Contents
When do you need a hash function for a string?
Comparing two strings is then an O ( 1) operation. For the conversion, we need a so-called hash function . The goal of it is to convert a string into an integer, the so-called hash of the string. The following condition has to hold: if two strings s and t are equal ( s = t ), then also their hashes have to be equal ( hash ( s) = hash ( t) ).
How to get a hash from a std string?
So, what are the easiest ways to get a hash from std::string using the C++ Standard Library? For a quick solution involving no external libraries, you can use hash to hash string s. It’s defined by including the header files hash_map or unordered_map (or some others too).
How to calculate the hashes of two substrings?
In most cases, rather than calculating the hashes of substring exactly, it is enough to compute the hash multiplied by some power of . Suppose we have two hashes of two substrings, one multiplied by and the other by . If then we multiply the first hash by , otherwise, we multiply the second hash by .
How to rotate the hash of a string?
One easy way to do that is to rotate the current result by some number of bits, then XOR the current hash code with the current byte. Repeat until you reach the end of the string. Note that you generally do not want the rotation to be an even multiple of the byte size either.
Which is an example of a hashing problem?
Here are some typical applications of Hashing: Calculating the number of palindromic substrings in a string. Problem: Given a string of length , consisting only of lowercase English letters, find the number of different substrings in this string. To solve this problem, we iterate over all substring lengths .
How is rolling hash used to calculate hash values?
Rolling hash is used to prevent rehashing the whole string while calculating hash values of the substrings of a given string. In rolling hash,the new hash value is rapidly calculated given only the old hash value.Using it, two strings can be compared in constant time.