In a previous post we learned about searching algorithms. We also learned that sorting is equally important; that sorting and searching go hand in hand. Sorting is deeply embedded in everything we do. Lets define sorting even though we all know what it is. Sorting is simply rearranging elements in a collection in increasing or decreasing order based on a property. That property could be a datatype like string or number. We could even sort based on some complex datatype. So if you have a class with many different properties, we could sort based on those properties. In the real word, we sort cards when we play poker, when buying online we like to search in order of price (sorted from lowest to highest or vice versa), or perhaps even sorting by product number reviews (my favorite type of sort when shopping on amazon). We sort emails, phone contacts, travel dates (you get the picture). Read More
Previously in other posts (on data structures) we learned about Queues. Particularly speaking, we discussed what priority queues were. What we didn’t know is that the underlying foundation of priority queue is the heap data structure (which we will be learning about in this post). Remember that a priority queue is a data structure that allows you to access an item in a collection with the highest priority. Accessing such an item can be done in so many different ways such as using arrays or lists (binary trees being one of them). However the best way to implement priority queues is to use the heap. When accessing, inserting or deleting into such data structures, the heap is the most performant (faster than arrays/lists/binary trees). What we really didn’t talk about in that post (on queues) was runtime complexity. We implemented methods such as enqueue, dequeue, etc. but we never considered how performant they were with basic operations such as adding and removing from the queue. Lets look at a diagram. Read More
Searching is fundamental to any programming language. It would be a waste to have data structures, and no way of searching through them. Searching is fundamental in in life (we even search for the meaning of our very own existence), so it only makes sense that after we create collections of data, that we want to search through them. By searching we are able to find answers to problems we are trying to solve. Take a phone book for example, or a telephone directory. Most of these come with tools that help make search relatively simple. For example, there are guides within the book that show you which letter you are currently on (making it easy to jump quickly to a section). If you wanted to call “Konstantin Addaquay” for example, you would just go the A section, which gives you listings of all people in the area with a last name starting with the letter A. Now imagine that telephone directories didn’t come already sorted alphabetically. It would be almost impossible trying to find someone in there (so many entries). Searching and sorting go hand in hand in computer science. Most of the time, the most efficient way to search is to have data that is already sorted. That is why this post and the next one will be covering searching and sorting algorithms. Read More
Graphs are mind blowing! The first time i studied graphs and its applications, i was simply in awe. The first thing one might think when they hear graphs is to think of some kind of chart or diagram. But that is not the case. They are actually derived from the study of networks and are an extremely hot topic these days. We have all used graph applications without even realizing it. Social media and social networks (Facebook, Twitter, Instagram, etc), the internet, etc. are applications of Graphs, and are used to model many real world systems, like airlines, road traffic, communication, etc. Graphs also fall under the non-linear data structures, but take things to a whole new level. You can even think of linked lists and the binary tree data structure as a form of graph (so we have been using graph applications without even realizing it). Linked lists have node connections from one node directly to another node, and binary trees have one node which could potentially connect to 1 or a maximum of 2 nodes. With graphs any node could potentially link to another node within the collection. So it is not limited as linked lists or Binary Trees. Lets look at our first graph. Read More
A tree data structure is very similar to a linked list. Where linked lists linear (meaning we have each node in the collection pointing to a next node in a fixed sequence), a tree data structure is non-linear. This means we can have each node potentially pointing to one or more nodes. Think of a trees as a way to order things hierarchically. Example are, graphical representations of data (like an organizational charts), files in a file system or even sorted collections of data. When working with trees, order of the elements is not important (if it is, you can store your collection in a linked list, stack, queue, etc). Lets take a look at an example of an organizational chart ( a type of tree structure showing hierarchical data ). Read More
