eeee

aaaaaaaaaa bb cc d e

a b c d eeee

What I have to do:

I have to store these strings into a 2D array of strings such that it looks like this:

arr[4][5] =  

[aa             bbb              cc     dddddddd        eeee]

[aa             bbbbbbbbbbbb     cc     ddddddddddd     eeee]

[aaaaaaaaaa     bb               cc     d               e   ]

[a              b                c      d               eeee]

I know how to create dynamic 2D array of integer and its working fine:

int** arr;
int* temp;

arr = (int**)malloc(row*sizeof(int*));
temp = (int*)malloc(row * col * sizeof(int));
for (int i = 0; i < row; i++)
{
    arr[i] = temp + (i * col);
}
int count = 0;
//setting values in 2-D array
for (int i = 0; i < row; i++)
{
    for (int j = 0; j < col; j++)
    {
        arr[i][j] = count++;
    }
}

But, when I am trying to do the same thing for strings, its crashing.

string** arr;
string* temp;

arr = (string**)malloc(row*sizeof(string*));
temp = (string*)malloc(row * col * sizeof(string));
for (int i = 0; i < row; i++)
{
    arr[i] = temp + (i * col);
}

//setting values in 2-D array
for (int i = 0; i < row; i++)
{
    for (int j = 0; j < col; j++)
    {
        arr[i][j].append("hello"); // CRASH here !!
    }
}

How to store each words in an array??

This is what I have written:

#include "stdafx.h"
#include <cstdlib>
#include <iostream>
#include <vector>
#include <map>
#include <fstream>
#include <string>
#include <algorithm>
#include <assert.h>     /* assert */
using namespace std;

vector<string> readFile(const string file, int& row, int& col)
{
    vector<string> buffer;

    ifstream read(file);
    string line;
    char * writable = NULL;

    if (read.is_open())
    {
        int temp_counter = 0;
        while (!read.eof())
        {
            std::getline(read, line);
            writable = new char[line.size() + 1];
            std::copy(line.begin(), line.end(), writable);
            writable[line.size()] = '\0'; // don't forget the terminating 0
            if (temp_counter == 0)//
            {
                row = std::stoi(line);
                ++temp_counter;
            }
            else if (temp_counter == 1)
            {
                col = std::stoi(line);
                ++temp_counter;
            }
            else
            {
                buffer.push_back(line);
            }       
        }
    }
    // don't forget to free the string after finished using it
    delete[] writable;
    return buffer;
}

void create2DDynamicArray(std::vector<string>&v, int row, int col)
{
    string** arr;
    string* temp;

    arr = (string**)malloc(row*sizeof(string*));
    temp = (string*)malloc(row * col * sizeof(string));
    for (int i = 0; i < row; i++)
    {
        arr[i] = temp + (i * col);
    }


    //setting values in 2-D array
    for (int i = 0; i < row; i++)
    {
        for (int j = 0; j < col; j++)
        {
            arr[i][j].append("hello");
        }
    }
}
int main()
{
    vector<string> myvector;
    int row=0;
    int col=0;

    myvector = readFile("D:\\input.txt", row, col);
    create2DDynamicArray(myvector, row, col);

    getchar();
    return 0;
}

txt file look like:

4

5

aa bbb cc dddddddd eeee

aa bbbbbbbbbbbb cc ddddddddddd eeee

aaaaaaaaaa bb cc d e

a b c d eeee

You change int to char. You allocate the space for string and not for int. — TeaNyan
– TeaNyan, Commented Aug 25, 2016 at 7:27
Add allocate an extra entry for the null-character of each string. — barak manos
– barak manos, Commented Aug 25, 2016 at 7:28
Why not choose one of C and C++? If you are going to use C, they say you shouldn't cast the result of malloc() in C. If you are going to use C++, why not use new[] instead of malloc()? — MikeCAT
– MikeCAT, Commented Aug 25, 2016 at 7:30
"But, when I am trying to do the same thing for strings, its crashing." How? Why not post a Minimal, Complete, and Verifiable example? — MikeCAT
– MikeCAT, Commented Aug 25, 2016 at 7:30
If you are going to use C++, why even care about allocating like this. — mtszkw
– mtszkw, Commented Aug 25, 2016 at 7:31

Adrian Jałoszewski · Accepted Answer · 2016-08-25 07:46:29Z

4

Don't use malloc in C++. It does not run the constructor of the strings, thus not allocating space for the dynamic char array stored in them. Try the new[] operator or smart pointers instead.

string **arr;
arr = new string*[height];
for (int i = 0; i < height; i++)
    arr[i] = new string[width];

A c++ string is just a kind of wrapper around a dynamic char array, which must be initialized (it should have memory assigned to it). By using malloc you don't call the constructor, resulting in accessing a non allocated memory area.

edited Aug 25, 2016 at 7:46

answered Aug 25, 2016 at 7:41

Adrian Jałoszewski

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Comments

David Ranieri · Accepted Answer · 2016-08-25 08:25:33Z

2

I suggest to avoid fragmentation and use a real 2d array.

