使用指针访问嵌套结构中的数据

Question

我正在做一项作业，要求我编写包含嵌套结构的代码。问题其实很简单。只是为了获得矩形角的坐标。我一直在网上研究，但大多数例子都没有 typedef 结构。我的问题主要是不确定访问和存储数据的正确语法是什么。我有如下结构模板：

typedef struct
{
    double x;
    double y;
} Point; // Create a Point datatype;

typedef struct // nested structure
{
    Point topLeft; // topLeft is the variable name for "Point" structure with x&y property
    Point botRight; // botRight is the variable name for "Point" structure with x&y property
} Rectangle; // Create a Rectangle datatype with Point structure within

void getRect(Rectangle *r); // function prototype
int main()
{
Rectangle r; // initializing rectangle;
getRect(&r); // call function to get inputs using call by reference
}

void getRect(Rectangle *r)
{
    printf("Enter top left points:\n");
    scanf("%lf %lf", r->topLeft.x,r->topLeft.y); // Not sure if this is correct as my program can compile and run up till this point and crashes.
    printf("Enter bottom right points:\n");
    scanf("%lf %lf", r->botRight.x,r->botRight.y);
}

求大家对此事的指点！

Answer 1

问题是调用scanf 的参数应该是指针。例如

scanf("%lf %lf", &r->topLeft.x, &r->topLeft.y);

和

scanf("%lf %lf", &r->botRight.x, &r->botRight.y);

那就是你需要通过引用传递对象 x 和 y，函数 scanf 可以处理原始对象而不是它们值的副本。

在 C 中，按引用传递意味着通过指向对象的指针间接传递对象。在这种情况下，被调用函数可以通过使用指针的取消引用操作来直接访问对象。

Answer 2

scanf() 不分配内存，因此您必须始终传递函数可以为每个说明符放置数据的地址，例如 %d 或 %lf。因此，在您的情况下，您必须使用运算符 &

的地址来传递每个 double 字段的地址

始终测试 scanf() 的 return。有一个点得不到就没有理由继续阅读数据

避免此类评论：

    void getRect(Rectangle *r); // function prototype

很明显这是一个原型。您的代码中有一些这样的注释。更喜欢使用描述用途、参数、参数、算法的注释...

一个例子

下面的小程序以一些常用的方式声明您的结构，包括一个函数 return 一个 Rectangle 用于每次调用和一个打印数据的函数。通常用于测试。这只是一个例子。我不是说它好或最好或什么。

SO Police：我总是将指针指向 malloc() 因为，好吧，我这样做了。

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

typedef struct
{
    double x;
    double y;

}   Point;


typedef struct
{
    Point tL; // top left
    Point bR; // bottom right

}   Rectangle;

Rectangle*  rFactory();
int         showR(Rectangle*);

int         main(void)
{
    Point   A = { 2., 3. };

    Point   B = 
    {
        .y = 3.4,
        .x = 0.
    };

    Point*  pP = &A;
    printf( "Point is (%lf,%lf)\n", pP->x, pP->y ); // using pointer
    printf( "Point is (%lf,%lf)\n", A.x, A.y ); // using data directly

    Rectangle   R0 = { A,B };

    Rectangle   R1 =
    {
        {
            .x = 0.,
            .y = 2.
        },  // tL

        {  3.,4. } // bR
    };

    Rectangle   R2 = 
    {
        .bR = { 1.,2. },
        .tL = { -4.,-5 }
    };

    Rectangle*  pR = &R0;
    pR = &R1;
    pR = &R2; // :) just to get rid of the unused-var warnings 
    printf( "(via pointer) Rectangle is [(%lf,%lf), (%lf,%lf)]\n",
        pR->tL.x, pR->tL.y,
        pR->bR.x, pR->bR.y ); // using pointer
    printf( "(via data) Rectangle is [(%lf,%lf), (%lf,%lf)]\n",
        R2.tL.x, R2.tL.y,
        R2.bR.x, R2.bR.y ); // using data directly

    R2.bR.x = 0.;
    R2.bR.y = 0.;
    printf( "(Changed bR) Rectangle is [(%lf,%lf), (%lf,%lf)]\n",
        pR->tL.x, pR->tL.y,
        pR->bR.x, pR->bR.y ); // using pointer

    printf( "(Using function)\n\n");
    showR( &R2);

    // using a 'factory function gets a new one
    printf( "\n\n(Using factory function)\n\n");
    Rectangle*  other = rFactory();
    showR( other );
    free(other); // this one was allocated

    return 0;

}


Rectangle*  rFactory()
{
    Rectangle* one = (Rectangle*) malloc(sizeof(Rectangle));
    if ( one == NULL ) return NULL; // could not alloc
    int res = 0;

    printf("\
\tEnter points like '3., -2.3' separated by at least one space\
\n\tTop Left point: ");
    res = scanf("%lf %lf", &one->tL.x,&one->tL.y);
    if ( res != 2 )
    {
        free(one);
        return NULL;
    };  // if()

    printf("\
\n\tBottom Right point: ");
    res = scanf("%lf %lf", &one->bR.x,&one->bR.y);
    if ( res != 2 )
    {
        free(one);
        return NULL;
    };  // if()
    
    return one;
};


int     showR( Rectangle* pR)
{
    if ( pR == NULL ) return -1;
    printf( "\n\t[using showR()] Rectangle is [(%lf,%lf), (%lf,%lf)]\n",
        pR->tL.x, pR->tL.y,
        pR->bR.x, pR->bR.y ); // using pointer
    return 0;
}

输出

PS C:\src\bases> gcc -o tst -std=c17 -Wall sh.c
PS C:\src\bases> ./tst
Point is (2.000000,3.000000)
Point is (2.000000,3.000000)
(via pointer) Rectangle is [(-4.000000,-5.000000), (1.000000,2.000000)]
(via data) Rectangle is [(-4.000000,-5.000000), (1.000000,2.000000)]
(Changed bR) Rectangle is [(-4.000000,-5.000000), (0.000000,0.000000)]
(Using function)


        [using showR()] Rectangle is [(-4.000000,-5.000000), (0.000000,0.000000)]


(Using factory function)

        Enter points like '3., -2.3' separated by at least one space
        Top Left point: 1. 2.

        Bottom Right point: 3. 4.

        [using showR()] Rectangle is [(1.000000,2.000000), (3.000000,4.000000)]
PS C:\src\bases>

使用指针访问嵌套结构中的数据

Accessing data in a nested structure using pointers

c

scanf

pass-by-reference

pass-by-value

一个例子

输出