timeNomad timeNomad - 5 months ago 69
Java Question

Recursive function to match a string against a wildcard pattern

So I've been trying to solve this assignment whole day, just can't get it.

The following function accepts 2 strings, the 2nd (not 1st) possibly containing

*
's (asterisks).

An
*
is a replacement for a string (empty, 1 char or more), it can appear appear (only in s2) once, twice, more or not at all, it cannot be adjacent to another
*
(
ab**c
), no need to check that.

public static boolean samePattern(String s1, String s2)


It returns true if strings are of the same pattern.

It must be recursive, not use any loops, static & global variables.
Can use local variables & method overloading.

Can use only these methods: charAt(i), substring(i), substring(i, j), length().

Examples:

1:
TheExamIsEasy
; 2:
The*xamIs*y
→ true

1:
TheExamIsEasy
; 2:
Th*mIsEasy*
→ true

1:
TheExamIsEasy
; 2:
*
→ true

1:
TheExamIsEasy
; 2:
TheExamIsEasy
→ true

1:
TheExamIsEasy
; 2:
The*IsHard
→ FALSE


I tried comparing the the chars one by one using charAt until an asterisk is encountered, then check if the asterisk is an empty one by comparing is successive char (i+1) with the char of s1 at position i, if true -- continue recursion with i+1 as counter for s2 & i as counter for s1; if false -- continue recursion with i+1 as counters for both.
Continue this until another asterisk is found or end of string.

I dunno, my brain loses track of things, can't concentrate, any pointers / hints? Am I in the right direction?

Also, it's been told that a backtracking technique is to be used to solve this.

My code so far (doesn't do the job, even theoretically):

public static boolean samePattern(String s1, String s2) {
if (s1.equals(s2) || s2 == "*") {
return true;
}
return samePattern(s1, s2, 1);
}
public static boolean samePattern(String s1, String s2, int i)
{
if (s1.equals(s2))
return true;
if (i == s2.length() - 1) // No *'s found -- not same pattern.
return false;

if (s1.substring(0, i).equals(s2.substring(0, i)))
samePattern(s1, s2, i+1);
else if (s2.charAt(i-1) == '*')
samePattern(s1.substring(0, i-1), s2.substring(0, i), 1); // new smaller strings.
else
samePattern(s1.substring(1), s2, i);
}

Answer

Here's some Python "psudocode" that may help

def samePattern(s1,s2):
    if s2=="*" or s1==s2: return True
    if s1=="": return False
    if s1[0]==s2[0]: return samePattern(s1[1:],s2[1:])
    if s2[0]=="*": return samePattern(s1,s2[1:]) or samePattern(s1[1:],s2)
    return False

Here is a rough guide for converting the code

s[0] = the first character
s[1:] = the string minus the first character
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