C++ Struct

type

在程式語言裡，型別 (type) 定義了：

  • 這個變數能儲存什麼樣的資料

  • 資料在記憶體中的大小與格式

  • 能對這個資料做什麼操作

e.g. int, string, float, char

自己定義一個type

如何自己創造一個新的type ?

(user-defined type)

Struct

struct type_name {
    member_type1 member_name1;
    member_type2 member_name2;
    member_type3 member_name3;
    .
    .
} object_names;

Syntax of struct

type_name: 你要創造的type

object_names: 在定義結構的同時，直接建立的一個或多個變數 (object)

Example

struct fruit {
  int weight;
  double price;
} ;

fruit apple;
fruit banana, melon;
struct fruit {
  int weight;
  double price;
} apple, banana, melon;

Remark: struct結尾要加 ;

Access its member

apple.weight = 5;

apple.price = 9.9;

banana.weight = 10;

cout << apple.weight + banana.weight <<"\n"

Initialize

struct Movie {
    string title;
    int year;
};

Movie m1 = {"Inception", 2010};
Movie m2{"Interstellar", 2014};

function

d091. 00476 - Points in Figures: Rectangles -

struct Point{
    int x, y;
};
Point add(Point a, Point b)
{
    a.x += b.x;
    a.y += b.y;
    return a;
}
signed main() {
    Point a = {1, 2};
    Point b = {3, 4};
    Point c = add(a, b);
    cout << c.x << " " << c.y <<"\n";
    return 0;
}

Pointer

struct Point{
    int x, y;
}*a;

Point b;

a = &b;

如何從a拿到b.x?

(*a).x

Pointer

struct Point{
    int x, y;
}*a;

Point b;

a = &b;

如何從a拿到b.x?

a -> x

Sort

自訂comparator

int cmp(int &x, int &y)
{
    return x > y;
}
signed main() {
    int arr[] = {1, 2, 6, 5, 3};
    sort(arr, arr + 5, cmp);
    for(int i = 0; i < 5; i++)
        cout << arr[i] << "\n";
    return 0;
}

使用call by ref: 避免不必要的複製

當 struct 裡有大物件（像 string 或大陣列）時，複製代價更高

Sort

struct Point {
    int x, y;
};
bool cmp(const Point &a, const Point &b) {
    if (a.x != b.x) return a.x < b.x;
    return a.y < b.y;
}
int main() {
    Point arr[] = {{3, 4}, {1, 2}, {3, 1}, {2, 5}};
    int n = sizeof(arr) / sizeof(arr[0]);
    sort(arr, arr + n, cmp); 
    for (int i = 0; i < n; i++) {
        cout << arr[i].x << " " << arr[i].y << "\n";
    }
}

練習:

  1. a915. 二维点排序

C++ Struct

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C++ Struct

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