线性搜寻

问题：给定n个元素的数组arr []，编写一个函数以搜索arr []中的给定元素x。

例子 ：

Input : arr[] = {10, 20, 80, 30, 60, 50, 
                     110, 100, 130, 170}
          x = 110;
Output : 6
Element x is present at index 6

Input : arr[] = {10, 20, 80, 30, 60, 50, 
                     110, 100, 130, 170}
           x = 175;
Output : -1
Element x is not present in arr[].

一种简单的方法是进行线性搜索，即

• 从arr []的最左边的元素开始，然后将x与arr []的每个元素一一比较
• 如果x与元素匹配，则返回索引。
• 如果x与任何元素都不匹配，则返回-1。

例子：

C++

// C++ code to linearly search x in arr[]. If x
// is present then return its location, otherwise
// return -1
 
#include 
using namespace std;
 
int search(int arr[], int n, int x)
{
    int i;
    for (i = 0; i < n; i++)
        if (arr[i] == x)
            return i;
    return -1;
}
 
// Driver code
int main(void)
{
    int arr[] = { 2, 3, 4, 10, 40 };
    int x = 10;
    int n = sizeof(arr) / sizeof(arr[0]);
   
    // Function call
    int result = search(arr, n, x);
    (result == -1)
        ? cout << "Element is not present in array"
        : cout << "Element is present at index " << result;
    return 0;
}

C

// C code to linearly search x in arr[]. If x
// is present then return its location, otherwise
// return -1
 
#include 
 
int search(int arr[], int n, int x)
{
    int i;
    for (i = 0; i < n; i++)
        if (arr[i] == x)
            return i;
    return -1;
}
 
// Driver code
int main(void)
{
    int arr[] = { 2, 3, 4, 10, 40 };
    int x = 10;
    int n = sizeof(arr) / sizeof(arr[0]);
   
    // Function call
    int result = search(arr, n, x);
    (result == -1)
        ? printf("Element is not present in array")
        : printf("Element is present at index %d", result);
    return 0;
}

Java

// Java code for linearly searching x in arr[]. If x
// is present then return its location, otherwise
// return -1
 
class GFG
{
    public static int search(int arr[], int x)
    {
        int n = arr.length;
        for (int i = 0; i < n; i++)
        {
            if (arr[i] == x)
                return i;
        }
        return -1;
    }
 
    // Driver code
    public static void main(String args[])
    {
        int arr[] = { 2, 3, 4, 10, 40 };
        int x = 10;
 
        // Function call
        int result = search(arr, x);
        if (result == -1)
            System.out.print(
                "Element is not present in array");
        else
            System.out.print("Element is present at index "
                             + result);
    }
}

Python3

# Python3 code to linearly search x in arr[].
# If x is present then return its location,
# otherwise return -1
 
 
def search(arr, n, x):
 
    for i in range(0, n):
        if (arr[i] == x):
            return i
    return -1
 
 
# Driver Code
arr = [2, 3, 4, 10, 40]
x = 10
n = len(arr)
 
# Function call
result = search(arr, n, x)
if(result == -1):
    print("Element is not present in array")
else:
    print("Element is present at index", result)

C#

// C# code to linearly search x in arr[]. If x
// is present then return its location, otherwise
// return -1
using System;
 
class GFG {
    public static int search(int[] arr, int x)
    {
        int n = arr.Length;
        for (int i = 0; i < n; i++)
        {
            if (arr[i] == x)
                return i;
        }
        return -1;
    }
 
    // Driver code
    public static void Main()
    {
        int[] arr = { 2, 3, 4, 10, 40 };
        int x = 10;
 
        // Function call
        int result = search(arr, x);
        if (result == -1)
            Console.WriteLine(
                "Element is not present in array");
        else
            Console.WriteLine("Element is present at index "
                              + result);
    }
}
 
// This code is contributed by DrRoot_

Javascript

 
// Javascript code to linearly search x in arr[]. If x
// is present then return its location, otherwise
// return -1
 
function search(arr, n, x)
{
    let i;
    for (i = 0; i < n; i++)
        if (arr[i] == x)
            return i;
    return -1;
}
 
// Driver code
 
    let arr = [ 2, 3, 4, 10, 40 ];
    let x = 10;
    let n = arr.length;
 
    // Function call
    let result = search(arr, n, x);
    (result == -1)
        ? document.write("Element is not present in array")
        : document.write("Element is present at index " + result);
 
// This code is contributed by Manoj
 

输出

Element is present at index 3

上述算法的时间复杂度为O(n)。

线性搜索实际上很少使用，因为其他搜索算法(例如二进制搜索算法和哈希表)可实现与线性搜索相比更快的搜索比较。

改善线性搜索最坏情况的复杂性

1. 如果元素最后在O(n)到O(1)处找到
2. 如果未找到元素O(n)至O(n / 2)

下面是实现：

C++ 14

// C++ program for linear search
#include
using namespace std;
 
void search(vector arr, int search_Element)
{
    int left = 0;
    int length = arr.size();
    int position = -1;
      int right = length - 1;
       
