数组与 C 编程语言中的结构和联合有何不同？

Question

I am unable to understand difference between them. When the same thing is done by three of them then when we should go for Array/Structure/Union?

Answer 1

In an array all the elements have the same size and type, so you can't use one for an int and the other one as a double value, and so on.

In struct s, every element can have a different size or type. You can use one as an int and the others for any data type you can use for a regular variable, you can also have arrays of structures.

The union s are used to use a single variable for possibly multiple data types. In a union the size of an instance equals the size of it's largest member, unlike in struct s where it equals the sum of individual member sizes.

Also, essentially the syntax is very much clearer if you use a struct even for members of the same type. For example, instead of having

float ****point3d;

You could have

struct point3d_s {
    float x, float y, float z;
};
point3d_s *point3d;

will declare a pointer to a 3 dimensional point, which in turn can be used as an array too.

Answer 2

Well, they are three totally different objects.

Use array when you should have many (well, at least two...) elements of the very same type. Mainly, when the number of them might vary. For example: Hold all the phone numbers of students in a class.

Use struct when you should aggregate a few variables together. For example: Hold, per a student, their name, their phone number and their address.

Use union when you should always use only one variable type out of a few possible ones. For example: Hold, per a student, either his phone number or their email address.

Answer 3

Array has no padding in between its elements as compared to structure. All elements of array and structure are considered for total size calculation, while union size is equal to its maximum sized element.

Array have all elements of same type, which is no prerequisite for structure and union.

Array uses index based access for accessing its elements, while structure and union uses .element_name for accessing its elements.