Lambda Expression

Made by 12527 Brine

Lambda Expression

Make by 12527 鄭竣陽

大綱 Outline

「函式」複習
什麼是 lambda expression?
一些酷酷的小知識

Made by 12527 Brine

Function

函式

就是將一堆指令包在一起
簡化重複結構
透過命名提升程式可讀性
函數內變數出來後就會消失
來看些特殊的用法吧

基本介紹＆複習

函式的特殊用法

可能是常識然後我是井底之蛙

函式多載 Function Overloading

#include <iostream>

using namespace std;

void print(int input) {
    cout << input << '\n';
}

void print(double input) {
    cout << input << '\n';
}

void print(string input) {
    cout << input << '\n';
}

void print() {
    cout << '\n';
}

int main() {
    print();
    print(15);
    print(1.5);
    print("16");
}

# PRESENTING CODE

建構函式 Constructor

#include <iostream>

using namespace std;

class Domain {
public:
    Domain() {}

    Domain(int sideLength, int middle): 
           l(middle - sideLength), r(middle + sideLength),
           u(middle - sideLength), d(middle + sideLength) {}
    Domain(int left, int right, int up, int down):
           l(left), r(right), u(up), d(down) {}

    long long getArea() { return (r - l) * (d - u); }
    void print() {
        cout << "l: " << l << '\n';
        cout << "r: " << r << '\n';
        cout << "u: " << u << '\n';
        cout << "d: " << d << '\n';
    }

private:
    int l = 0;
    int r = 0;
    int u = 0;
    int d = 0;
};

int main() {
    Domain defaultDomain;
    Domain squareDomain(12, 5);
    Domain rectangleDomain(5, 13, 2, 12);
    Domain* declareWithNew = new Domain(2, 3);

    cout << defaultDomain.getArea();
    cout << squareDomain.getArea();
    rectangleDomain.print();
    declareWithNew->print();
}

# PRESENTING CODE

解構函式 Destructor

#include <iostream>
#include <climits>
#include <vector>

using namespace std;

struct Temp {
    Temp(int v): value(v) {}
    Temp(int v, int p): value(v), priority(p) {}

    ~Temp() { cout << "deleted object with priority " << this->priority << '\n'; }

    int value;
    int priority = 0;
};

int main() {
    Temp(5, INT32_MAX);
    if (true) {
        vector<Temp> myList;
        for (int i = 10; i > 0; i--) myList.push_back(Temp(5, i));
        cout << "done\n";
    }
    Temp current(15, 0);
    cout << current.value << '\n';
    cout << "hello world!\n";
}

# PRESENTING CODE

一般函式的問題

在程式邊緣
很浪費空間
引數不能使用 auto

Lambda Expression

進入正題

匿名函式 aka λ

since C++ 11
直接宣告在程式碼內
像是變數

一個 λ 的結構

[]() mutable throw() -> TYPE {
    doSomething();
    return aTYPEObject;
}

[] Capture Clause

抓取範圍內變數用
開始一個 lambda 的關鍵

[&] [=] 

[&, a] [=, &a]

[a, &b]

[]

() Parameter List

宣告丟在函式內的變數
長度不可變
全部都得命名
可省略

(int &x)

(string name, int age)

(auto output)

(vector<int> v)

mutable

說明可不可修改傳值外部變數
可省略

throw()

說明是否會輸出例外
可省略

-> TYPE

回傳值類別
可省略

{} body

跟一般函式一樣

範例 Examples

講那麼多誰記得起來
來看實作比較快
來看看他的威力多強大吧

範例 Examples(1)

#include <iostream>

using namespace std;

int main() {
    []{ cout << "hello world!\n"; }();
}

蛤？

範例 Examples(1)

#include <iostream>

using namespace std;

int main() {
    auto sayHi = []{ cout << "hello world!\n"; };
    sayHi();
}

範例 Examples(2)

#include <iostream>
#include <vector>

using namespace std;

int main() {
    vector<int> v;
    auto initializeVector = [&](int length, int value) {
        v.resize(length);
        for (int& n: v) n = value++;
    };
    
    initializeVector(4, 6);

    for (int& n: v) cout << n << ' ';
}

範例 Examples(3)

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main() {
    vector<int> v{60,12,30,21,71,50,48,90,87,69,47};

    int standard;
    cin >> standard;

    stable_partition(v.begin(), v.end(), [&](int x) {
        return x < standard;
    });

    for (int& n: v) cout << n << ' ';
}

範例 Examples(4)

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

void quicksort(vector<int>::iterator head, vector<int>::iterator tail) {
    if (tail - head <= 1) return;

    int pivot = *head;
    auto mid = stable_partition(head, tail, [&](int x) {
        return x < pivot;
    });

    quicksort(head, mid);
    quicksort(mid + 1, tail);
}

int main() {
    vector<int> v{60,12,30,21,71,50,48,90,87,69,47};

    quicksort(v.begin(), v.end());

    for (int& n: v) cout << n << ' ';
}

範例 Examples(5)

#include <iostream>

using namespace std;

int main() {
    auto print = [](auto x) -> void {
        cout << x << '\n';
    };

    print("hello");
    print(420.69);
    print(true);
}

範例 Examples(6)

#include <iostream>

using namespace std;

int main() {
    string a, b;
    cin >> a >> b;

    auto sum = [](auto a, auto b) {
        return a + b;
    };

    cout << sum(a, b);
}

範例 Examples(7)

#include <iostream>

using namespace std;

int main() {
    int m, n;
    cin >> m >> n;

    int o = [&, n]() mutable { return ++m + ++n; }();

    cout << m << ", " << n << ", " << o;
}

小知識時間

一些酷酷的知識

Why lambda?

所以為什麼要叫這個名字？

Lambda Calculus

不是微積分
Alonzo Church, 1936
圖靈完備
所以原因是什麼？

偶然啦

Lambda Calculus

x ↦ f(x)

(x, y) ↦ x^2 + y^2

x ↦ 2^x \small(5) = 32

(x, y) ↦ x^y \small(3, 2) = 9

基本上就是和程式裡面的寫法長差不多

課程結束

感謝聆聽～

建中電研 42nd Lambda Expression

By Brine

建中電研 42nd Lambda Expression

Made in 2022

Brine

A certain person trying to make his way in the universe.

Lambda Expression

Made by 12527 Brine

Lambda Expression

大綱 Outline

Made by 12527 Brine

Function

基本介紹＆複習

函式的特殊用法

函式多載 Function Overloading

建構函式 Constructor

解構函式 Destructor

一般函式的問題

Lambda Expression

匿名函式 aka λ

一個 λ 的結構

[] Capture Clause

() Parameter List

mutable

throw()

-> TYPE

{} body

範例 Examples

範例 Examples(1)

蛤？

範例 Examples(1)

範例 Examples(2)

範例 Examples(3)

範例 Examples(4)

範例 Examples(5)

範例 Examples(6)

範例 Examples(7)

小知識時間

Why lambda?

Lambda Calculus

Lambda Calculus

課程結束

建中電研 42nd Lambda Expression

More from Brine