In C, since C99 you can use VLA's (variable length arrays):

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    int rows = 4, cols = 5;
    char *(*arr)[cols];
    int i, j;

    arr = malloc(sizeof(*arr) * rows);
    for (i = 0; i < rows; i++) {
        for (j = 0; j < cols; j++) {
            arr[i][j] = "hello"; /* For read only, to be writable use strdup */
        }
    }
    for (i = 0; i < rows; i++) {
        for (j = 0; j < cols; j++) {
            printf("%s\t", arr[i][j]);
        }
        printf("\n");
    }
    free(arr);
    return 0;
}

Output:

hello   hello   hello   hello   hello   
hello   hello   hello   hello   hello   
hello   hello   hello   hello   hello   
hello   hello   hello   hello   hello

arr[i][j].append("hello"); // CRASH here !!

There are not methods in C and this wont compile, why are you mixing C and C++? pick one of them.

edited Aug 25, 2016 at 8:25

answered Aug 25, 2016 at 7:55

David Ranieri

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6 Comments

Lundin Over a year ago

Even if C99 isn't available, this method works even on ancient C compilers or C++ compilers, as long as rows and cols are compile-time constants. I personally prefer to use the style arr = malloc(sizeof(char*[rows][cols])); as that makes the intent clearer, it is self-documenting code.

David Ranieri Over a year ago

@Lundin are you sure? with gcc -std=c89 : ISO C90 forbids variable length array ‘arr’

David Ranieri Over a year ago

The warning disapears when cols is defined as enum {cols = 5}; or #define cols 5 but is still there using with int cols = 5; even using const: related question: stackoverflow.com/q/436300/1606345

Lundin Over a year ago

Yes that's what I meant, you have to make it a compile-time constant. Unfortunately, C doesn't treat const variables as compile-time constants (but that might work in C++).

David Ranieri Over a year ago

@Lundin Ah, ok ;)

|

Jarod42 · Accepted Answer · 2016-08-25 09:05:47Z

1

Don't use malloc, avoid new/new[] and use RAII containers:

std::vector<std::vector<std::string>> readFile(const std::string& filename)
{
    std::ifstream file(filename);
    int row;
    int col;
    file >> row >> col;

    std::vector<std::vector<std::string>> words(row, std::vector<std::string>(col));

    for (auto& rows : words) {
        for (auto& word : rows) {
            file >> word;
        }
    }
    return words;
}

answered Aug 25, 2016 at 9:05

Jarod42

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Comments

Software_Designer · Accepted Answer · 2016-08-25 10:59:17Z

Vectors are also Dynamic Arrays , but with all the work of keeping track of pointers hidden from the user.

If you decide to use vectors instead then coding a 2D dynamic array is as easy as this:

#include <iostream>
#include <vector>
#include <string>
#include <fstream>       
using namespace std;


int main () {

    cout<<"\nDynamic 2D Array.\n\n";

    // create string vector
    vector<string> vArray;

    // create one line string
    string line;

    // open file for reading
    ifstream fileToRead("d2d.txt");
    while (getline(fileToRead, line)){

        //  fuse (store) line from file in vector
        vArray.push_back(line);
    }
    fileToRead.close();

    // display results
    for (int i=0; i< vArray.size();i++){
        cout<<" [ "<< vArray[i] <<" ] \n";
    }

cout<<"\nPress ANY key to close.\n\n";
cin.ignore(); cin.get();
return 0;
}

VolAnd · Accepted Answer · 2016-08-25 07:50:04Z

If you really want to use 2D array with memory allocated with malloc my suggestion is to shift from string ** to string *** type, like:

    ifstream f(your_file_name);

    string*** arr;

    arr = (string***)malloc(row*sizeof(string**));

    for (int i = 0; i < row; i++)
    {
        arr[i] = (string**)malloc(col * sizeof(string*));
    }

    //setting values in 2-D array
    for (int i = 0; i < row; i++)
    {
        for (int j = 0; j < col; j++)
        {
            arr[i][j] = new string();
            f >> *arr[i][j]; // or arr[i][j] -> append("hello");
        }
    }

But if it really C++ project consider using vector<vector<string>> or even use new instead of malloc, like:

    ifstream f("tmp.txt");

    string** arr;

    arr = new string*[row];

    for (int i = 0; i < row; i++)
    {
        arr[i] = new string[col];
    }

    //reading 2-D array from file
    for (int i = 0; i < row; i++)
    {
        for (int j = 0; j < col; j++)
        {
            f >> arr[i][j];
        }
    }
    // show file content
    for (int i = 0; i < row; i++)
    {
        for (int j = 0; j < col; j++)
        {
            cout << arr[i][j] << " ";
        }
        cout << endl;
    }

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