printf根据输入在C中打印多维数组的意外的最后一个元素？

Question

I'm learning to use multidimensional arrays in C and I'm unable to understand why printf() sometimes gives an unexpected result in the following program.

The idea in this program is that I wish to initialize a 5x2 array and accept 5 integers from the user with scanf to populate the 2nd index, then print the array:

#include <stdio.h>
#include <conio.h>

int main(void)
{
    int i=0, mat[5][2] = {
        {0, 0},
        {0, 0},
        {0, 0},
        {0, 0},
        {0, 0}
    };

    printf("Please enter 5 integers then press enter: \n");

    do {
        scanf("%i", &mat[i][2]);
        i++; 
       } while (getchar() != '\n');


    printf("Here's what the 5x2 array looks like: \n");

    for(i = 0; i < 5; i++){
        printf("%i %i", mat[i][1], mat[i][2]);
        printf(" \n");
    }

    return 0;   
}

If I enter certain integers as the user, then the output is as expected:

C:\Users\hackr>tmp.exe
Please enter 5 integers then press enter:
0 1 2 3 4
Here's what the 5x2 array looks like:
0 0
0 1
0 2
0 3
0 4

However, if I enter different integers, then the last line of output is not what I expected:

C:\Users\hackr>tmp.exe
Please enter 5 integers then press enter:
1 2 3 4 5
Here's what the 5x2 array looks like:
0 1
0 2
0 3
0 4
0 4

C:\Users\hackr>tmp.exe
Please enter 5 integers then press enter:
9 8 7 6 5
Here's what the 5x2 array looks like:
0 9
0 8
0 7
0 6
0 4

In fact, as you can see above, it looks like the final element of index 2 is arbitrarily "4".

Perhaps this is due to a misunderstanding on my part regarding how the array values are indexed or referenced?

Answer 1

Always the array starts with the index 0.

You can try the following snippet of code:

#include <stdio.h>
#include <conio.h>

int main(void)
{
    int i=0, mat[5][2] = {
        {0, 0},
        {0, 0},
        {0, 0},
        {0, 0},
        {0, 0}
    };

    printf("Please enter 5 integers then press enter: \n");

    do {
        scanf("%i", &mat[i][1]);
        i++; 
       } while (getchar() != '\n');


    printf("Here's what the 5x2 array looks like: \n");

    for(i = 0; i < 5; i++){
        printf("%i %i", mat[i][0], mat[i][1]);
        printf(" \n");
    }

    return 0;   
}

while declaration you can use mat[5][2], but for access you should use

mat[0][0] mat[0][1]
mat[1][0] mat[1][1]
...
..
mat[4][0] mat[4][1]

I think now it should work.

Answer 2

mat is defined as:

int mat[5][2];

Valid elements are mat[x][y] for x in range 0 ~ 4 and y in range 0 ~ 1, while you are trying to access out of bound elements in

scanf("%i", &mat[i][2]);

and

printf("%i %i", mat[i][1], mat[i][2]);