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Question

我是C的初学者，我有一个程序，我试图在函数内设置结构的值，但我不确定如何将结构传递给函数并让它修改它的原始值

我有一个简单的结构，如下所示：

struct player {

    char name[40];
    int acceleration;

};

只是为了玩，我这样做：

struct player terry;
terry.acceleration = 20;
strcpy(terry.name, "John Terry");

我想将此功能移至某个功能，以便我可以执行以下操作：

createPlayer(terry, 20, "John Terry");

到目前为止，我的功能看起来像这样：

void createPlayer(struct player currentPlayer, char name[], int acceleration) {

     strcpy(currentPlayer.name, name);

     currentPlayer.acceleration = acceleration;

}

但是当我打印出来时：

printf("The speed of %s is %d \n\n", terry.name, terry.acceleration);

我明白这一点：

The speed of  is 0

我在这里做错了什么？请建议我的代码/样式的任何更改违反通常惯例。

非常感谢你！

Answer 1

您需要通过指针传递结构：

将功能更改为：

void createPlayer(struct player *currentPlayer, char name[], int acceleration) {

     strcpy(currentPlayer->name, name);

     currentPlayer->acceleration = acceleration;

}

并称之为：

createPlayer(&terry, 20, "John Terry");

编辑：

在原始代码中，将结构传递给函数时。它是“按价值”完成的。这意味着将副本复制到函数中。因此，您在函数内对其进行的更改将在本地副本上进行，并且不会应用于原始副本。

Answer 2

您可能还需要考虑“createPlayer”功能感觉好像它正在创建一个供应用程序使用的新播放器。只有当您需要在代码中使用它时才为结构分配内存，并在完成后丢弃内存，这是习惯且非常强大的。此外，这样做可以让您在运行代码时根据需要创建任意数量的“播放器”，并根据需要使用它们。

下面是一篇文章，展示如何使用动态内存分配（在执行程序时分配内存）来完成。让我们创建一个包含我们定义的头文件（.h）

player.h：

/* this player.h header file is used by other parts of our program to access
 * the player functionality definitions and interface 
 */

/* the #ifndef / #define below protects against inadvertent syntax errors
 * that may occur by double inclusion of the same file.
 */

#ifndef _PLAYER_H 
#define _PLAYER_H

struct player {

    char name[40];
    int acceleration;

};

extern struct player * create_player(const char *name, int acceleration);
void delete_player(struct player *p);
const char * get_player_name(struct player *p);
void set_player_name(struct player *p, const char *name);
int get_player_accel(struct player *p);
void set_player_accel(struct player *p, int accel);

/* you can add other functions here and then define their implementation in player.c
   shown below */

#endif /* _PLAYER_H */

让我们创建一个player.c，它将包含player.h中定义的函数的实现。

player.c：

/* include memory management functions */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "player.h" /* we created this above */


/* Define the structure */

定义管理结构分配的功能

/* create_player function allocates memory for a new player structure 
 *  and returns the pointer to it upon succcess. It returns NULL on failure 
 */
struct player * create_player(const char *name, int acceleration) {
       struct player *new_player = malloc(sizeof(struct player));
       /* malloc allocates memory for the sizeof the structure and returns
          the pointer to it */
       if ( !new_player ) {
             /* typically we don't put error messages in production code
              * it is handled elsewhere but verbosity 
              * is okay for learning, so we add an error message */
             fprintf(stderr, "unable to allocate memory for a new player\n");
             return NULL;
       }
} 

/* delete_player deallocates memory that was allocated in create_player
 * only pass it the pointers that were created by create_player structure
 */

void delete_player(struct player *p) {
     if (p)
        free(p); 
} 

/* let's do some house keeping functions */


const char * get_player_name(struct player *p) {
      if ( p ) 
           return p->name;
      else 
           return NULL;
}

void set_player_name(struct player *p, const char *name) {
     if ( p ) {
           strncpy(p->name, name, sizeof(p->name)); /* only 40 characters */
     }
}

int get_player_accel(struct player *p) {
    return p->acceleration;
} 

void set_player_accel(struct player *p, int accel) {
    if ( p ) 
         p->acceleration = accel;
}

现在代码中的其他地方你可以使用上面的函数：

main.c中：

#include <stdio.h>
#include "player.h"

int main() {
      struct player *player = create_player("john", 30);

      if ( player ) { 
               printf("%s's acceleration is %d\n", player->name, player->acceleration);
      }
}

使用c编译器使用两个c文件编译程序（如果使用的是C IDE，则将它们添加到项目中）。建立项目......

如果您使用命令行，则：

cc -o player_test player.c main.c

应该做的伎俩

注

现在我在没有使用交流编译器的情况下全部写出来，所以我可能会有一些语法错误...请从中获取C的感觉，而不是字面上使用它:)以防万一有愚蠢的错误。

在C中修改函数中的变量

2 个答案:

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