user1136342 user1136342 - 5 days ago 6
Java Question

Explain the use of a bit vector for determining if all characters are unique

I am confused about how a bit vector would work to do this (not too familiar with bit vectors). Here is the code given. Could someone please walk me through this?

public static boolean isUniqueChars(String str) {
int checker = 0;
for (int i = 0; i < str.length(); ++i) {
int val = str.charAt(i) - 'a';
if ((checker & (1 << val)) > 0) return false;
checker |= (1 << val);
}
return true;
}


Particularly, what is the
checker
doing?

Answer

int checker is used here as a storage for bits. Every bit in integer value can be treated as a flag, so eventually int is an array of bits (flag). Each bit in your code states whether the character with bit's index was found in string or not. You could use bit vector for the same reason instead of int. There are two differences between them:

  • Size. int has fixed size, usually 4 bytes which means 8*4=32 bits (flags). Bit vector usually can be of different size or you should specify the size in constructor.

  • API. With bit vectors you will have easier to read code, probably something like this:

    vector.SetFlag(4, true); // set flag at index 4 as true

    for int you will have lower-level bit logic code:

    checker |= (1 << 5); // set flag at index 5 to true

Also probably int may be a little bit faster, because operations with bits are very low level and can be executed as-is by CPU. BitVector allows writing a little bit less cryptic code instead plus it can store more flags.

For future reference: bit vector is also known as bitSet or bitArray. Here are some links to this data structure for different languages/platforms:

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