动态内存分配以反向打印浮点数数组

Question

所以我是 C 的新手，正在研究数组的内存分配。我正在尝试创建一个程序，该程序将使用 malloc 动态分配 space 来反转浮点数数组。

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

    struct Rec {
         float * x;
         int size;
    };

    int main(){
    struct Rec a[50]; 
    int i, y;  
    printf("Enter the number of floating point numbers: ");
    scanf("%d", &y);
    x = malloc(y * sizeof(struct));

    printf("Enter 5 floating point numbers: \n");
    for(i = 0; i < sizeof(struct); i++){
    scanf("%.3f", &x[i]);
    }

    printf("The numbers in reverse order are: \n");
    for(i = --sizeof(struct); i >= 0; i--){
    printf("%f \n", a[i]);
    }
}

编译过程中出现以下错误：

error: use of undeclared identifier 'x'
*x = malloc(y * sizeof(struct);
^

test.c:14:25: error: declaration of anonymous struct must be 
a definition
*x = malloc(y * sizeof(struct);
                       ^

test.c:14:32: error: type name requires a specifier or qualifier
*x = malloc(y * sizeof(struct);
                              ^

test.c:14:31: error: type name requires a specifier or qualifier
x = malloc(y * sizeof(struct));
                             ^

test.c:14:24: note: to match this '('
*x = malloc(y * sizeof(struct);
                   ^

test.c:25:3: error: expected '}'
}       
        ^

test.c:9:11: note: to match this '{'
int main(){
      ^

Answer 1

您的指针 x 是存储在数组中的结构的一部分。您可能想通过该结构访问您的 "x"。所以而不是

x = malloc(y * sizeof(struct));

你可能想要

a[some index].x = malloc(y * sizeof(struct));

上面这行代码可以编译，但很可能会给您不正确的结果。既然你想分配它，你希望它是你计划存储在那里的变量的大小，而不是结构的大小。

我应该提到还有其他问题。您不能以这种方式遍历结构。您想改为遍历数组（结构）的长度。

Answer 2

您的代码有很多问题。我建议你在尝试这样做之前多练习 C 基础知识。以下是您可能希望通过代码实现的近似值：

#include <stdio.h>
#include <string.h>

// This structure can hold array of floats - and their size
struct Rec
{
     float * x;
     int size;
};

int main()
{
 // Declare variable of type rec
 struct Rec a;  
 int i, y;  

 // How many floats to store? This could also be stored in a.size instead of y
 printf("Enter the number of floating point numbers: ");        
 scanf("%d", &y);  

 // Create and populate dynamic array
 a.x = malloc(y * sizeof(float));  
 printf("Enter floating point numbers: \n");
 for(i = 0; i < y; i++)
 {
   scanf("%.3f", &a.x[i]);       
 }

 // Print
 printf("The numbers in reverse order are: \n");
 for(i = y-1; i >= 0; i--)
 {
     printf("%f \n", a.x[i]);
 }

  free(a.x);
  return 0;
 }

动态内存分配以反向打印浮点数数组

Dynamic Memory Allocation to Reverse Print Array of Floating Point Numbers

c

arrays

floating-point

malloc

struct