Question

我正在创建一个群众中心计划，我正在努力推广它，它最多只适用于3D。使用具有3D和质量元素的结构。类型对象的变量数量，我希望由用户而不是我定义。但我找不到使用循环创建变量的方法。也许有一种不同的方式去做，我只是不知道它。这是我的代码的一个例子，它不是整个代码，但它是所有方程式的相同逻辑。

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

struct objects 
{
    int Xcdnt;
    int Ycdnt;
    int Zcdnt;
    int Mass;
};

int main()
{

    int objectcount = 0;
    int dimensioncount = 0;

    printf("How many objects are there?");
    scanf(" %d", &objectcount);

    printf("How many dimensions are used?");
    scanf(" %d", &dimensioncount);

    for(int i = 0; i < objectcount; i++)
    {
        struct objects (i);
    }

    for(int i = 0; i < objectcount; i++)
    {
        printf("X coordinate is %d.\n", (i).Xcdnt);
    }

    return 0;

}

Answer 1

这是用C处理记忆的一个基本部分。这并不困难，但在灯泡眨眼之前，你必须与之交朋友之一。（注意：如果你的编译器支持C99可变长度数组对象，你可以使用VLA而不是动态分配，这里没有解决）

您有三种工具可动态分配内存，malloc，calloc和realloc。（calloc与malloc做同样的事情，但是将所有字节初始化为零 - 当你想要确保所有内存都被初始化并且只比malloc稍慢时

对于您的示例，您可以动态为对象分配内存，如下所示（注意以下/* comments */中的其他信息）：

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

typedef struct {    /* a typedef will do (and make things easier)  */
    int xcdnt;      /* C generally uses all lowercase variable and */
    int ycdnt;      /* function names, avoiding MixedCase or       */
    int zcdnt;      /* camelCase, reserving all uppercase for      */
    int mass;       /* constants and macros.                       */
} object;

int main (void) {

    int objectcount = 0;
    int dimensioncount = 0;
    object *objects = NULL; 

    printf ("How many objects are there?: ");
    if (scanf (" %d", &objectcount) != 1) {  /* validate ALL user input */
        fprintf (stderr, "error: invalid conversion or EOF.\n");
        return 1;
    }

    printf ("How many dimensions are used?: ");
    if (scanf (" %d", &dimensioncount) != 1) {
        fprintf (stderr, "error: invalid conversion or EOF.\n");
        return 1;
    }

    /* create objectcount objects using calloc */
    objects = calloc (objectcount, sizeof *objects);

    if (!objects) { /* validate all memory allocations */
        fprintf (stderr, "error: virtual memory exhausted.\n");
        return 1;
    }

    /* create some random values */
    for (int i = 0; i < objectcount; i++) {
        objects[i].xcdnt = i;
        objects[i].ycdnt = i + 1;
        objects[i].zcdnt = i + 2;
        objects[i].mass = i % 10 + 5;
    }

    printf ("\ndimensioncount: %d\n\n", dimensioncount);

    /* output the random data in objects */
    for (int i = 0; i < objectcount; i++) {
        printf ("X coordinate is %d.\nY coordinate is %d.\n"
                "Z coordinate is %d.\nmass is: %d\n\n", 
                objects[i].xcdnt, objects[i].ycdnt, objects[i].zcdnt,
                objects[i].mass);
    }

    free (objects); /* don't forget to free the memory you allocate */

    return 0;
}

示例使用/输出

$ /bin/dynamicobjects
How many objects are there?: 10
How many dimensions are used?: 3

dimensioncount: 3

X coordinate is 0.
Y coordinate is 1.
Z coordinate is 2.
mass is: 5

X coordinate is 1.
Y coordinate is 2.
Z coordinate is 3.
mass is: 6

<snip... objects[2-7]>

X coordinate is 8.
Y coordinate is 9.
Z coordinate is 10.
mass is: 13

X coordinate is 9.
Y coordinate is 10.
Z coordinate is 11.
mass is: 14

内存使用/错误检查

在你编写的动态分配内存的任何代码中，你有2个职责关于任何分配的内存块：（1）总是保留一个指向起始地址的指针内存块，（2）当不再需要时，它可以释放。

您必须使用内存错误检查程序，以确保您不会尝试在已分配的内存块的范围之外/之外进行写入，尝试读取或基于未初始化值的条件跳转，最后，确认您释放了已分配的所有内存。

对于Linux valgrind是正常的选择。每个平台都有类似的记忆检查器。它们都很简单易用，只需通过它运行程序即可。

$ valgrind ./bin/dynamicobjects
==9105== Memcheck, a memory error detector
==9105== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==9105== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==9105== Command: ./bin/dynamicobjects
==9105==
How many objects are there?: 10
How many dimensions are used?: 3

dimensioncount: 3

X coordinate is 0.
Y coordinate is 1.
Z coordinate is 2.
mass is: 5

<snip... remaining>

==9105==
==9105== HEAP SUMMARY:
==9105==     in use at exit: 0 bytes in 0 blocks
==9105==   total heap usage: 1 allocs, 1 frees, 160 bytes allocated
==9105==
==9105== All heap blocks were freed -- no leaks are possible
==9105==
==9105== For counts of detected and suppressed errors, rerun with: -v
==9105== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

始终确认已释放已分配的所有内存并且没有内存错误。

仔细看看，如果您有任何其他问题，请告诉我。

Answer 2

你需要malloc一个对象数组。您当前的代码只是挖掘创建和销毁相同的代码

struct objects* objects = malloc(objectcount * sizeof(struct objects));
for(int i = 0; i < objectcount; i++)
{
   objects[i].Xcdnt = 42;
   .....

}

Answer 3

要创建一个对象数组，而不是循环，

con <- DBI::dbConnect(odbc::odbc(),
                      driver = "/Library/ODBC/FileMaker ODBC.bundle/Contents/MacOS/FileMaker ODBC",
                      server = "127.0.0.1",
                      database = "/Users/bradcannell/Dropbox/Filemaker Pro/Notes/test_r.fmp12",
                      uid = "Admin",
                      pwd = "password")

其中struct objects foo[objectcount];是数组的名称，而foo是数组中的项目数。

然后访问每个元素的代码将是：

objectcount

虽然您需要在此之前执行一些其他步骤，才能在打印之前将值分配给for(int i = 0; i < objectcount; i++) { printf("X coordinate is %d.\n", foo[i].Xcdnt); }。

Answer 4

使用malloc动态创建数组

package.json

你也可以像这样创建一个本地数组：

struct objects* objs = malloc(sizeof *objs * objectcount);

但如果它太大，你就有可能炸毁堆栈。

c结构使用for循环初始化

4 个答案: