[英]How to remove error in 2-d array declaration using pointers?
If I declare a two dimensional array as如果我将二维数组声明为
#include <stdio.h>
#include <stdlib.h>
int main() { int c=5;int r=6;
int **a=(int **) malloc (c*sizeof(int *));
int i,j;
for (i=0;i<c;i++){
*(a+i)=(int *) malloc (r*sizeof (int));
}
}
The above program works successfully.上述程序成功运行。
#include <stdio.h>
#include <stdlib.h>
int main() { int c=5;int r=6;
int **a;
**a=(int **) malloc (c*sizeof (int *));
int i,j;
for (i=0;i<c;i++){
*(a+i)=(int *) malloc (r*sizeof (int));
}
}
But the compiler shows an error in the above program.但是编译器在上述程序中显示错误。
Why so?为什么这样? Any help would be greatly appreciated.
任何帮助将不胜感激。
You declared a pointer of the type int **
that is not initialized and has an indeterminate value.您声明了一个未初始化且具有不确定值的
int **
类型的指针。
int **a;
Then in the next statement you are dereferencing the pointer two times然后在下一条语句中,您将取消引用指针两次
**a=(int **) malloc (c*sizeof (int *));
The expression **a
has the type int
while the right hand side expression has the type int **
.表达式
**a
具有int
类型,而右侧表达式具有int **
类型。
So the compiler issues a message that the operands have different types.因此编译器发出一条消息,指出操作数具有不同的类型。
Moreover dereferencing an uninitialized pointer results in undefined behavior if such a program will be run.此外,如果将运行这样的程序,则取消引用未初始化的指针会导致未定义的行为。
You should at least write你至少应该写
a=(int **) malloc (c*sizeof (int *));
Pay attention to that if in the first program the variable r
means rows and the variable c
means columns then you should allocate arrays by rows that is the program should look like请注意,如果在第一个程序中,变量
r
表示行,变量c
表示列,那么您应该按行分配 arrays,程序应该看起来像
#include <stdio.h>
#include <stdlib.h>
int main() { int c=5;int r=6;
int **a=(int **) malloc (r*sizeof(int *));
int i;
for (i=0;i<r;i++){
*(a+i)=(int *) malloc (c*sizeof (int));
}
}
Otherwise the expression a[i]
will yield a column instead of a row.否则,表达式
a[i]
将产生一列而不是一行。
After you will allocate arrays as shown above then the expression **a
is equivalent to the expression a[0][0]
and will yield the object of the type int
that is stored in the first column of the first row.如上所示分配 arrays 后,表达式
**a
等效于表达式a[0][0]
并将产生存储在第一行第一列中的int
类型的 object。
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