Shulou(Shulou.com)06/01 Report--

Naive pattern matching algorithm for Review string

Pattern matching:

Substring positioning operation to find out the location of the substring in the main string.

In string matching, the main string S is called target (string) and the substring T is called pattern (string). If the substring T can be found in the main string S, the matching is said to be successful, and the sequence number of the first character equal to the first character in the substring T is returned in the main string S. otherwise, if the matching fails, 0 is returned.

Algorithm idea:

Start from the pos character of the main string S and compare it with the first character of the mode T, if it is the same, then the two will compare each subsequent character in sequence, otherwise the character of the mode T will be compared again from the next character of the main string S. The reason why it is simple is that it is stupid, every time the factor string is compared with the main string, when it is found that the match is wrong, the pointer of the main string should be traced back to the next character at the character where the comparison began last time, to compare it again! Painstaking).

Detailed illustration

Given two strings, S and T, the length is known.

-"

The initial ab is the same, can be compared sequentially, when 3, do not match. Then j goes back to T1, I goes back to S2, the next character of S, and compares it with T1 from the beginning.

-"

B and a do not match, j goes back to 1 (the position remains the same), I goes back to the next character, that is, 3, continue to compare, match, compare sequentially. To the bottom.

The j of the pattern string goes back to 1Magi I to 4 again, the comparison continues, the mismatch continues, and the j of T continues to trace the I of 1BI S to the next character, and the comparison continues until iCombined 6, matching.

Continue to compare in sequence until the T ratio is over, that is, when j and I continue to + + after jackers 5 and I continue to + 10, it is necessary to judge that the comparison is over, as shown in the figure. This is the whole process. The important thing of the algorithm is the thought, understanding the thought, is the first step, there are clear ideas and perfect scene reproduction in the mind, then the code implementation is a natural thing.

Write it in code as follows:

1 int getLength (char * str) 2 {3 int I = 0; 4 5 while ('\ 0'! = str [I]) {6 return 10} 11 12 int strCompare (char * strMain, char * strSub, int index) 13 {14 int iMain = index;15 int jSub = 0ten 16 int lenMain = getLength (strMain); 17 int lenSub = getLength (strSub) 18 19 while ((iMain > = 0 & & iMain = 0 & & jSub lenSub-1) {30 return iMain-lenSub;// is obtained after matching ok, the position of the first character in the main string that matches the first character of the pattern string 31} else {32 return 0spur / match failure 33} 34} 35 36 int main (int argc, const char * argv []) {37 char * str1 = "sawtsafvda" 38 char * str2 = "safv"; 39 40 int I = strCompare (str1, str2, 0); 41 42 printf ("% d\ n", I); 43 44 return 0 return 45}

four

Program ended with exit code: 0

Analyze time complexity

At worst, the match is successful, for example, 0000000000001 and 00001. Each comparison does not match at the beginning of 00001. The pointer goes back to the beginning, and the main string also traces back to i-j+1. If the length of the pattern substring is m and the length of the target string is n, the worst case is that each comparison occurs at the end, that is, each comparison is compared for a maximum of m times and n-m+1 times, and the total number of comparisons is m (n-m+1). Therefore, the simple pattern matching algorithm is o (m match n), although the time complexity of simple pattern matching is relatively large, but in practice, in general (unless there are a lot of partial matches between the pattern string and the main string, because there are many times of comparison each time, the multiplication can not be approximated), the real execution time is approximate to o (nymph), so it still has his use today!

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