朴素的模式搜索算法

给定文本txt [0..n-1]和模式pat [0..m-1] ，编写一个函数search(char pat []，char txt []) ，将所有出现的pat []都打印在txt中[] 。您可以假设n> m 。
例子：

Input:  txt[] = "THIS IS A TEST TEXT"
        pat[] = "TEST"
Output: Pattern found at index 10

Input:  txt[] =  "AABAACAADAABAABA"
        pat[] =  "AABA"
Output: Pattern found at index 0
        Pattern found at index 9
        Pattern found at index 12

模式搜索是计算机科学中的一个重要问题。当我们在记事本/单词文件或浏览器或数据库中搜索字符串时，将使用模式搜索算法来显示搜索结果。

天真的模式搜索：
一张一张地在文本上滑动图案，然后检查是否匹配。如果找到匹配项，则再次滑动1以检查后续匹配项。

C

// C program for Naive Pattern Searching algorithm
#include 
#include 
 
void search(char* pat, char* txt)
{
    int M = strlen(pat);
    int N = strlen(txt);
 
    /* A loop to slide pat[] one by one */
    for (int i = 0; i <= N - M; i++) {
        int j;
 
        /* For current index i, check for pattern match */
        for (j = 0; j < M; j++)
            if (txt[i + j] != pat[j])
                break;
 
        if (j == M) // if pat[0...M-1] = txt[i, i+1, ...i+M-1]
            printf("Pattern found at index %d \n", i);
    }
}
 
/* Driver program to test above function */
int main()
{
    char txt[] = "AABAACAADAABAAABAA";
    char pat[] = "AABA";
    search(pat, txt);
    return 0;
}

C++

// C++ program for Naive Pattern
// Searching algorithm
#include 
using namespace std;
 
void search(char* pat, char* txt)
{
    int M = strlen(pat);
    int N = strlen(txt);
 
    /* A loop to slide pat[] one by one */
    for (int i = 0; i <= N - M; i++) {
        int j;
 
        /* For current index i, check for pattern match */
        for (j = 0; j < M; j++)
            if (txt[i + j] != pat[j])
                break;
 
        if (j == M) // if pat[0...M-1] = txt[i, i+1, ...i+M-1]
            cout << "Pattern found at index "
                 << i << endl;
    }
}
 
// Driver Code
int main()
{
    char txt[] = "AABAACAADAABAAABAA";
    char pat[] = "AABA";
    search(pat, txt);
    return 0;
}
 
// This code is contributed
// by Akanksha Rai

Java

// Java program for Naive Pattern Searching
public class NaiveSearch {
 
    public static void search(String txt, String pat)
    {
        int M = pat.length();
        int N = txt.length();
 
        /* A loop to slide pat one by one */
        for (int i = 0; i <= N - M; i++) {
 
            int j;
 
            /* For current index i, check for pattern
              match */
            for (j = 0; j < M; j++)
                if (txt.charAt(i + j) != pat.charAt(j))
                    break;
 
            if (j == M) // if pat[0...M-1] = txt[i, i+1, ...i+M-1]
                System.out.println("Pattern found at index " + i);
        }
    }
 
    public static void main(String[] args)
    {
        String txt = "AABAACAADAABAAABAA";
        String pat = "AABA";
        search(txt, pat);
    }
}
// This code is contributed by Harikishore

Python3

# Python3 program for Naive Pattern
# Searching algorithm
def search(pat, txt):
    M = len(pat)
    N = len(txt)
 
    # A loop to slide pat[] one by one */
    for i in range(N - M + 1):
        j = 0
         
        # For current index i, check
        # for pattern match */
        while(j < M):
            if (txt[i + j] != pat[j]):
                break
            j += 1
 
        if (j == M):
            print("Pattern found at index ", i)
 
# Driver Code
if __name__ == '__main__':
    txt = "AABAACAADAABAAABAA"
    pat = "AABA"
    search(pat, txt)
 
# This code is contributed
# by PrinciRaj1992

C#

// C# program for Naive Pattern Searching
using System;
 
class GFG {
 
    public static void search(String txt, String pat)
    {
        int M = pat.Length;
        int N = txt.Length;
 
        /* A loop to slide pat one by one */
        for (int i = 0; i <= N - M; i++) {
            int j;
 
            /* For current index i, check for pattern
            match */
            for (j = 0; j < M; j++)
                if (txt[i + j] != pat[j])
                    break;
 
            // if pat[0...M-1] = txt[i, i+1, ...i+M-1]
            if (j == M)
                Console.WriteLine("Pattern found at index " + i);
        }
    }
 
    // Driver code
    public static void Main()
    {
        String txt = "AABAACAADAABAAABAA";
        String pat = "AABA";
        search(txt, pat);
    }
}
// This code is Contributed by Sam007

Javascript

    // Javascript program for Naive Pattern Searching
     
    function search(txt, pat)
    {
        let M = pat.length;
        let N = txt.length;
   
        /* A loop to slide pat one by one */
        for (let i = 0; i <= N - M; i++) {
            let j;
   
            /* For current index i, check for pattern
            match */
            for (j = 0; j < M; j++)
                if (txt[i + j] != pat[j])
                    break;
   
            // if pat[0...M-1] = txt[i, i+1, ...i+M-1]
            if (j == M)
                document.write("Pattern found at index " + i + "</br>");
        }
    }
     
    let txt = "AABAACAADAABAAABAA";
    let pat = "AABA";
    search(txt, pat);
                                

输出：

Pattern found at index 0 
Pattern found at index 9 
Pattern found at index 13 

最好的情况是什么?
最好的情况是当模式的第一个字符根本不在文本中出现时。

C

txt[] = "AABCCAADDEE";
pat[] = "FAA";

在最佳情况下，比较次数为O(n)。
最坏的情况是什么?
朴素模式搜索的最坏情况发生在以下情况下。
1)当文字和图案的所有字符都相同时。

C

txt[] = "AAAAAAAAAAAAAAAAAA";
pat[] = "AAAAA";

2)当仅最后一个字符不同时，也会发生最坏情况。

C

txt[] = "AAAAAAAAAAAAAAAAAB";
pat[] = "AAAAB";

最坏情况下的比较次数为O(m *(n-m + 1))。尽管具有重复字符的字符串不太可能出现在英文文本中，但是它们很可能出现在其他应用程序中(例如，在二进制文本中)。 KMP匹配算法将最坏情况改善为O(n)。我们将在下一篇文章中介绍KMP。另外，我们将写更多的文章来介绍所有模式搜索算法和数据结构。