    // Run loop from 0 to right
    for(left = 0; left <= right;)
    {
         
        // If search_element is found with
        // left variable
        if (arr[left] == search_Element)
        {
             
            position = left;
            cout << "Element found in Array at "
                 << position + 1 << " Position with "
                 << left + 1 << " Attempt";
                
            break;
        }
       
        // If search_element is found with
        // right variable
        if (arr[right] == search_Element)
        {
            position = right;
            cout << "Element found in Array at "
                 << position + 1 << " Position with "
                 << length - right << " Attempt";
                
            break;
        }
        left++;
        right--;
    }
 
    // If element not found
    if (position == -1)
        cout << "Not found in Array with "
             << left << " Attempt";
}
 
// Driver code
int main()
{
    vector arr{ 1, 2, 3, 4, 5 };
    int search_element = 5;
     
    // Function call
    search(arr, search_element);
}
     
// This code is contributed by mayanktyagi1709

Java

// Java program for linear search
 
import java.io.*;
 
class GFG
{
 
    public static void search(int arr[], int search_Element)
    {
        int left = 0;
        int length = arr.length;
        int right = length - 1;
        int position = -1;
 
        // run loop from 0 to right
        for (left = 0; left <= right;)
        {
             
            // if search_element is found with left variable
            if (arr[left] == search_Element)
            {
                position = left;
                System.out.println(
                    "Element found in Array at "
                    + (position + 1) + " Position with "
                    + (left + 1) + " Attempt");
                break;
            }
           
            // if search_element is found with right variable
            if (arr[right] == search_Element)
            {
                position = right;
                System.out.println(
                    "Element found in Array at "
                    + (position + 1) + " Position with "
                    + (length - right) + " Attempt");
                break;
            }
             
            left++;
            right--;
        }
 
        // if element not found
        if (position == -1)
            System.out.println("Not found in Array with "
                               + left + " Attempt");
    }
   
    
    // Driver code
    public static void main(String[] args)
    {
        int arr[] = { 1, 2, 3, 4, 5 };
        int search_element = 5;
       
        // Function call
        search(arr,search_element);
    }
}

Python3

# Python3 program for linear search
def search(arr, search_Element):
    left = 0
    length = len(arr)
    position = -1
    right = length - 1
 
    # Run loop from 0 to right
    for left in range(0, right, 1):
 
        # If search_element is found with
        # left variable
        if (arr[left] == search_Element):
            position = left
            print("Element found in Array at ", position +
                  1, " Position with ", left + 1, " Attempt")
            break
 
        # If search_element is found with
        # right variable
        if (arr[right] == search_Element):
            position = right
            print("Element found in Array at ", position + 1,
                  " Position with ", length - right, " Attempt")
            break
        left += 1
        right -= 1
 
    # If element not found
    if (position == -1):
        print("Not found in Array with ", left, " Attempt")
 
# Driver code
arr = [1, 2, 3, 4, 5]
search_element = 5
 
# Function call
search(arr, search_element)
 
# This code is contributed by Dharanendra L V.

C＃

// C# program for linear search
using System;
class GFG
{
 
    public static void search(int []arr,
                              int search_Element)
    {
        int left = 0;
        int length = arr.Length;
        int right = length - 1;
        int position = -1;
 
        // run loop from 0 to right
        for (left = 0; left <= right;)
        {
             
            // if search_element is found with left variable
            if (arr[left] == search_Element)
            {
                position = left;
                Console.WriteLine(
                    "Element found in Array at "
                    + (position + 1) + " Position with "
                    + (left + 1) + " Attempt");
                break;
            }
           
            // if search_element is found with right variable
            if (arr[right] == search_Element)
            {
                position = right;
                Console.WriteLine(
                    "Element found in Array at "
                    + (position + 1) + " Position with "
                    + (length - right) + " Attempt");
                break;
            }
             
            left++;
            right--;
        }
 
        // if element not found
        if (position == -1)
            Console.WriteLine("Not found in Array with "
                               + left + " Attempt");
    }
   
    
    // Driver code
    public static void Main(String[] args)
    {
        int []arr = { 1, 2, 3, 4, 5 };
        int search_element = 5;
       
        // Function call
        search(arr,search_element);
    }
}
 
// This code is contributed by 29AjayKumar

Java脚本

     
// Javascript program for linear search
 
function search(arr, search_Element)
{
    let left = 0;
    let length = arr.length;
    let right = length - 1;
    let position = -1;
 
    // Run loop from 0 to right
    for(left = 0; left <= right;)
    {
         
        // If search_element is found
        // with left variable
        if (arr[left] == search_Element)
        {
            position = left;
            document.write(
                "Element found in Array at " +
                (position + 1) + " Position with " +
                    (left + 1) + " Attempt");
            break;
        }
        
        // If search_element is found
        // with right variable
        if (arr[right] == search_Element)
        {
            position = right;
            document.write(
                "Element found in Array at " +
                (position + 1) + " Position with " +
                (length - right) + " Attempt");
            break;
        }
        left++;
        right--;
    }
 
    // If element not found
    if (position == -1)
        document.write("Not found in Array with " +
                       left + " Attempt");
}
      
// Driver code
let arr = [ 1, 2, 3, 4, 5 ];
let search_element = 5;
 
// Function call
search(arr, search_element);
 
// This code is contributed by code_hunt
 

输出

Element found in Array at 5 Position with 1 Attempt

另请参阅–二进制搜索