Week 10: Enums, Structures, Unions

Tabs (space separated): C

10.1.1 Enumerations: Enumerations

Constant symbolication

Use symbols instead of concrete numbers to represent numbers in programs

enumerate

Use enums instead of defining separate const int variables
An enumeration is a user-defined data type that is declared with the keyword enum in the following syntax:
- enum enumeration type name{name0,...,namen};
Enumeration type names are usually not really used, the names in curly braces are used, because they are constant symbols, their type is int, and their values are in order from 0 to n. Such as:
- enum colors{red,yellow,green};
Three constants are created, the value of red is 0, the value of yellow is 1, and the value of green is 2.
When you need some constant values that can be arranged, the point of defining an enumeration is to give these constant values names.
Enumerators can be used as values
Enumeration types can keep up with enum as a type
But in fact, integers are used for internal calculations and external input and output.

Routine: Automatically counted enumeration

enum colors{red,yellow,green，NumCOLORS};
- This is very convenient when you need to traverse all enumerators or need to create an array with enumerators as subscripts

Enumeration amount

You can specify a value when declaring an enumerator
- enum color{red=1, yellow,green=5};

enums are just int

There is no warning or error even if a non-existing integer value is assigned to a variable of an enum type
- Although enumeration types can be used as types, they are actually very (bu) rarely (hao) used
- Enum is more convenient than const int if it makes sense to rank names
- Enums are better than macros because enums have type int

10.2.1 Structs: Struct Types

declare structure type

#include <stdio.h>

int main(int argc, char const *argv[])
{
    struct date{
        int month;
        int day;
        int year;
    };

    struct date today;

    today.month = 04;
    today.day = 16;
    today.year = 2018;

    printf("Today's date is %i-%i-%i.\n",
        today.year,today.month,today.day);

    return 0;
 } 
 Today's date is 2018-4-16.

inside/outside the function?

Like local variables, struct types declared inside a function can only be used inside the function
So usually the struct type is declared outside the function so that it can be used by multiple functions

The form of the declaration structure

 struct point{
    int x;
    int y;
 };

 struct point p1,p2;

 p1和p2都是point，里面有x和y的值 

 struct{
    int x;
    int y;
} p1,p2;

p1和p2都是一种无名结构，里面有x和y

struct point{
    int x;
    int y;
} p1,p2;
p1和p2都是point，里面有x和y的值

structure variable

struct date today;
today.month = 04;
today.day = 16;
today.year = 2018;

structure initialization

struct date today{04,16,2018};
struct date thismonth{.month=4, .year=2018};

structure member

Structs are a bit like arrays
Arrays use the [] operator and subscripts to access their members
- a[0] = 10;
Structs use the . operator and name to access their members
- today.day
- student.firstName
- p1.x
- p1.y

Structural operations

To access the entire structure, use the name of the structure variable directly
For the entire structure, you can do assignments, take addresses, or pass them to function parameters
- p1 = (struct point){5,10}; //equivalent to p1.x=5, p1.y=10;
- p1=p2; //equivalent to p1.x=p2.x; p1.y=p2.y; arrays cannot do these two operations

structure pointer

Unlike arrays, the name of a structure variable is not the address of the structure variable, and the & operator must be used
struct date *pDate = &today;

#include <stdio.h>

int main(int argc, char const *argv[])
{
    struct date{
        int month;
        int day;
        int year;
    };

    struct date today;  
    today=(struct date){04,16,2018};    
    struct date *pDate = &today; 

    printf("Today's date is %i-%i-%i.\n",
        today.year,today.month,today.day);
    printf("address of today is %p\n", pDate);

    return 0;
 }

10.2.2 Structures: Structures and Functions

struct as function parameter

The entire structure can be passed into the function as the value of the parameter
At this time, create a new structure variable in the function and copy the value of the caller's structure
can also return a struct
This is completely different from an array

input structure

There is no direct way to scanf one structure at a time
If we were to write a function to read in the struct

#include <stdio.h>

struct point {
    int x;
    int y;
}; 

void getStruct(struct point);
void output(struct point);

int main(int argc, char const *argv[])
{
    struct point y = {0,0};
    getStruct(y);
    output(y);
}

void getStruct(struct point p)
{
    scanf("%d", &p.x);
    scanf("%d", &p.y);
    printf("%d, %d\n", p.x, p.y); 
}

void output(struct point p)
{
    printf("%d, %d", p.x, p.y);
}
12 23
12,23
0,0

But how is the read structure sent back?
Remember that C is pass-by-value when calling a function
- So p in the function is different from y in main
- After the function reads the value of p, nothing goes back to main, so y is still {0,0}

solution

In the previous scheme, a structure was passed into the function, and then operated in the function, but there was no return
- The problem is that what is passed into the function is a clone of that struct outside, not a pointer
- Passing in a structure is different from passing in an array
In this input function, it is entirely possible to create a temporary structure variable, and then return this structure to the caller

