[英]Is it dumb to make a dynamic 2d array where the second dimension is a constant?
I am trying to make an array with a variable number of rows, but it will always have 4 columns.我正在尝试创建一个行数可变的数组,但它总是有 4 列。 Is doing something like:
正在做类似的事情:
int** numGrades = new int* [num_exams];
for (int i = 0; i < num_exams; ++i)
{
numGrades[i] = new int[4];
}
a good way to do this?这样做的好方法吗? I feel like there's an easier way, but I can't think of one.
我觉得有一种更简单的方法,但我想不出一个。 Also, the array keeps giving me memory leaks so I'm wondering if that is because I'm doing something I shouldn't be.
此外,该数组不断给我内存泄漏,所以我想知道这是否是因为我在做我不应该做的事情。 Vectors are banned for this program fyi.
仅供参考,该程序禁止使用载体。
You could make an array of rows.你可以制作一个行数组。
struct Row{
int values[4];
};
Row* numGrades = new Row[num_exams];
You could skip the for loop:您可以跳过 for 循环:
int* numGrades = new int[num_exams*4];
int firstElement = numGrades[x];
int secondElement = numGrades[x+1];
int thirdElement = numGrades[x+2];
int fourthElement = numGrades[x+3];
By skipping the for loop you gain this:通过跳过 for 循环,您可以获得:
You don't have to have a for-loop for freeing the memory:您不必使用 for 循环来释放内存:
delete[] numGrades;删除[] numGrades;
The heap does not fragment so much because you don't call "new" so many times.堆没有那么多碎片,因为您不会多次调用“新”。
BUT it all depends what you are using it for.但这一切都取决于您使用它的目的。 In modern C++ is not such a good idea to use dynamic but make a struct in a std::vector.
在现代 C++ 中,使用动态但在 std::vector 中创建结构并不是一个好主意。
Maybe you can try this.也许你可以试试这个。
typedef int row[4];
//or
using row = int[4];
row *numGrades = new row[num_exams];
Allocating some number of arrays of fixed size is fine and advantageous in many cases.在许多情况下,分配一定数量的固定大小的数组是好的和有利的。
In addition to a struct
(which is a very good option), another option is to declare a Pointer-To-Array of a fixed number of elements.除了
struct
(这是一个很好的选择)之外,另一种选择是声明一个包含固定数量元素的Pointer-To-Array 。 The benefit there is you have a Single-Allocation and Single-Free for the block of memory.这样做的好处是您可以为内存块提供单一分配和单一空闲。 (as you do with an array of struct ) If you need to grow the block of memory (with a -- declare bigger block, copy existing to bigger, delete existing reallocation), it simplifies the process.
(就像使用struct 数组一样)如果您需要增加内存块(使用 -- 声明更大的块,将现有复制到更大,删除现有重新分配),它会简化过程。 In your case:
在你的情况下:
int (*numGrades)[4] = new int[num_exams][4];
Which will allocate num_exams
number of arrays of 4 int
all at once.这将一次分配
num_exams
数量的 4 个int
数组。 That provides the benefit of a single delete[] numGrades;
这提供了单个
delete[] numGrades;
when you are done with the memory.当你完成记忆时。
A short example that uses a std::istringstream
to hold example values to be read into a block of memory containing fixes size arrays could be:使用
std::istringstream
保存示例值以读取到包含修复大小数组的内存块的简短示例可能是:
#include <iostream>
#include <sstream>
int main (void) {
std::istringstream iss { "1 2 3 4 5 6 7 8 9" };
int npoints = 3,
(*points)[3] = new int[npoints][3],
n = 0;
while (n < 3 && iss >> points[n][0] >> points[n][1] >> points[n][2])
n++;
for (int i = 0; i < n; i++)
std::cout << points[i][0] << " " << points[i][1] << " " << points[i][2] << '\n';
delete[] points;
}
( note: you should avoid using new
and delete
in favor of a container such as std::vector if this is for other than educational purposes) (注意:如果不是出于教育目的,您应该避免使用
new
和delete
来支持诸如std::vector之类的容器)
Example Use/Output示例使用/输出
$ ./bin/newptr2array3
1 2 3
4 5 6
7 8 9
Worth noting, the benefit of the struct
is that it will allow you to overload >>
and <<
with std::istream
and std::ostream
to provide a convenient functions to read and write the data you need.值得注意的是,
struct
的好处是它允许您使用std::istream
和std::ostream
重载>>
和<<
以提供方便的函数来读取和写入您需要的数据。
So either way, a Pointer-to-Array of fixed elements, or creating a struct
and then an Array of struct
is perfectly fine.所以无论哪种方式,一个指针到阵列固定元件,或创建一个
struct
,然后数组struct
是完全没有问题。
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