c - 为什么这个矩阵加法代码给出了错误的答案？

Question

如果我输入以下代码，下面的代码将给出错误的答案:

1st Matrix 
1 2 3
4 5 6
7 8 9

2nd Matrix
2 2 2
2 2 2
2 2 2

它给了我这个矩阵和输出:

9 10 11
9 10 11
9 10 11

这显然是错误的!我似乎找不到原因？

#include <stdlib.h>
#include <stdio.h>

int main(void)
{

    int r,c;
    int ir,ic;
    int matrix1[r][c];
    int matrix2[r][c];
    int finalmatrix[r][c];

    printf("Insert number of rows of the matrixes (max 10):  ");
    scanf("%d", &r);
    printf("Insert number of columns of the matrixes (max 10):  ");
    scanf("%d", &c);


    while(r!=c)
    {
    printf("The number of rows and columns are not equal, retry:\n");
    printf("Insert number of rows of the Matrix (max 10):  ");
    scanf("%d", &r);
    printf("Insert number of columns of the Matrix (max 10):  ");
    scanf("%d", &c);
    }

    for(ir=1; ir<=r; ir++)
    {
        for(ic=1; ic<=c; ic++)
        {
        printf("Insert element row %d and column %d of the first matrix: ", ir, ic);
        scanf("%d", &matrix1[ir][ic]);
        }
    }

    for(ir=1; ir<=r; ir++)
    {
        for(ic=1; ic<=c; ic++)
        {
        printf("Insert element row %d and column %d of the second matrix: ", ir, ic);
        scanf("%d", &matrix2[ir][ic]);
        }
    }

    for(ir=1; ir<=r; ir++)
    {
        for(ic=1; ic<=c; ic++)
        {
        finalmatrix[ir][ic]=matrix1[ir][ic]+matrix2[ir][ic];
        }
    }

    printf("The sum Matrix is:\n");

    for(ir=1; ir<=r; ir++)
    {
        for(ic=1; ic<=c; ic++)
        {
        printf("%d ", finalmatrix[ir][ic]);
        }
    printf("\n");
    }

    return 0;
}

Answer 1

您的代码在声明 VLA 之前无法初始化变量 r 和 c。您无法知道这些数组的大小，或者 r 和 c 是否为正值!我很惊讶这段代码竟然还能运行。我运行了您发布的代码，并收到了此运行时错误:

runtime error: variable length array bound evaluates to non-positive value -1208010352

我进行了以下更改，并且代码有效:

int r,c;
int ir,ic;

printf("Insert number of rows of the matrixes (max 10):  ");
scanf("%d", &r);
printf("Insert number of columns of the matrixes (max 10):  ");
scanf("%d", &c);

int matrix1[r][c];
int matrix2[r][c];
int finalmatrix[r][c];

示例的输出:

The sum Matrix is:
3 4 5 
6 7 8 
9 10 11 

根据标准中关于数组声明符的部分:

the [ and ] may delimit an expression.... If they delimit an expression (which specifies the size of an array), the expression shall have an integer type. (ISO/IEC 9899:1999 6.7.5.2/1)

If the size is an expression that is not an integer constant expression... each time it is evaluated it shall have a value greater than zero. (ISO/IEC 9899:1999 6.7.5.2/5)

因此，r 或 c 的负值很可能会导致未定义的行为，这意味着任何事情都可能发生!当我运行代码时，出现运行时错误。您运行相同的代码，却得到了不同的结果。该行为未定义，因此不知道会发生什么。