hashing vs hash function, don't know the difference

Question

For example, "Consistent hashing" and "Perfect hash function", in wikipedia, I click "hashing" and the link direct to "hash function", so it seems that they have the same meaning, but why does another exist? And is there any difference when using "hashing" or "hash function"? And is it ok to call "consistent hashing" as "consistent hash function"? Thanks!

Answer 1

A hash function takes some input data (typically a bunch of binary bytes, but could be anything - whatever you make it to) and calculates a hash value, which is typically an integer number (but, again, can be anything). The process of doing this is called hashing.

The hash value is always the same size, no matter what the input looks like. Well, I suppose you cold make a hash function that has a variable-size output, but I haven't seen one in the wild yet. It wouldn't be very practical. Thus, by its very nature, hashing is usually a one-way calculation. You can't normally get the original data back from the hash value, because there are many more possible input data combinations than there are possible hash values.

The main advantages are:

• The hash value is always the same size
• The same input will always generate the same output.
• If it's a good hash function, different inputs will usually generate different outputs, but it's still possible that two different inputs generate the same output (this is called a hash collision).

If you have a cryptographical hash function you also get one more advantage:

• From having only the hash value, it's impossible (unfeasible) to come up with input data that would hash to this value. Never mind that it's not the original input data, any kind of input data that would hash to the given output value is impossible to find in a useful timeframe.

The results of a hash function can be used in various ways. As mentioned in other answers, hash tables are one common use-case. Verifying data integrity is another case - for example, you download a file, then hash it, then check the hash value against the value that was specified in the webpage where you downloaded the file from. If they don't match, the file was not downloaded correctly. If you combine hash values with public-key cryptography you can get digital signatures. And I'm sure there are other uses to which the principle can be put.

Answer 2

you can write a hash function and what it does is to hash keys to bins.

In other words the hash function is doing the hashing.

I hope that clarifies it.

Answer 3

HashTable is a data Structure in which a given value is mapped with a particular key for faster access of elements. - Process of populating this data structure is known as hashing.

To do hashing , you need a function which will provide logic for mapping values to keys. This function is hash function

I hope this clarifies your doubt.