#include <stdio.h>

struct point {
    int x;
    int y;
}; 

struct point getStruct(void);
void output(struct point);

int main(int argc, char const *argv[])
{
    struct point y = {0,0};
    y = getStruct();
    output(y);
}

struct point getStruct(void)
{
    struct point p;
    scanf("%d", &p.x);
    scanf("%d", &p.y);
    printf("%d, %d\n", p.x, p.y); 
    return p;
}

void output(struct point p)
{
    printf("%d, %d", p.x, p.y);
}
12 23
12, 23
12, 23

Structure pointer as parameter

K&R p131
- "If you have a large structure variable that needs to be passed into a function, it is usually more efficient to pass a pointer instead of copying the entire structure."

pointer to structure

struct date {
    int month;
    int day;
    int year;
} myday;

struct date *p = &myday;

(*p).month = 12;
p -> month = 12;

Use -> to represent the member in the structure variable pointed to by the pointer

10.2.3 Structures: Structures within Structures

array of structures

struct date dates[100];
struct date dates[] = { {4,5,2005},{2,4,2005} };

structure within structure

struct dateAndTime {
    struct date sdate;
    struct time stime;
};

nested structure

struct point {
    int x;
    int y;
};
struct rectangle {
    struct point pt1;
    struct point pt2;
};

如果有变量
    struct rectangle r;
就可以有：
    r.pt1.x、 r.pt1.y， 
    r.pt2.x 和 r.pt2.y 

如果有变量定义：
    struct rectangle r, *rp;
    rp = &r;

那么下面的四种形式是等价的：
    r.pt1.x
    rp->pt1.x
    (r.pt1).x
    (rp->pt1).x
但是没有rp->pt1->x （因为pt1不是指针）

array of structs within structs

#include <stdio.h>

struct point{
    int x;
    int y;
};

struct rectangle {
    struct point p1;
    struct point p2;
};

void printRect(struct rectangle r)
{
    printf("<%d, %d> to <%d, %d>\n", r.p1.x, r.p1.y, r.p2.x, r.p2.y);
}

int main(int argc, char const *argv[])
{
    int i;
    struct rectangle rects[] = {
        {{1,2}, {3,4}},
        {{5,6}, {7,8}}      
    };// 2 rectangles
    for(i=0;i<2;i++) {
        printRect(rects[i]);
    }
}
<1, 2> to <3, 4>
<5, 6> to <7, 8>

10.3.1 Unions: Type Definitions

Custom data type (typedef)

The C language provides a function called typedef to declare a new name for an existing data type. for example:
- typedef int Length;
- Makes Length an alias for type int.
In this way, the name Length can appear in variable definitions and parameter declarations instead of int:
- Length a,b,len;
- Length numbers[10];

Typedef

Declare the name of the new type
- The new name is an alias of some kind
- Improved program readability

typedef long int64_t; //重载已有的类型名字，新名字的含义更清晰，具有可移植性
typedef struct ADate {
    int month;
    int day;
    int year;
} Date; //简化了复杂的数字

int 64_t i = 1000000000;
Date d = {4,17,2018};

typedef int Length;//Length 就等价于int类型

typedef *char[10] Strings;//Strings 是10个字符串的数组的类型

typedef struct node {
    int data;
    struct node *next;
} aNode;

或

typedef struct node aNode;//这样用aNode就可以代替struct node

10.3.2 Union: Union

joint

union AnElt {
    int i;
    char c;
} elt1,elt2;

elt1.i = 4;
elt2.c = 'a';
elt2.i = 0xDEADBEEF;

choose:
- member is
  - an int i or
  - a char c
    sizeof(union...) = maximum value of sizeof(each member)
storage
- All members share a space
- Only one member is active at a time
- The size of a union is its largest member
initialization
- Initialize the first member

The usefulness of union

#include <stdio.h>

typedef union {
    int i;
    char ch[sizeof(int)];
} CHI; 

int main(int argc, char const *argv[])
{
    CHI chi;
    int i;
    chi.i = 1234;
    for (i=0; i<sizeof(int); i++) {
        printf("%02hhX", chi.ch[i]);
    }
    printf("\n");

    return 0;
}
FFD2040000

C programming language: enums, structures, unions