结构体的属性是否有占位符?(C)

Question

我正在尝试我的手在CS50的过滤器，我在像素部分的模糊。我可以使用

来访问像素的颜色

image[row][pixel].rgbtRed
image[row][pixel].rgbtGreen
image[row][pixel].rgbtBlue

我希望能够调用一个函数来计算周围像素的平均值，并传入我想要的平均值的颜色。是否有某种占位符，以便我可以访问结构体的特定元素/属性?(对不起，我不确定它的正确名称)。

我还是一个新手，试着把它放在括号之间，但是没有效果。

这里是我试图获得传递的颜色的值的函数。

float calcAverage(int height, int width, RGBTRIPLE image[height][width], int row, int pixel, string color)
{
float sum =  image[row][pixel].color + image[row][pixel - 1].color; 
return 0;
}

函数是这样调用的

redAverage = calcAverage(height, width, image, row, pixel, "rgbtRed");

现在我的。color计划不起作用，因为我知道他正在寻找一个名为color的属性。这是我得到的错误

error: no member named 'color' in 'RGBTRIPLE'
float sum =  image[row][pixel].color + image[row][pixel - 1].color;

为了测试的目的，我把总和保持得很短。先谢谢你，我开始觉得这是不可能的，我应该离开它。如果我用了错误的术语来描述我要找的东西，再次表示抱歉。

Answer 1

这可以通过对成员进行偏移来实现，如下面的代码所示。从某种意义上说，使用这种方法有些笨拙。然而，在某些情况下，它可能是有用和适当的。由于使用了显式转换，覆盖了关于类型的常见编译器警告，因此必须非常小心。

typedef struct { float red, green, blue; } RGBTRIPLE;

#include <stdio.h>
#include <stddef.h>

/*  Get the offset of a member in an RGBTRIPLE using the C standard "offsetof"
feature.  This could be a function, preferably a static inline function
visible where it is used.
*/
#define OffsetOf(member)    (offsetof(RGBTRIPLE, member))
/*  Get a member of an RGBTRIPLE by its offset.
This uses the offset to locate the member and then converts the address to
a pointer to the member type, which we then use with "*" to refer to the
member.  That produces an lvalue for the member which can be used to read
(use the value of) or write (assign to) the member.
This must be a macro because a function cannot return an lvalue.
*/
#define MemberByOffset(Structure, Offset) 
(*(float *)((char *) &Structure + Offset))

static float Average(RGBTRIPLE *Array, size_t Offset)
{
static const size_t N = 2;  // For demonstration only.
float sum = 0;
for (size_t i = 0; i < N; ++i)
sum += MemberByOffset(Array[i], Offset);
return sum/N;
}

int main(void)
{
RGBTRIPLE Array[] = {{ 10, 20, 30 }, { 100, 200, 300 }};
printf("Red average = %g.n",   Average(Array, OffsetOf(red  )));
printf("Green average = %g.n", Average(Array, OffsetOf(green)));
printf("Blue average = %g.n",  Average(Array, OffsetOf(blue )));
}

Answer 2

不能使用字符串变量代替成员名。您需要检查字符串的值，并根据该值选择字段。

float calcAverage(int height, int width, RGBTRIPLE image[height][width], 
int row, int pixel, string color)
{
if (!strcmp(color, "rgbtRed")) {
return image[row][pixel].rgbtRed + image[row][pixel - 1].rgbtRed;
} else if (!strcmp(color, "rgbtGreen")) {
return image[row][pixel].rgbtGreen + image[row][pixel - 1].rgbtGreen;
} else if (!strcmp(color, "rgbtBlue")) {
return image[row][pixel].rgbtBlue + image[row][pixel - 1].rgbtBlue;
} else {
// invalid color
return 0;
}
}