lambda.cpp

//
// Created by renbin jiang on 2021/11/27.
//
// even_lambda.cpp
// compile with: cl /EHsc /nologo /W4 /MTd
#include <algorithm>
#include <iostream>
#include <vector>

#include "gtest/gtest.h"
using namespace std;
//声明Lambda表达式
//        Lambda表达式完整的声明格式如下：
//
//[capture list] (params list) mutable exception-> return type { function body }
//
int for_each_test() {
    // Create a vector object that contains 9 elements.
    vector<int> v;
    for (int i = 1; i < 10; ++i) {
        v.push_back(i);
    }

    // Count the number of even numbers in the vector by
    // using the for_each function and a lambda.
    int evenCount = 0;
    for_each(v.begin(), v.end(), [&evenCount](int n) {
        cout << n;
        if (n % 2 == 0) {
            cout << " is even " << endl;
            ++evenCount;
        } else {
            cout << " is odd " << endl;
        }
    });

    // Print the count of even numbers to the console.
    cout << "There are " << evenCount << " even numbers in the vector." << endl;
    return 0;
}
#include <algorithm>
#include <cmath>

bool abssort(float* x, unsigned n) {
    std::sort(x, x + n,
              // Lambda expression begins
              [](float a, float b) {
                  return (std::abs(a) < std::abs(b));
              } // end of lambda expression
    );
    return true;
}
#include <algorithm> // std::sort
#include <iostream>  // std::cout
#include <vector>    // std::vector
//以普通函数的方式实现自定义排序规则
bool mycomp(int i, int j) {
    return (i < j);
}
//以函数对象的方式实现自定义排序规则
class mycomp2 {
public:
    bool operator()(int i, int j) { return (i < j); }
};
int lambda3_test() {
    std::vector<int> myvector{32, 71, 12, 45, 26, 80, 53, 33};
    //调用第一种语法格式，对 32、71、12、45 进行排序
    //    std::sort(myvector.begin(), myvector.begin() + 4); //(12 32 45 71) 26 80 53 33
    //    //调用第二种语法格式，利用STL标准库提供的其它比较规则（比如 greater<T>）进行排序
    //    std::sort(myvector.begin(), myvector.begin() + 4,
    //              std::greater<int>()); //(71 45 32 12) 26 80 53 33
    //
    //    //调用第二种语法格式，通过自定义比较规则进行排序
    //    std::sort(myvector.begin(), myvector.end(), mycomp2()); // 12 26 32 33 45 53 71 80
    std::sort(myvector.begin(), myvector.end(), [](int i, int j) { return i > j; });
    //输出 myvector 容器中的元素
    for (std::vector<int>::iterator it = myvector.begin(); it != myvector.end(); ++it) {
        std::cout << *it << ' ' << std::endl;
    }
    return 0;
}
int sum() {
    using namespace std;

    // Assign the lambda expression that adds two numbers to an auto variable.
    auto f1 = [](int x, int y) { return x + y; };

    cout << "f1(2, 3) = " << f1(2, 3) << endl;

    // Assign the same lambda expression to a function object.
    function<int(int, int)> f2 = [](int x, int y) { return x + y; };

    cout << "f2(3, 4) = " << f2(3, 4) << endl;
    return 0;
}

//以下示例显示一个通过值捕获局部变量 i 并通过引用捕获局部变量 j 的 lambda 表达式。 由于 lambda
//表达式通过值捕获 i，因此在程序后面部分中重新指派 i 不影响该表达式的结果。 但是，由于 lambda
//表达式通过引用捕获 j，因此重新指派 j 会影响该表达式的结果。
int sum1() {
    using namespace std;

    int i = 3;
    int j = 5;

    // The following lambda expression captures i by value and
    // j by reference.
    function<int(void)> f = [i, &j] { return i + j; };

    // Change the values of i and j.
    i = 22;
    j = 44;

    // Call f and print its result.
    cout << f() << endl;
    return 0;
}
//你可以将 lambda 表达式嵌套在另一个中，如下例所示。 内部 lambda 表达式将其自变量与 2
//相乘并返回结果。 外部 lambda 表达式通过其自变量调用内部 lambda 表达式并在结果上加 3。
int test2() {
    using namespace std;

    // The following lambda expression contains a nested lambda
    // expression.
    int timestwoplusthree = [](int x) { return [](int y) { return y * 2; }(x) + 3; }(5);

    // Print the result.
    cout << "timestwoplusthree = " << timestwoplusthree << endl;
    return 0;
}
int test3() {
    using namespace std;
    int n = [](int x, int y) { return x + y; }(5, 4);
    cout << __FUNCTION__ << " " << __LINE__ << " n = " << n << endl;
    return 0;
}
//以下示例将 Lambda 表达式作为自变量传递给 find_if 函数。 如果 lambda 表达式 true
//的参数是一个均匀数字，则返回 。
#include <algorithm>
#include <iostream>
#include <list>

int test4() {
    using namespace std;

    // Create a list of integers with a few initial elements.
    list<int> numbers;
    numbers.push_back(13);
    numbers.push_back(17);
    numbers.push_back(42);
    numbers.push_back(46);
    numbers.push_back(99);

    // Use the find_if function and a lambda expression to find the
    // first even number in the list.
    const list<int>::const_iterator result =
            find_if(numbers.begin(), numbers.end(), [](int n) { return (n % 2) == 0; });

    // Print the result.
    if (result != numbers.end()) {
        cout << "The first even number in the list is " << *result << "." << endl;
    } else {
        cout << "The list contains no even numbers." << endl;
    }
    return 0;
}
//由于 lambda 表达式已类型化，因此你可以将其与 C++ 模板一起使用。 下面的示例显示 negate_all 和
// print_all 函数。 negate_all函数将一元 operator- 应用于 对象中的每个 vector 元素。 print_all
// 函数将 vector 对象中的每个元素打印到控制台。
#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

// Negates each element in the vector object. Assumes signed data type.
template <typename T>
void negate_all(vector<T>& v) {
    for_each(v.begin(), v.end(), [](T& n) { n = -n; });
}

// Prints to the console each element in the vector object.
template <typename T>
void print_all(const vector<T>& v) {
    for_each(v.begin(), v.end(), [](const T& n) { cout << n << endl; });
}

int test6() {
    cout << "--->test6:\n";
    // Create a vector of signed integers with a few elements.
    vector<int> v;
    v.push_back(34);
    v.push_back(-43);
    v.push_back(56);

    print_all(v);
    negate_all(v);
    cout << "After negate_all():" << endl;
    print_all(v);
    return 0;
}
int a = 4;
int& f(int x) {
    a = a + x;
    return a;
}
int test7() {
    int t = 5;
    cout << "1:" << f(t) << endl; // a = 9
    f(t) = 20;                    // a = 20
    cout << "2:" << f(t) << endl; // t = 5,a = 20  a = 25
    t = f(t);                     // a = 30 t = 30
    cout << "3:" << f(t) << endl; // t = 60
    return 1;
}

TEST(FOREACH, FOREACH_demo) {
    EXPECT_EQ(0, for_each_test());
    float x = 2.33333;
    EXPECT_TRUE(abssort(&x, 4));
    EXPECT_EQ(0, lambda3_test());
    EXPECT_EQ(0, sum());
    EXPECT_EQ(0, sum1());
    EXPECT_EQ(0, test3());
    EXPECT_EQ(0, test4());
    EXPECT_EQ(0, test2());
    EXPECT_EQ(0, test6());
    EXPECT_EQ(1, test7());
}