Multidimensional Arrays initialized,What is differnce of Initialize with declarations and Initialize after declaration?

Question

What is the difference of multidimensional array initialization?

This is 'Longest Common Subsequence' problem.

string str1, str2;
getline(cin, str1);
getline(cin, str2);
int alphaCount[26] = { 0, };
int str1Len = str1.length();
int str2Len = str2.length();
int** arr = new int* [str1Len+1];
for (int i = 0; i < str1Len+1; i++)
{
    arr[i] = new int[str2Len+1];
    //Method One
    for (int j = 0; j < str2Len+1; j++)
    {
        arr[i][j] = 0;
    }
}
for (int i = 0; i < str1Len; i++)
{
    for (int j = 0; j < str2Len; j++)
    {
        int y = i + 1;
        int x = j + 1;
        if (str1[i] == str2[j])
            arr[y][x] = arr[y - 1][x - 1] + 1;
        else
        {
            if (arr[y][x - 1] > arr[y - 1][x])// using uninitialized memory ERROR
                arr[y][x] = arr[y][x - 1];
            else
                arr[y][x] = arr[y - 1][x];
        }
    }
}
cout << arr[str1.length()][str2.length()] << "\n";

// Method Two
// Not Error
for (int i = 0; i < str1Len + 1; i++)
{
    for (int j = 0; j < str2Len + 1; j++)
    {
        arr[i][j] = 0;
    }
}

// Method Three
//global valiable, Not Error
int arr[1001][1001];

Why Method One has error message warning C6001: using uninitialized memory .

What is the difference between method 1 and method 2?

method1: Initialize with declarations. . method2: Initialize after declaration — Hui Chan Park, Jul 27 '19 at 13:04
You need to post a [mcve]. This includes a complete program that anyone can compile as is, and any input necessary to reproduce the problem. — n. m. could be an AI, Jul 27 '19 at 13:05

gsamaras · Accepted Answer · 2019-07-27T13:40:12.767

If there are more elements than numbers in the list, C++ pads the list with zeros. Thus this static array:

int alphaCount[26] = { 0, };

will have all its members initialized to zeroes (explicitly setting the first element to zero, and letting the others get automatically initialized).

This:

int** arr = new int* [str1Len+1];
for (int i = 0; i < str1Len+1; i++)
{
    arr[i] = new int[str2Len+1];
    for (int j = 0; j < str2Len+1; j++)
    {
        arr[i][j] = 0;
    }
}

will also initialize all elements of the array to 0. However, in this case, the array is a 2D array, and is dynamically allocated (don't forget to free it afterwards). Typically you should check if new succeeded.

Note: Static array vs. dynamic array in C++.

The second method will initialize all the elements of the 2D array to zero, by using a double for loop.

The third method will initialize the global array's elements to zeroes, since Why are global and static variables initialized to their default values?

Multidimensional Arrays initialized,What is differnce of Initialize with declarations and Initialize after declaration?

1 Answers1