递归Java程序忽略if​​ else语句返回并返回3次

Question

所以我遇到了递归程序的问题。该程序的作用是将文本文件作为输入，其中包含未排序的数字列表。然后它要求用户使用递归二进制搜索在数组中找到一个数字。如果找到数字，则返回该数字的索引，否则返回 -1。现在这个程序在这里为任何输入返回-1，即使它在数组中。我不明白的部分是 return mid 语句被调用，然后 return -1 被调用了几次。所以我对我的程序如何运行返回语句但忽略它然后返回-1感到非常困惑。这是我使用的文本文件中的数字。

31 70 5 71 140 187 162 98 153 8 109 103 145 157 27 23 136 54 19 168 114 25 139 129 94

这是程序本身。

import java.io.*;
import java.util.*;

public class Lab2
{
public static void main (String[] args) throws Exception
{
    if (args.length < 1)
    {
        System.out.println( "Fatal Error. Enter a filename on the command line!\n");

        System.exit(0);
    }

    int[] arr = new int[30];  // don't do the resizing  thing. Leave that to Project#1
    int cnt=0;

    Scanner file1 = new Scanner( new FileReader(args[0]) );
    while ( file1.hasNextInt() )
        arr[cnt++]= file1.nextInt();
    file1.close();

    // print the array as it came from the file

    printArray( "original array: ", arr, cnt );

    // sort using Arrays.sort  (see utils API)

    Arrays.sort(  arr, 0, cnt );  // 2nd index non inclusive - i.e. cnt-1

    // re-print it - should come out sorted

    printArray( "sorted array: ", arr, cnt );

    // now search the sorted array using YOUR bSearch

    Scanner kbd = new Scanner( System.in );
    do
    {
        System.out.print("Enter number to search for: ");
        int key = kbd.nextInt();
        if ( key <= 0) break; // ENTER ZERO OR NEGATIVE TO QUIT LOOP
            int index=bSearch( arr, 0, cnt-1, key );
        if ( index < 0 )
            System.out.println( key + " not found at index: " + index);
        else
            System.out.println( key + " found at index: " + index);
    }
    while ( true ); // infinite loop. Must break to get out


} // END main

// ======================================================================
//                  M    E   T    H    O    D   S
// ======================================================================

// return the index where key was found
// else return -1 for not found
static int bSearch(int[] array, int low, int high, int key)
{
    int mid;

    if (low <= high)
    {
        mid = (low+high)/2;

        if (array[mid] < key)
        {
            bSearch(array, mid+1, high, key);
        }
        else if (array[mid] > key)
        {
            bSearch(array, low, mid-1, key);
        }
        else {
            System.out.println("this is true");
            return mid;
        }
    }
    System.out.println("why is this returning");
    return -1;
}

// USE THIS METHOD AS GIVEN: DO NOT CHANGE

private static void printArray( String label, int[] array, int count )
{
    System.out.print(label);
    for( int i=0 ; i<count ;++i )
        System.out.print(array[i] + " " );
    System.out.println("\n");
}

Answer 1

您错过了 2 个退货声明。

    if (array[mid] < key)
    {
        return bSearch(array, mid+1, high, key);
    }
    else if (array[mid] > key)
    {
        return bSearch(array, low, mid-1, key);
    }

Answer 2

您正在调用 bsearch，但您没有返回答案。说bsearch(... 的两行都应该说return bsearch(...。