help with string method

[PvtBillPilgrim]

Hi. I just started Java and need some help with a method involving strings.

I need to return the index of the start of the first occurence of one string (parameter two) in another string (parameter one).

I tried this:

public static int findInString (String text1, String text2)
{
int length1 = text1.length();
int length2 = text2.length();
int index;
char x = text2.charAt(0);
for (index = 0; index < length1; index++)
{
char y = text1.charAt(index);
if (x == y)
{
text 1 = text1.substring(index, index + length2);
if (text1 == text2)
{
return index;
}
}
if (index == length1)
{
return -1;
}
}
 

NOTE: I cannot use the indexOf method. It's for a programming class at a local university and he won't let us use it. So I need to find an alternative way to basically do what it does.

My code compiles; it's just wrong semantically. Any help would be appreciated.

[RedSon]

Try using .equals instead of ==

[PvtBillPilgrim]

Can you just tell me if this makes any sense?

I think it's fairly intelligent in just thinking about it. But I'm not sure if it's doing exactly what I want it to do.

[r035198x]

Can you just tell me if this makes any sense?

I think it's fairly intelligent in just thinking about it. But I'm not sure if it's doing exactly what I want it to do.

Use the .equals method for camparing Strings as has been suggested then test your code and you'll see whether it's correct or not.

[JosAH]

NOTE: I cannot use the indexOf method. It's for a programming class at a local university and he won't let us use it. So I need to find an alternative way to basically do what it does.

My code compiles; it's just wrong semantically. Any help would be appreciated.

If you're planning to use the substring() method why not let that method do all
the work? Suppose you have a String S of length n and a pattern P of length m.

If m > n you're sure you can't find the pattern in String S. otherwise, for all
positions i in [0, n-m] take the substring of length m starting at position i and
compare that substring with your pattern P.

In other words, there's no need to search for that first character in S and P to
compare that for equality.

kind regards,

Jos

[PvtBillPilgrim]

OK. I'm trying this with no luck (but it looks fine to me).

I have the easy cases in my program; I'm just not going to include them here:

String mutation;
int index = 0;
while (index < (lengthStr - lengthSubstring))
{
mutation = str.substring(index, index + lengthSubstring);
if (mutation.equals(substring))
{
position = index;
break;
}
index++;
}
position = -1;
}
return position;
}
 

I get -1 everytime except for when the strings are equal (since the cases not included here work fine).

What am I doing wrong? It compiles, just doesn't work.

[JosAH]

OK. I get -1 everytime except for when the strings are equal (since the cases not included here work fine).

What am I doing wrong? It compiles, just doesn't work.

Even if you set your 'position' variable to the correct value in your while loop
you destroy it again just after the loop again, i.e. you explicitly set it to -1.

Do something like this instead;

int subPosition(String s, String p) {
 
   int sn= s.length();
   int pn= p.length();
 
   for (int i= 0; i <= sn-pn; i++) {
      if (s.substring(i, i+pn).equals(p))
         return i;
      }
   return -1;
}

[PvtBillPilgrim]

OK. Sorry about this.
Everything works now, except for when the substring does not exist within the string and the string has more characters than the substring (i.e. the following loop).

.............. (I again omitted the if and else if at the beginning because I'm sure they work)
else
{
for (int i = 0; i < lengthStr; i++)
{
String mutation;
mutation = str.substring (i, i + lengthSubstring);
if (mutation.equals(substring))
{
position = i;
return position;
}
}
position = -1;
return position;
}
}

Again it compiles. But if I try something like "hamburger" for the larger string and "dog" for the substring, it gives me like ten red lines telling me my string index is out of range (10 with this instance).

Is there an easy way to solve this without jeopardizing the code that does work?
Thanks again (I'm just frustrated at this point.)

[JosAH]

OK. Sorry about this.<snip>
Again it compiles. But if I try something like "hamburger" for the larger string and "dog" for the substring, it gives me like ten red lines telling me my string index is out of range (10 with this instance).

Is there an easy way to solve this without jeopardizing the code that does work?
Thanks again (I'm just frustrated at this point.)

No need to apologize; you're working on your problem and there's nothing wrong
with that. Think of it, suppose you have a String of five characters "abcde" and
suppose you have a pattern of two characers: "de". What would the maximum
value of i be from where you start your substring? Remember positions start at
zero. In order to rip out a substring of two characters you can rip out your
substring starting at position 5-2 == 3 at most. Further than that position you
can't build a substring of two characters.

This basically translates to: your for loop shouldn't loop over all possible values
less than the String's length. It should take the length of the pattern in account
too; reread my code snippet in my previous reply.

kind regards,

Jos