C++ std::bsearch用法及代码示例

std::bsearch在排序数组中搜索元素。在ptr指向的数组中查找与key指向的元素相等的元素。
如果数组包含comp表示与搜索到的元素相等的多个元素，则未指定函数将作为结果返回的元素。
用法:

void* bsearch( const void* key, const void* ptr, std::size_t count,
               std::size_t size, * comp );

参数：
key     -    element to be found
ptr     -    pointer to the array to examine
count    -    number of element in the array
size    -    size of each element in the array in bytes
comp    -    comparison function which returns ?a negative integer value if 
             the first argument is less than the second,
             a positive integer value if the first argument is greater than the second
             and zero if the arguments are equal.

返回值：
Pointer to the found element or null pointer if the element has not been found.

实现二进制谓词comp：

// Binary predicate which returns 0 if numbers found equal 
int comp(int* a, int* b) 
{ 
  
    if (*a < *b) 
        return -1; 
  
    else if (*a > *b) 
        return 1; 
  
    // elements found equal 
    else
        return 0; 
}

Implementation

// CPP program to implement 
// std::bsearch 
#include <bits/stdc++.h> 
  
// Binary predicate 
int compare(const void* ap, const void* bp) 
{ 
    // Typecasting 
    const int* a = (int*)ap; 
    const int* b = (int*)bp; 
  
    if (*a < *b) 
        return -1; 
    else if (*a > *b) 
        return 1; 
    else
        return 0; 
} 
  
// Driver code 
int main() 
{ 
    // Given array 
    int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8 }; 
  
    // Size of array 
    int ARR_SIZE = sizeof(arr) / sizeof(arr[0]); 
  
    // Element to be found 
    int key1 = 4; 
  
    // Calling std::bsearch 
    // Typecasting the returned pointer to int 
    int* p1 = (int*)std::bsearch(&key1, arr, ARR_SIZE, sizeof(arr[0]), compare); 
  
    // If non-zero value is returned, key is found 
    if (p1) 
        std::cout << key1 << " found at position " << (p1 - arr) << '\n'; 
    else
        std::cout << key1 << " not found\n"; 
  
    // Element to be found 
    int key2 = 9; 
  
    // Calling std::bsearch 
    // Typecasting the returned pointer to int 
    int* p2 = (int*)std::bsearch(&key2, arr, ARR_SIZE, sizeof(arr[0]), compare); 
  
    // If non-zero value is returned, key is found 
    if (p2) 
        std::cout << key2 << " found at position " << (p2 - arr) << '\n'; 
    else
        std::cout << key2 << " not found\n"; 
}

输出：

4 found at position 3
9 not found

使用位置：二进制搜索可用于要查找关键字的已排序数据。它可以用于诸如排序列表中键的计算频率之类的情况。

为什么选择二进制搜索？
二进制搜索比线性搜索更有效，因为它使每一步的搜索空间减半。这对于长度为9的数组而言并不重要。这里，线性搜索最多需要9个步骤，而二进制搜索最多需要4个步骤。但是考虑具有1000个元素的数组，此处线性搜索最多需要1000步，而二进制搜索最多需要10步。
对于10亿个元素，二进制搜索最多可以在30个步骤中找到我们的关键字。

相关用法

C++ cin get()用法及代码示例

注：本文由纯净天空筛选整理自 std::bsearch in C++。非经特殊声明，原始代码版权归原作者所有，本译文未经允许或授权，请勿转载或复制。