Question

I need to write method that will chek “String str” on other string, and return the index that the str starts.

That’s sound like homework, and it is some of homework but for my use to learn for a test…

i’ve tried:

public int IndexOf (String str) {

   for (i= 0;i<_st.length();i++)
   {
        if (_st.charAt(i) == str.charAt(i)) {
            i++;
            if (_st.charAt(i) == str.charAt(i)) {
                return i;
            }
        }
   }
   return -1;
 }

but i dont get the right return. why? i’m on the right way or don’t even close?

Answer 1

I am afraid, you are not close.

Here’s what you have to do:

• Loop on the characters of the string (the one on which you are supposed to do an indexOf, I will call this the master) (you are going this right)
• For every character check whether your other string’s character and this character are the same.
• If they are (a potential start of the same sequence) check whether the next characters in the master match with your String to check (You might want to loop through the elements of the string and check one by one).
• If they don’t match, continue with the characters in the master string

Something like:

Loop master string
for every character (using index i, lets say)
   check whether this is same as first character of the other string
   if it is
      //potential match
      loop through the characters in the child string (lets say using index j)

      match them with the consecutive characters in the master string 
      (something like master[j+i] == sub[j])

       If everything match, 'i' is what you want
       otherwise, continue with the master, hoping you find a match

Some other points:

• In java, method names start with a
  lower case letter by convention
  (meaning, the compiler won’t
  complain, but your fellow programmers
  may). So IndexOf should actually be
  indexOf
• Having instance variables
  (class level variables) start with a
  _ (as in _st) is not a really good
  practice. If your professor insists,
  you may not have many options, but
  keep this in mind)