In the last post we talked about Maps. We learned that Maps can be referred to as dictionaries, associative arrays, etc. Sometimes you will also hear Maps or Sets being referred to as a Hash ( You will hear the likes of HashMaps or HashSets ). These types of key/value pair collections, are “Hash-bashed Data Structures”. What is hashing? In my very first post on data structures, i mentioned that Linked Lists are faster at adding data quicker to a collection, than it is at retrieving. Hashing is a way of storing data that makes retrieval and access extremely fast (Runtime complexity of O(1) for both retrieval, access, and deletion operations). Our collection is stored in a data structure known as a hash-table. This table is what makes data retrieval and storage so fast. When we looked at Maps in the last post (or even linked lists), anytime we wanted to find data, we had to traverse the whole collection in order to find it. Given a key, we would enumerate the whole thing for its value. Therefore it would take a long time we had a huge collection. Hash tables can be used to solve this problem. When using hash tables we can find data in the shortest amount of time because we already know which position in the table the value is stored. Like an array, we can retrieve the value at position 4 instantly. But as we said in the previous post, writing a statement like state does not convey meaning (number index based structure, what if we don’t know its location?). When we tried to solve this problem with maps, we looked at how to write data structures so we could retrieve them using syntax that looked more like this state[‘NY’]. The only disadvantage here is we still would have to enumerate the entire object to get its value (though the syntax was nicer). Hashing employs a technique using something called a hash function. Given a key, it is passed to a function, and a value is returned right away (the key is hashed into a value). That value is used as an array index(in the hash table to retrieve values). The value of the index can be retrieved instantly because of its array nature. Read More
I recently moved my blog off Godaddy because i was having a whole bunch of issues when writing posts. I believe Godaddy sets a block on how many times you can publish. Perhaps their servers takes a hit from all the publishers writing content. If they are like me they save every 2 minutes! What happens is that after a few saves on your post, you begin to get server internal errors(504 Gateway Timeout Error). Go daddy claims it was my ISP but reading complaints on other forums i found out that many bloggers had this issue. So it wasn’t an ISP issue. I took action right away. I decided to host my own WordPress (and boy was it a nightmare). Found a VPS solution and installed WordPress from a backup i have made when i was using Godaddy. The only problem i had was my posts now had urls that were not so friendly. Updating permalinks to use the postname format wasn’t working. I would get a 404 error. I decided to document the steps i took to resolve this issue. Hope it works for you (worth a try if you have having similar issues). If you are using a VPS and want to install or reinstall WordPress, here are some steps that might help you. I will talk about reinstallation as well at the end of the post. I assume you are comfortable with the CLI (of course if you are using a VPS solution, that is the only way). Read More
As we journey down the lane of data structures, you may or may not have noticed a pattern for how we access data. Since the underlying data storage we have used have been arrays, we have had to rely heavily on using numeric type indexing to retrieve values. Even though in stacks and queues we were only interested items at the top or front, indirectly we were still using some sort of indexing to get those item values. Also when we did traversals in our data structure, it was based on indexing. While this has served us well, sometimes you want to put more meaning into how you retrieve values. Asking for items, doesn’t really give a clear definition of what is being asked for. In this and the next post, we will be talking about accessing data based on meaningful keys/names. Think of a real world dictionary for example. Every word in a dictionary has a meaning. You can think of a word as a key, and the meaning as a value. So in essence you are using one unique word to look up a meaning. A key mapped to a value ( key/value pairs). Notice that dictionaries do not have duplicate keys. If you need the meaning for a word, for e.g engineer, you will not find the key twice. You may however find multiple meanings under that key word. Lets think of another scenario where you might want to use a dictionary. What if you wanted a data structure where you could find a state name based on abbreviation? Like NY (key) for New York (value), or CA for California. We are not using an index here. We might access it like this instead state[“NY”]. We are using a unique identifier to refer to a particular state. This is when we use the dictionary data structure. Dictionaries are also referred to as maps, and may be the preferred word to use depending on the programming language. Dictionaries, associative arrays, maps, … they all mean the same thing! We will be referring to them as maps from now on.