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Showing posts from October, 2018

Kotlin Language Features Related to Null Handling

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Any software engineer with a Java background would find the null handling features in the Kotlin language interesting. Let's summarize this topic with some examples. Nullable types: In Kotlin, types are non-nullable by default. If you want a variable to be able to hold a null value, you need to explicitly declare its type as nullable using the Type? syntax. For example, String? denotes a nullable string, while String represents a non-nullable string. Safe calls (?.): Kotlin introduces the safe call operator (?.) for handling nullable types. It allows you to safely invoke a method or access a property on a nullable object. If the object is null, the expression returns null instead of throwing a NullPointerException. Example: data class Person(val name: String, val age: Int, val address: String?) fun main() {     // Create a person with a nullable address     val person1 = Person("John Doe", 25, "123 Main Street")     val person2 = Person("Jane Doe", 30,
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Dynamic Connectivity Problem and the Union-Find Algorithm

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Lets say we need to connect some objects and query if some objects are connected or not. Below is an example test: @Test     public void testConnectivity() {         MyConnectionDb connectDb = new MyConnectionDb();         connectDb.connect(4, 3);         connectDb.connect(3, 8);         connectDb.connect(6, 5);         connectDb.connect(9, 4);         connectDb.connect(2, 1);         assertTrue(connectDb.isConnected(8, 9));         assertFalse(connectDb.isConnected(5, 4));         connectDb.connect(5, 0);         connectDb.connect(7, 2);         connectDb.connect(6, 1);         connectDb.connect(7, 3);         assertTrue(connectDb.isConnected(5, 3));     } So when some numbers are connected, they form a set and we enlarge that set by applying union operations. And of course we query if given two numbers are connected or not.  I started with a solution using Java collections like HashSet. It is good to get a feel of the problem and c
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DEVOPS

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This table shows imporant aspects of DEVOPS and related software products. The ones that I have used in the past are painted in red. I think this is a very nice table to look at alternative solutions to use for new projects. Of course some of them are essential: Version control. (Subversion,  Git, ..) Build tool. (Maven, Gradle, ..) Testing. Unit Test, Stress Test, UI TEst. (JUnit, JMeter, Selenium) Continuous Integration. (Bamboo, Continuum, ..) The others seem to depend on the size of the organization and the project. Some use AWS, others manage their own servers. 
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Do you speak MATLAB?

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MATLAB is a programming platform designed specifically for engineers and scientists. I first used it for a masters degree course on image processing. And now, I'm looking at it for the purpose of machine learning. Now lets remember what we can do with it: Matlab has a command line and also you can save your scripts as .m files. You should define your functions and save them as .m files. Note: If you put a semicolon to the end of the expression, the output is not shown in the console. Lets create a 3 x 3 matrix: A = [1, 2, 3; 0, 0, 0; -1, -1, -1] Lets create a vector: V = [2, 3, 6, 7] Lets create a column vector: V = [2; 3; 4; 5] Lets draw a diagram with some values: X = [1,2,3,4,5]; Y= [0,-1,3,4,8]; plot(X,Y) How to define a function? function y = myFunction(x)     y = x^2 + x + 1; end From command line: result = myFunction(6); Lets create a uniformly spaced vector.  V1 = 1:10 V2 = 1:10 plot(V1, V2) We can also specify the interval:
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Trie Data Structure and Finding Patterns in a Collection of Words

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 I faced a very hard problem recently about finding substrings based on a collection of patterns.  Example For  words = ["Apple", "Melon", "Orange", "Watermelon"]  and  parts = ["a", "mel", "lon", "el", "An"] , the output should be  findSubstrings(words, parts) = ["Apple", "Me[lon]", "Or[a]nge", "Water[mel]on"] . While  "Watermelon"  contains three substrings from the  parts  array,  "a" ,  "mel" , and  "lon" ,  "mel"  is the longest substring that appears first in the string. Note: Number of words and number of parts can be huge! A brute force method or careless usage of Java String methods would make it impossible to handle a large number of words and parts (patterns). Solution Approach Whenever we want to search patterns inside a string, we should think about the Trie data structure:  https://en.wikipedia.
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swapLexOrder: Finding lexicographically largest string

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The following coding question is really amazing: Given a string  str  and array of  pairs  that indicates which indices in the string can be swapped, return the  lexicographically largest  string that results from doing the allowed swaps. You can swap indices any number of times. Example For  str = "abdc"  and  pairs = [[1, 4], [3, 4]] , the output should be swapLexOrder(str, pairs) = "dbca" . By swapping the given indices, you get the strings:  "cbda" ,  "cbad" ,  "dbac" ,  "dbca" . The lexicographically largest string in this list is  "dbca" . Input/Output [execution time limit] 3 seconds (java) [input] string str A string consisting only of lowercase English letters. Guaranteed constraints: 1 ≤ str.length ≤ 10 4 . [input] array.array.integer pairs An array containing pairs of indices that can be swapped in  str  (1-based). This means that for each  pairs[i] , you can swap elements in  str  that have the indices  p
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A Graph Application in Java: Using WordNet to Find Outcast Words

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I'll explain our intent with an example: Let's say we have the following words: horse, zebra, cat, bear, table Which one is the outcast? The table.. But how can a computer system find this? We are talking about the whole English language and any word can be in this list. The number of words in the list can be huge, so the solution should use appropriate data structures and algorithms.  We'll use WordNet which is a graph of English words. It contains the semantic relationships between words and our concern is the "is a" relationship. For our example, we know that horse and cat are both animals. "Table" is not an animal and it is the outcast in this situation. But we even have the ability to tell that horse and zebra is very close. Below is a unit test that shows a high level view of our application in Java: public class OutcastTest {     private WordNet wordNet;     private Outcast outcast;     @Before  
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