为什么在Java中未初始化的数组上没有'For each'循环

Question

我有一个用Java进行二进制搜索的程序.在为数组输入后,'for-each'循环似乎没有增加计数器变量.但是,它确实适用于常规的'for'循环.为什么'for-each'循环在这种情况下不能增加计数器？

import java.util.Scanner;

public class binarySearch {
    public static int rank(int key, int[] a) {
        int lo = 0;
        int hi = a.length - 1;

        while (lo <= hi) {
            int mid = lo + (hi - lo) / 2;
            if (key > a[mid])
                lo = mid + 1;
            else if (key < a[mid])
                hi = mid - 1;
            else
                return mid;
        }
        return -1;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.println("Enter the key to be searched");
        int key = in.nextInt();
        System.out.println("\nEnter the number of elements in the array");
        int num = in.nextInt();
        int[] array = new int[num];

        for (int counter : array) {
            System.out.println("Enter the element of the array!");
            array[counter] = in.nextInt();
        }
        int result = rank(key, array);
        if (result == -1) {
            System.out.println("\n The given key is not found!\n");
        } else {
            System.out.println("\n The given key is found at position : " + (result + 1));
        }
    }
}

Answer 1

你刚刚创建了数组而没有填充它,所以它将充满默认值.然后,您将迭代数组元素的值,这意味着counter每次的值都将为0.这个循环:

for(int counter : array )
{
    System.out.println("Enter the element of the array!");
    array[counter] = in.nextInt();
}

......大致相当于:

for (int i = 0; i < array.length; i++) {
    // Note: this will always be zero because the array elements are all zero to start with
    int counter = array[i]; 
    System.out.println("Enter the element of the array!");
    array[counter] = in.nextInt();
}

你根本不想迭代数组中的原始值 - 你只想迭代从0到数组的长度(独占),这很容易用for循环完成:

for (int i = 0; i < array.length; i++) {
    System.out.println("Enter the element of the array!");
    array[i] = in.nextInt();
}