在矩阵中从左、右、上、下查找岛

Question

I have been trying for some time now, but i can't figure it out. I have to make a program where in a matrix, it finds the top, bottom, left and right numbers and prints them out. I made it where it prints the bottom, left and right numbers but can't figure out how to print the top one.

#include <iostream>

using namespace std;

int main()
{
    int a[10][10],i,j,m,n,zb,zb2,zb3,zb4;
    cin>>m>>n;
    
    for(i=0;i<n;i++){
        for(j=0;j<m;j++){
            cin>>a[i][j];
        }    
    }
    
    for(i=0;i<n;i++){
        for(j=0;j<m;j++){
            if(i+j<m-1){
                zb=a[i][j]; // first
            }
            if(i+j<m+1){
                zb2=a[i][j]; // second
            }
            if(i<j){
                zb3=a[i][j]; // third
            }        
            if(n+m<j+1){
                zb4=a[i][j]; // fourth
            }  
        }    
    }
   
    

    cout<<zb<<endl;
    cout<<zb2<<endl;
    cout<<zb3<<endl;
    cout<<zb4<<endl;

    return 0;
}

A Graph of how the program works Example of program

Thank you in advance!

Answer 1

Hmmm, the midpoint is defined as:
length >> 1 or (length + 1) / 2

The index of the right most column is (column quantity) - 1 .
The index of the bottom most row is (row quantity) - 1 .

So, the locations are:

const unsigned int mid_column = MAXIMUM_COLUMNS / 2;
const unsigned int mid_row    = MAXIMUM_ROWS / 2;
std::cout << matrix[0][mid_column] << "\n"
          << matrix[mid_row][MAXIMUM_COLUMNS - 1] << "\n"
          << matrix[MAXIMUM_ROWS - 1][mid_column] << "\n"
          << matrix[mid_row][0] << "\n";

Answer 2

The for -loop is extremely unnecessary. If you already now the numbers of column and row, the midpoint can easily be calculated:

#include <iostream>

const unsigned int maxn = 10;
int a[maxn][maxn];

int main()
{
    int row,col;
    std::cin>>row>>col;

    for(int i = 0; i < row; i++)
    {
        for(int j = 0; j < col; j++) { std::cin>>a[i][j]; }
    }

    int midcol = col/2;
    int midrow = row/2;

    std::cout << "Top row : " << a[0][midcol];
    if (col%2==0) { std::cout << " " << a[0][midcol-1]; }
    std::cout << "\n";

    std::cout << "Bottom row : " << a[row-1][midcol];
    if (col%2==0) { std::cout << " " << a[row-1][midcol-1]; }
    std::cout << "\n";

    std::cout << "Left column: " << a[midrow][0];
    if (row%2==0) { std::cout << " " << a[midrow-1][0]; }
    std::cout << "\n";

    std::cout << "Right column: " << a[midrow][col-1];
    if (row%2==0) { std::cout << " " << a[midrow-1][col-1]; }
    std::cout << "\n";

    return 0;
}

Example:

3 3
1 2 3
4 5 6
7 8 9
Top row : 2
Bottom row : 8
Left column: 4
Right column: 6

Example (2):

4 4
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
Top row : 3 2
Bottom row : 15 14
Left column: 9 5
Right column: 12 8

Example (3):

3 4
1 2 3 4
5 6 7 8
9 10 11 12
Top row : 3 2
Bottom row : 11 10
Left column: 5
Right column: 8

It should also be noticed that:

• As @ThomasMatthews mentioned, you can use #define or const variable to declare your array capacities. That way, when you change the size of the matrix, you'll only have to change 1 variable.

• Don't declare a bunch of variables that is one-character or have no meaning like i,j,m,n,zb,zb2,zb3,zb4 . A few is OK, but having more of those will cause a lot of debugging problems when your code gets longer. Also, while using i,j,k as iterator variables in loops are somewhat common within programmers (I do it frequently myself), as a beginner, they can cause a lot of confusion for newbies, for example:

for (int i = 0; i < n; i++)
{
    for (int j = 0; j < m; i++) //mistake here
    {

    }
}

• See Why is "using namespace std;" considered bad practice?