c中2d数组的5 * 1矩阵

Question

when i am working with 2d arrays. i had given an input of 5*1 matrix and the it results like this.

#include<stdio.h>
int main(){
int rows=5,cols=1;
int arr[rows][cols];
for(int i=0;i<rows;i++){
    for(int j=0;j<cols;j++){
        scanf("%d",&arr[i][j]);
    }
}
printf("%d\n",arr[1][1]);
printf("%d\n",arr[2][0]);
printf("%d\n",arr[0][2]);
return 0;
}

Input: 1 2 3 4 5

Output : 3 3 3

How thus it works?

Answer 1

Change below snippet

printf("%d\n",arr[1][1]);
printf("%d\n",arr[2][0]);
printf("%d\n",arr[0][2]);

to following one:

printf("%d\n",arr[1][0]);
printf("%d\n",arr[2][0]);
printf("%d\n",arr[0][0]);

As Yunnosch mentioned in comment, you are trying to access outside of array and observing undefined behaviour.

Answer 2

With (effectively)

int arr[5][1];

The only array members you can meaningfully access are

arr[0][0]
arr[1][0]
arr[2][0]
arr[3][0]
arr[4][0]

You might get lucky that your accesses

arr[r][c]

with c!=0 are treated as

arr[r+c]

but there is no guarantee.

You probably have intentionally designed your access to all be r+c==2.
And the value of arr[2] is 3.

Answer 3

Here

int arr[rows][cols];

cols value is 1 means in each arr[rows] you can store only one elements as array index starts from zero . Hence arr[1][1] doesn't exists at all, there is only arr[1][0] exists.

Here in all printf statement you are accessing array elements out of bounds which invokes undefined baheviour .

printf("%d\\n",arr[1][1]);/* result is UB, it may prints some junk data */

Answer 4

Accessing an array subscript out of range is undefined behavior .

arr is a 2D array with dimension 5x1 . These statements

printf("%d\n",arr[1][1]); // UB --> accessing second row second column element
printf("%d\n",arr[2][0]); // Valid --> accessing third row first column elemnt
printf("%d\n",arr[0][2]); // UB --> accessing first row third column element

The in-memory view of 5x1 array would be something like this:

arr 5x1
        col 0
 row 0  +----+
 [0][0] |    |
        |    |
 row 1  +----+
 [1][0] |    |
        |    |
 row 2  +----+
 [2][0] |    |
        |    |
 row 3  +----+
 [3][0] |    |
        |    |
 row 4  +----+
 [4][0] |    |
        |    |
        +----+

In 5x1 array, the valid value of rows is from 0 to 4 and the only valid value of column is 0 (as you have only one column in your 2D array). Trying to access any value of row and column beyond these will lead to undefined behavior.