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Efficient String Matching Techniques: Haystack Needle Algorithms

Learn about the basic algorithm for finding a needle in a haystack and the advanced Boyer-Moore algorithm for efficient string searching. Explore space/time tradeoffs and best practices for maximizing performance in string matching applications.

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Efficient String Matching Techniques: Haystack Needle Algorithms

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  1. String Matching By Joshua Yudaken

  2. Terms • Haystack • A string in which to search • Needle • The string being searched for • find the needle in the haystack

  3. Basic Algorithm • For each letter in the haystack • Check if the needle is present for (int k = 0; k < (strlen(haystack) - strlen(needle) + 1); k++) if (memcmp(haystack,needle,strlen(needle)) == 0) return true; return false;

  4. Boyer-Moore Algorithm Haystack = “jim saw it at a barbershop” Needle = “barber” JIM SAW IT AT THE BARBERSHOP | BARBER

  5. Boyer-Moore Algorithm Haystack = “jim saw it at a barbershop” Needle = “barber” JIM SAW IT AT THE BARBERSHOP | | BARBER | BARBER

  6. Boyer-Moore Algorithm Haystack = “jim saw it at a barbershop” Needle = “barber” JIM SAW IT AT THE BARBERSHOP | | | BARBER | | BARBER | BARBER

  7. Boyer-Moore Algorithm Haystack = “jim saw it at a barbershop” Needle = “barber” JIM SAW IT AT THE BARBERSHOP | | | | BARBER | | | BARBER | | BARBER | BARBER

  8. Boyer-Moore Algorithm Haystack = “jim saw it at a barbershop” Needle = “barber” JIM SAW IT AT THE BARBERSHOP | | | | | BARBER | | | | BARBER | | | BARBER | | BARBER | BARBER

  9. Boyer-Moore Algorithm Haystack = “jim saw it at a barbershop” Needle = “barber” JIM SAW IT AT THE BARBERSHOP | | | | | BARBER | | | | BARBER | | | BARBER | | BARBER | BARBER

  10. Space/Time Tradeoff • Create a ‘shift table’ for the search For “barber”: [a: 4, b: 2, c: 6, d: 6, e: 1,…,r: 3,…, z: 6, ‘ ‘: 6] • Set every element in the shift table to the length of the needle • Go through each letter (from the second last to the first) of the needle • If the distance from the letter to the end of the needle, is less than the letters current value in the shift table • update the shift table with the distance to the end of the needle 3 1 2 3 4 2

  11. Best when • The “alphabet” used in the haystack is much larger than that of the needle. • The haystack is long • The same needle is to be used in many different searches • In other cases, use strstr()! • Or a different available function.

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