C++ 和JAVA中的Foreach

foreach循环用于快速迭代容器（数组、向量等）的元素，而不执行初始化、测试或增加/减少。foreach循环通过为每个元素执行操作来工作，而不是执行n次操作。虽然C中没有foreach循环，但它受到C++和Java的支持。它首次在C++ 11中引入，并在Java的JDK 1.5.0中引入。在C++和Java中，foreach循环的关键字是”for”。

语法

for (data_type  variable_name : container_type) {
     operations using variable_name
}

由于C++引入了auto关键字，Java引入了var关键字，我们不再需要在foreach循环中指定变量的数据类型。类型推断会检测容器的数据类型，并将用于遍历的变量设置为相同的数据类型。

下面的代码演示了在不同的容器中使用foreach循环以及C++/Java中的auto/var关键字。

// C++ program to demonstrate use of foreach for array
#include <iostream>
using namespace std;

int main()
{
    int arr[] = { 10, 20, 30, 40 };

    // Printing elements of an array using
    // foreach loop
    // Here, int is the data type, x is the variable name
    // and arr is the array for which we want to iterate foreach
    cout<<"Traversing the array with foreach using array's data type: ";
    for (int x : arr)
        cout<<x<<" ";

    // data type of x is set as int
    cout<<"\nTraversing the array with foreach using auto keyword    : ";
    for (auto x : arr)
    cout<<x<<" ";
}

Java代码

// Java program to demonstrate use of foreach
public class Main {
    public static void main(String[] args)
    {
        // Declaring 1-D array with size 4
        int arr[] = { 10, 20, 30, 40 };

        // Printing elements of an array using
        // foreach loop
        // Here, int is the data type, x is the variable name
        // and arr is the array for which we want to iterate foreach
        System.out.print("Traversing the array with foreach using array's data type: ");
        for (int x : arr)
            System.out.print(x+" ");

        // data type of x is set as int
        System.out.print("\nTraversing the array with foreach using auto keyword     : ");
        for (var x : arr)
            System.out.print(x+" ");
    }
}

输出

Traversing the array with foreach using array's data type: 10 20 30 40 
Traversing the array with foreach using auto keyword     : 10 20 30 40

Vector C++程序

#include 
#include 
using namespace std;

int main() {

    vector value{"This", "is", "foreach", "example", "using", "vector."};

    cout<<"Traversing the vector with foreach using vector's data type: ";
    for (string v : value) {
cout<

输出

Traversing the vector with foreach using vector's data type: This is foreach example using vector. 
Traversing the vector with foreach using auto keyword      : This is foreach example using vector.

C++/Java集合程序

#include 
#include 
using namespace std;

int main() {

    set value = {6, 2, 7, 4, 10, 5, 1};

    cout<<"Traversing the set with foreach using set's data type: ";
    for (int v : value) {
cout<

Java代码

import java.util.*;

public class GFG {

    public static void main(String[] args)
    {
        Set hash_Set = new HashSet();
        hash_Set.add("Geeks");
        hash_Set.add("For");
        hash_Set.add("Geeks");
        hash_Set.add("Foreach");
        hash_Set.add("Example");
        hash_Set.add("Set");

        System.out.print("Traversing the set with foreach using set's data type: ");
        for(String hs : hash_Set) {
            System.out.print(hs+" ");
        }

        System.out.print("\nTraversing the set with foreach using auto keyword : ");
        for (var hs : hash_Set) {
            System.out.print(hs+" ");
        }

    }
}

输出

Traversing the set with foreach using set's data type: 1 2 4 5 6 7 10 
Traversing the set with foreach using auto keyword   : 1 2 4 5 6 7 10

对于数组、向量和集合，我们可以在 foreach 中使用不同的数据类型。

C++/Java Map程序

#include <iostream>
#include <map>
using namespace std;

int main() {

    map<int, string> mapExample;
    mapExample.insert(pair<int, string>(1, "Geeks"));
    mapExample.insert(pair<int, string>(2, "4"));
    mapExample.insert(pair<int, string>(3, "Geeks"));
    mapExample.insert(pair<int, string>(4, "Map"));
    mapExample.insert(pair<int, string>(5, "Foreach"));
    mapExample.insert(pair<int, string>(6, "Example"));

    cout<<"Traversing the map with foreach using map's data type\n";
    for (pair<int, string> mpEx : mapExample ) {
        cout<<mpEx.first<<" "<<mpEx.second<<endl;
    }

    cout<<"\nTraversing the map with foreach using auto keyword\n";
    for (auto mpEx : mapExample){
        cout<<mpEx.first<<" "<<mpEx.second<<endl;
    }

    return 0;
}

Java代码

import java.io.*;
import java.util.Map;
import java.util.HashMap;

class GFG {
    public static void main (String[] args) {
        Map gfg = new HashMap();

        gfg.put(1, "Geeks");
        gfg.put(2, "4");
        gfg.put(3, "Geeks");
        gfg.put(4, "Map");
        gfg.put(5, "Foreach");
        gfg.put(6, "Example");

        System.out.println("Traversing the map with foreach using map's data type");
        for (Map.Entry entry : gfg.entrySet())
                System.out.println(entry.getKey() + " " + entry.getValue());

        System.out.println("\nTraversing the map with foreach using auto keyword");
        for (var entry : gfg.entrySet())
                System.out.println(entry.getKey() + " " + entry.getValue());
    }
}

输出

Traversing the map with foreach using map's data type
1 Geeks
2 4
3 Geeks
4 Map
5 Foreach
6 Example

Traversing the map with foreach using auto keyword
1 Geeks
2 4
3 Geeks
4 Map
5 Foreach
6 Example