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Friday, January 14, 2011

IBM Interview Questions

Qu1: What is hashing and how it works???

Answer: Hashing means using some function or algorithm to map object data to some representative integer value. This so-called hash code (or simply hash) can then be used as a way to narrow down our search when looking for the item in the map.

How hashing works

Purely as an example to help us grasp the concept, let's suppose that we want to map a list of string keys to string values (for example, map a list of countries to their capital cities). So let's say we want to store the data in Table 1 in the map.
Table 1: Example data to put in a map
And let's suppose that our hash function is to simply take the length of the string. For simplicity, we'll have two arrays: one for our keys and one for the values. So to put an item in the hash table, we compute its hash code (in this case, simply count the number of characters), then put the key and value in the arrays at the corresponding index. For example, Cuba has a hash code (length) of 4. So we store Cuba in the 4th position in the keys array, and Havana in the 4th index of the values array etc. And we end up with the following:
(hash code = key length)
Keys arrayValues array
Now, in this specific example things work quite well. Our array needs to be big enough to accommodate the longest string, but in this case that's only 11 slots. And we do waste a bit of space because, for example, there's no 1-letter keys in our data, nor keys between 8 and 10 letters. But in this case, the waste isn't so bad either. And taking the length of a string is nice and fast, so so is the process of finding the value associated with a given key (certainly faster than doing up to five string comparisons)1.
We can also easily see that this method wouldn't work for storing arbitrary strings. If one of our string keys was a thousand characters in length but the rest were small, we'd waste the majority of the space in the arrays. More seriously, this model can't deal with collisions: that is, what to do when there is more than one key with the same hash code (in this case, one than more string of a given length). For example, if our keys were random words of English, taking the string length would be fairly useless. Granted, the word psuedoantidisestablishmentarianistically would probably get its own place in the array. But on the other hand, we'd be left with thousands of, say, 6-letter words all competing for the same slot in the array