KMP algorithm Learning

1. What is KMP algorithm?

The knuth-Morris-Pratt string search algorithm (often referred to as the "KMP algorithm") is used in a "main text string"SSearch for a "word" in"WTo avoid re-checking the previously matched characters. (Match the pattern string in the original string)

II,KMP demo

Http://staff.ustc.edu.cn /~ Ypb/jpk/ Flash/find_kmp.swf

Iii. KMP principles

KMP is the most common improved algorithm. It can effectively jump a few characters to the backend when the matching process is not matched, thus speeding up the matching.

In the KMP algorithm, there is an array called a prefix array, and some are called. Each pattern string has a fixed next array. Of course, it also describes the degree of symmetry of the substring, the higher the degree, the larger the value. Of course, the chance of re-matching may be greater before.

For an understanding of the next array, see http://blog.csdn.net/yearn520/article/details/6729426#t0

 

[CPP]View plaincopy

1. Void setprefix (const char * pattern, int prefix [])
2. {
3. Int I;
4. Int Len = strlen (pattern); // The length of the pattern string.
5. Prefix [0] = 0;
6. For (I = 1; I <Len; I ++)
7. {
8. Int K = prefix [I-1];
9. // Recursively determine whether sub-symmetry exists. If K is set to 0, sub-symmetry is no longer available. Pattern [I]! = Pattern [k] indicates that although symmetric, the value after symmetric is not equal to the current character value, so the recurrence is continued.
10. While (pattern [I]! = Pattern [k] & K! = 0) // when I is equal to 14, evaluate the value of prefix [14]
11. K = prefix [k-1]; // continue Recursion
12. If (pattern [I] = pattern [k]) // finds the sub-symmetry, or directly inherits the symmetry above, both of which are based on ++
13. Prefix [I] = k + 1;
14. Else
15. Prefix [I] = 0; // if all the sub-symmetry is traversed, this new character is not symmetric.
16. }
17. Prefix [0] =-1;
18. }

Iv. test example [CPP]View plaincopy

1. # Include <iostream>
2. # Include <cstring>
3. # Include <string>
4. Using namespace STD;
6. String SORG;
7. String spat;
8. Int prefix [10000];
9. Int result [20];
11. Void Init ()
12. {
13. SORG = "";
14. Spat = "";
15. Memset (prefix, 0, sizeof (INT) * 10000 );
16. // Memset (result, 0, sizeof (INT) * 20 );
17. }
18. Void setprefix (string temp, int next [])
19. {
20. Int Len = temp. Size ();
21. Next [0] = 0;
23. For (INT I = 1; I <Len; I ++)
24. {
25. Int K = next [A I-1];
27. While (temp [k]! = Temp [I] & K! = 0)
28. K = next [k-1];
30. If (temp [k] = temp [I])
31. Next [I] = k + 1;
32. Else
33. Next [I] = 0;
34. }
36. Next [0] =-1;
38. For (INT I = 0; I <Len; I ++)
39. If (next [I]> = 1)
40. Next [I] = next [I]-1;
41. }
43. Int KMP (string S1, string S2)
44. {
45. Int number = 0;
46. Int I = 0;
47. Int J = 0;
48. While (I <(INT) s1.size () & J <(INT) s2.size ())
49. {
50. If (j =-1 | S1 [I] = S2 [J])
51. {
52. I ++;
53. J ++;
54. }
55. Else
56. J = prefix [J];
58. If (j = s2.size ())
59. {
60. I = I-j + 1;
61. J = 0;
62. Number ++;
63. }
64. }
66. Return number;
67. }
68. Int main ()
69. {
70. Int T;
71. Cin> T;
72. Memset (result, 0, sizeof (INT) * 20 );
73. For (INT I = 0; I <t; I ++)
74. {
75. Init ();
76. // Cout <"input spat string:" <Endl;
77. Cin> spat;
79. // Cout <"input SORG string:" <Endl;
80. Cin> SORG;
82. Setprefix (spat, prefix );
83. Result [I] = KMP (SORG, spat );
84. }
86. For (Int J = 0; j <t; j ++)
87. Cout <result [J] <Endl;
90. Return 0;
91. }

KMP algorithm Learning