[英]Pointer syntax usage in Array
I have some problem understanding the pointers syntax usage in context with two dimensional arrays, though I am comfortable with 1-D array notation and pointers, below is one of the syntax and I am not able to understand how the following expression is evaluated. 我对使用二维数组的上下文中的指针语法用法有一些问题,虽然我对1-D数组符号和指针感到满意,但下面是语法之一,我无法理解下面的表达式是如何计算的。
To access the element stored at third row second column of array a we will use the subscripted notation as a[2][1] other way to access the same element is 要访问存储在数组a的 第三行第二列的元素,我们将使用下标表示法作为[2] [1]其他方式来访问相同的元素是
*(a[2]+1)
and if we want to use it as pointers we will do like this 如果我们想用它作为指针,我们会这样做
*(*(a+2)+1)
Though I am able to understand the replacement of *(a[2]+1)
as *(*(a+2)+1)
but I don't know how this is getting evaluated. 虽然我能够理解*(a[2]+1)
替换为*(*(a+2)+1)
但我不知道如何评估它。 Please explain with example if possible. 如果可能,请举例说明。 Assume array is stored in row wise order and contain the following elements 假设数组按行顺序存储并包含以下元素
int a[5][2]={
21,22,
31,32
41,42,
51,52,
61,62
};
and base address of array is 100(just assume) so the address of a[2] is 108 (size of int =2(another assumption)) So the expression *(*(a+2)+1). How does it gets evaluated does it start from the inside bracket and if it does then after the first bracket we have the value to which 1 is being added rather than the address... :/
数组的基址是100(只是假设)所以a [2]的地址是108(int的大小= 2(另一个假设))所以表达式*(*(a+2)+1). How does it gets evaluated does it start from the inside bracket and if it does then after the first bracket we have the value to which 1 is being added rather than the address... :/
*(*(a+2)+1). How does it gets evaluated does it start from the inside bracket and if it does then after the first bracket we have the value to which 1 is being added rather than the address... :/
To start of with 开始
a[i] = *(a+i);
So 所以
a[i][j] = *(a[i] +j)
and 和
a[i][j] = *(*(a+i) + j)
How a[i] = *(a+i): a [i] = *(a + i)如何:
If a
is a array then the starting address of the array is given by &a[0]
or just a
如果a
是一个数组,那么数组的起始地址由&a[0]
或只是a
So when you specify 所以当你指定时
a[i]
this will decay to a pointer operation *(a+i)
which is, start at the location a
and dereference the pointer to get the value stored in the location. a[i]
这将衰减到指针操作*(a+i)
,即从位置a
开始并取消引用指针以获得存储在该位置的值。
If the memory location is a
then the value stored in it is given by *a
. 如果存储器位置是a
则存储在其中的值由*a
给出。
Similarly the address of the next element in the array is given by 类似地,数组中下一个元素的地址由下式给出
&a[1] = (a+1); /* Array decays to a pointer */
Now the location where the element is stored in given by &a[1]
or (a+1)
so the value stored in that location is given by *(&a[1])
or *(a+1)
现在存储元素的位置由&a[1]
或(a+1)
因此存储在该位置的值由*(&a[1])
或*(a+1)
For Eg: 对于Eg:
int a[3];
They are stored in the memory as shown below: 它们存储在内存中,如下所示:
a a+1 a+2
------------------
| 100 | 102 | 104|
------------------
&a[0] &a[1] &a[2]
Now a is pointing to the first element of the array. 现在a指向数组的第一个元素。 If you know the pointer operations a+1
will give you the next location and so on. 如果您知道指针操作a+1
将为您提供下一个位置,依此类推。
In 2D arrays the below is what the access is like : 在2D数组中,下面是访问的样子:
int arr[m][n];
arr:
will be pointer to first sub array, not the first element of first sub
array, according to relationship of array & pointer, it also represent
the array itself,
arr+1:
will be pointer to second sub array, not the second element of first sub
array,
*(arr+1):
will be pointer to first element of second sub array,
according to relationship of array & pointer, it also represent second
sub array, same as arr[1],
*(arr+1)+2:
will be pointer to third element of second sub array,
*(*(arr+1)+2):
will get value of third element of second sub array,
same as arr[1][2],
A 2D array is actually a consecutive piece of memory. 2D数组实际上是一块连续的内存。 Let me take a example : int a[3][4]
is representend in memory by a unique sequence of 12 integer : 让我举一个例子: int a[3][4]
在内存中由12个整数的唯一序列表示:
a00 a01 a02 a03 a04 a10 a11 a12 a13 a14 a20 a21 a22 a23 a24
| | |
first row second row third row
(of course it can be extended to any muti-dimensional array) (当然它可以扩展到任何多维数组)
a
is an array of int[4]
: it decays to a pointer to int[4]
(in fact, it decays to &(a[0])
) a
是int[4]
的数组:它衰减到指向int[4]
的指针(事实上,它衰减为&(a[0])
)
a[1]
is second row. a[1]
是第二行。 It decays to a int *
pointing to beginning of first row. 它衰减到指向第一行开头的int *
。
Even if arrays are not pointer, the fact that they decay to pointers allows to use them in pointer arithmetic : a + 1
is a pointer to second element of array a. 即使数组不是指针,它们衰减到指针的事实允许在指针算术中使用它们: a + 1
是指向数组a的第二个元素的指针。
All that explains why a[1] == *(a + 1)
所有这些都解释了为什么a[1] == *(a + 1)
The same reasoning can be applied to a[i] == *(a+i)
, and from there to all the expressions in your question. 同样的推理可以应用于a[i] == *(a+i)
,并从那里应用到你问题中的所有表达式。
Let's look specifically to *(*(a+2)+1)
. 让我们具体看看*(*(a+2)+1)
。 As said above, a + 2
is a pointer to the third element of an array of int 4
. 如上所述, a + 2
是指向int 4
的数组的第三元素的指针。 So *(a + 2)
is third row, is an array of int[4]
and decays to a int *
: &(a[2][0])
. 所以*(a + 2)
是第三行,是int[4]
的数组,并衰减为int *
: &(a[2][0])
。
As *(a + 2)
decays to an int *
, we can still use it as a base for pointer arithmetic and *(a + 2) + 1
is a pointer to the second element of third row : *(a + 2) + 1 == &(a[2][1])
. 当*(a + 2)
衰减到int *
,我们仍然可以将它用作指针算术的基础,而*(a + 2) + 1
是指向第三行的第二个元素的指针: *(a + 2) + 1 == &(a[2][1])
。 Just dereference all that and we get 只是取消引用所有这些,我们得到
*(*(a + 2) + 1) == a[2][1]
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