CSC 301.01, Class 38: Basics of string matching

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Approximate substring matching
• Exact substring matching
• The brute-force approach
• Review: The hash-code approach

News / Etc.

• Warning! Our Chair is visiting class this week.
• Cut/Close/Balance period finishes on Friday at 5:00 p.m.
  • You may start Add/Drop on Monday the 4th.

Upcoming work

• Homework 9 due tonight.
  • Please use the folder attached to my bulletin board.
• Homework 10 due next Wednesday.
• Final is in-class on the morning of Wednesday, December 13.

Extra Credit (Academic/Artistic)

• CS Extras Thursday: Anya’s Advisor.

Extra credit (Peer)

• Swim meet this coming weekend.
• Chamber Ensembles Saturday at 4:00 p.m.
• One Acts this weekend (Saturday at 8, Sunday at 2).

Extra Credit (Misc)

• Mental Health Campus Resource Fair. Friday at 4pm in JRC 209.

Other good things

• Jazz Ensemble Concert Friday at 7:30 p.m.
• YGB Saturday at 2:00 p.m.
• Collegium Concert Sunday at 2:00 p.m.
• Festival of trees at Drake Friday afternoon

Questions

What should we do on the tourist problem if an edge includes an invalid city?

The preconditions of the problem have been violated.

For the tourist problem, can we print as we go or do we have to wait until we’ve read all the input?

Whatever is easier.

Should we try it online?

Sure. Send me the results of your tests. (Optional.)

Approximate substring matching: review

Reminder: We use dynamic programming to match strings.

          source
  C  "" s1 s2 s3 s4 ...
    +--+--+--+--+--+--
t ""|  |  |  |  |  | ...
a   +--+--+--+--+--+--
r t1|  |  |  |  |  | ...
g   +--+--+--+--+--+--
e t2|  |  |  |  |  | ...
t    .   .  .  .  .

Meaning: C[i,j] is the cost of converting substring(source,i) to substring(target,j).

The algorithm

C[0,0] = 0
C[i,0] = d + C[i-1,0] (for i>0)
C[0,j] = a + C[0,j-1] (for j>0)
C[i,j] = if (s[i] == t[j]) then
           min(d+C[i-1,j], C[i-1,j-1], a+C[i,j-1]) 
         else 
           min(d+C[i-1,j], r+C[i-1,j-1], a+C[i,j-1]) 

          source
  C  "" s1 s2 s3 s4 ...
    +--+--+--+--+--+--
t ""| 0| d|2d|3d|4d| ...
a   +--+--+--+--+--+--
r t1| a|  |  |  |  | ...
g   +--+--+--+--+--+--
e t2|2a|  |  |  |  | ...
t    .   .  .  .  .

Note that we could also make a, d, and r functions of the position and character.

Approximate substring matching: Additional issues

Variant one

How would you update the algorithm to accommodate *substring* matching rather than whole string matching? (E.g., to figure out the best place to align "habit" in "alphabetical"?)

A brute force strategy

• Build the table for source vs “”
• Build the table for source vs t[1]
• Build the table for source vs t[1..2]
• Build the table for source vs t[1..3]
• …
• Build the table for source vs t[1..m]
• Build the table for source vs t[2]
• Build the table for source vs t[2..3]
• Build the table for source vs t[2..4]
• …
• Build the table for source vs t[2..m]
• Build the table for source vs t[3]
• …

This should work, in that it looks at every possible solution, but it’s really inefficient.

A better solution

• Column 0 is all 0’s. Represents “we don’t care about leading characters”
• Minimize across the last column to find the cost of the best match.

Variant two

The whole string matching problem only gives us a cost. How do we update the algorithm so that each cell includes both cost and steps to achieve that cost? S[i,j] are the steps to achieve C[i,j].

C[0,0] = 0
S[0,0] = <>
C[i,0] = d + C[i-1,0] (for i>0)
S[i,0] = S[i-1,0] ++ <D.i>
C[0,j] = a + C[0,j-1] (for j>0)
S[0,j] = S[0, j-1] ++ <A.j>
C[i,j] = if (s[i] == t[j]) then
           min(d+C[i-1,j], C[i-1,j-1], a+C[i,j-1]) 
         else 
           min(d+C[i-1,j], r+C[i-1,j-1], a+C[i,j-1]) 
S[i,j] = if (C[i,j] == d+C[i-1,j])
           S[i-1,j] ++ <D.i>
         ...

Exact substring matching

Approximate matching is hard. Best algorithm is O(mn). Exact substring matching is easier.

• Find the first instance of source in target.
• Find all instances of source in target.
• Find any instance of source in target.

The brute-force approach

for (s = 0; s < length(target) - length(source), s++)
  {
    for (i = 0; i < length(source) && target[s+i] == source[i]; i++)
      ;
    if (i == length(source)
      return s;
  } // for s
return -1;      // Not found

What is the asymptotic upper bound on the algorithm and what is an input that reaches that upper bound?

Worst case O(mn).

• source: habit
• target: habihabihabihabihabihabihabihabihabihabihabihabihabihabihabihabihabihabihabihabihabihabihabihabit.

A worse one

• source: hhhhhhhhhhi
• target: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhi

Our question for next class: Can we do better than O(mn) and, if so, how?

Review: The hash-code approach

This doesn’t solve our problem in general, although it does in most cases.