cpp 2D阵列会自行改变吗？

Question

In the following piece of code i create a 2D array, tab. Let len_a=5,len_b=4 After the first for loop, tab[0][5] should be equal to 5, and it is. But, when i print the row, it changes value to 1. What's happening?

int len_a=a.length(),len_b=b.length();

    int tab[len_a+1][len_b+1];
    tab[0][0]=0;


    for(int i=1;i<=len_a;i++)//Init first row
        tab[0][i]=i;

    cout<<tab[0][5]<<endl;


    for(int j=1;j<=len_b;j++)//Init first column
        tab[j][0]=j;

    for(int i=0;i<=len_a;i++)//Print row
        cout<<tab[0][i]<<" | "<<i<<endl;

    cout<<endl;
    for(int i=0;i<=len_b;i++)
        cout<<tab[i][0]<<" | "<<i<<endl;    

Outputs:

5 (first cout<< tab[0][5])

0 | 0

1 | 1

2 | 2

3 | 3

4 | 4

1 | 5 (second cout<< tab[0][5] ???)

0 | 0

1 | 1

2 | 2

3 | 3

4 | 4

Answer 1

C++ has 0 indexed arrays, which means that first element is 0 and last is N-1.

Example:

tab[2][3]; //creating array

tab[0][0]; //first element of array
tab[0][1];
tab[0][2]; //last element in the first row
tab[1][0];
tab[1][1];
tab[1][2]; //last element in the second row and last element in array

Any other element not mentioned above, like tab[2][...] or tab[...][3] is outside declared bounds array.

Since in example there was tab[6][5] created, cout << tab[0][5]; is accessing something that is out of tab bounds. As @Bob__ mentioned arrays allocate continuous memory, so by accessing tab[0][5] in fact tab[1][0] is accessed. Second access to tab[1][0] element is after //Init first column occurred which overwrite value in tab[1][0] element.

BTW: In the example there is first bracket swap with second. There should be:

int tab[len_a][len_b];

for(int i=0; i<=len_a; i++)//Init first row
    tab[i][0]=i; //not tab[0][i]