[英]C subscripted value is neither array nor pointer nor vector
int * matrixsum(int *a,int *b,int n,int m)
{
int *p=NULL,i,j;
p=malloc(sizeof(int)*n*m);
if(p==NULL)
{
printf("Error!\n");
exit(1);
}
for(i=0;i<n;i++)
{
for(j=0;j<m;j++)
{
*(p+i*n+j)=*(a+i*n+j)+*(b+i*n+j);
}
}
return p;
}
My question is about the line *(p+i*n+j)=*(a+i*n+j)+*(b+i*n+j);
我的问题是关于行*(p+i*n+j)=*(a+i*n+j)+*(b+i*n+j);
: if I replace it with p[i][j]=a[i][j]+b[i][j];
:如果我将其替换为p[i][j]=a[i][j]+b[i][j];
I get the following error 3 times: 我收到以下错误3次:
error: subscripted value is neither array nor pointer nor vector 错误:下标值既不是数组也不是指针也不是向量
Why? 为什么? From my knowledge they are the same thing. 据我所知,它们是同一回事。
My compiler is gcc version 4.6.3. 我的编译器是gcc版本4.6.3。
They're not the same thing at all — which is why the compiler is complaining! 它们根本不是一回事,这就是编译器抱怨的原因! You could write: 你可以写:
p[i*n+j] = a[i*n+j] + b[i*n+j];
The type of p
is int *
; p
的类型为int *
; therefore the type of p[i]
is int
, and you can't subscript an int
. 因此p[i]
的类型是int
,并且不能下标int
。 You'd have to be passing a 2D-array of int
, or an array of pointers to int
, to be able to use the p[i][j]
notation. 您必须传递一个int
的2D数组或一个指向int
的指针数组,才能使用p[i][j]
表示法。 For example, in C99 (using variable-length arrays — and note the reordering of the parameters): 例如,在C99中(使用可变长度数组-并注意参数的重新排序):
int *matrixsum(int n, int m, int a[m][n], int b[m][n])
{
...
p[i][j] = a[i][j] + b[i][j];
...
}
Or, with some considerable care in the setup, you could use: 或者,在设置时要格外小心,您可以使用:
int *matrixsum(int **a, int **b, int m, int n)
{
...
p[i][j] = a[i][j] + b[i][j];
...
}
Note that for this latter example, you can't simply write: 请注意,对于后一个示例,您不能简单地编写:
int a[4][4] = { ... };
int b[4][4] = { ... };
int r = matrixsum(a, b, 4, 4);
The memory allocation for the 2D array is quite different from what is required for the int **
notation. 2D数组的内存分配与int **
表示法所需的内存分配有很大不同。
The compiler is simply telling you that you can't de-reference an integer. 编译器只是告诉您不能取消引用整数。 p
is only an int
pointer - similarly for a
, b
, and c
. p
只是一个int
指针-与a
, b
和c
相似。 You can simulate p[i][j]
by doing the tricky pointer arithmetic into your buffer, but you can't de-reference an int
. 您可以通过对缓冲区执行棘手的指针算法来模拟 p[i][j]
,但不能取消引用int
。
These variables must be int **
before you can use them as a 2D array, using array subscripts. 在使用数组下标将它们用作2D数组之前,这些变量必须为int **
。
from my knowledge they are the same thing 据我所知,他们是同一回事
Yes, you've converted from linear indexing to subscripts correctly, but all 3 of those variables are of the wrong type to apply those subscripts. 是的,您已正确地从线性索引转换为下标,但是所有这三个变量的类型都不正确,无法应用这些下标。 They need to be of type int **
for you to be able to do 2-D array indexing. 它们必须是int **
类型,才能进行二维数组索引。
In the second form, how can the compiler guess that n is the length of the first dimension of YOUR tab (not his one) ? 在第二种形式中,编译器如何猜测n是YOUR选项卡(不是他的)的第一维的长度? to compute &t[i][j] == *(t + i * n + j) 计算&t [i] [j] == *(t + i * n + j)
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