Recursion string in JAVA

Question

I'm writing with JAVA, and I wrote a recursive boolean function that gets 2 strings and is supposed to return if the first one is prefix of the second one. My code:

public static boolean prefix(String s, String t)
{
    int i = s.length()-1;
    if (s.charAt(i) == t.charAt(i) && s.length() >= 0)
    {
        return true;
    }
    return false;
}

If I'm writing in the main for example:

s = "Del";
f = "Delight";

The function is working as well.

But if I'm writing s = "Dellll", f = "Dell", it says "Out of bounds". Why?

The second thing is that it's not working with big and small characters. For example:

s ="Dell"
f ="dell"

It will return true for the above.

Thank you.

This does not look recursive? Also you use s.length()-1 to get the last character of s and t however t may be shorter, in which case it will throw IndexOutOfBoundsException — Mark
– Mark, Commented Dec 20, 2018 at 15:45
If you want to check a prefix, why not just use startsWith method? — OneCricketeer
– OneCricketeer, Commented Dec 20, 2018 at 15:47

Oleg Cherednik · Accepted Answer · 2018-12-20 16:27:00Z

3

What about using out-of-box solution?

public static boolean prefix(String str, String prefix) {
    return str.startsWith(prefix);
}

In case you really need recursion:

public static boolean prefix(String str, String prefix) {
    if (str == null || str.isEmpty())
        return prefix == null || prefix.isEmpty();
    if (prefix == null || prefix.isEmpty())
        return true;
    if (str.charAt(0) != prefix.charAt(0))
        return false;
    // remove first character of each string and go to next iteration
    return prefix(str.substring(1), prefix.substring(1));
}

edited Dec 20, 2018 at 16:27

answered Dec 20, 2018 at 15:46

Oleg Cherednik

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3 Comments

liem M Over a year ago

Thanks! I used the 2nd code. Its not working as well.

Oleg Cherednik Over a year ago

@liemM which test case is not working for my solution?

liem M Over a year ago

If srt = "sssss", prefix = "f" it will work, one problem solved. But if str = "Dell", prefix = "Dell" -> It will return false

GBlodgett · Accepted Answer · 2018-12-20 15:46:36Z

2

This is because you declare i as:

int i = s.length()-1;

And then call the ith char in both Strings

if (s.charAt(i) == t.charAt(i) && s.length() >= 0)

But if the first String is bigger than the second then i will be out of bounds. An easier way to do this would be to use the built in method startsWith, which:

Tests if this string starts with the specified prefix.

So your method could be as simple as:

return t.startsWith(s);

Or if you can't use built in methods, then you'll need to check if the first String is larger that the second:

if (s.charAt(i) == t.charAt(i) && s.length() >= 0 && s.length() < t.length())

answered Dec 20, 2018 at 15:46

GBlodgett

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Comments

Gixuna · Accepted Answer · 2018-12-20 15:46:38Z

0

Out of bounds means that, being the second string shorter than the first one, you are trying to access a character that doesn't exist. Just check the two lengths at the beginning of the function, and switch the parameters internally if needed.

answered Dec 20, 2018 at 15:46

Gixuna

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