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Showing posts from January, 2012

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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A Code Analyzer for a Subset of Java Language (J-) Using ANTLR - PART 2

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As we know, semantic analysis of a compiler includes checking the types of expressions. To do that, we have to maintain a Symbol Table which maps variable names to their types and locations. There maybe many variables with the same name which are in different scopes. This is legal in most languages and we have to handle this situation. We have to know when following scopes are entered in: - class scope - method scope - no inner block scopes (my language does not support this currently) We will do type-checking in two phases. First we'll populate the Symbol Table and it will contain the symbols with their scopes. Then, we'll check the types of symbols that are subject to addition operation by querying the symbol table. ANTLR allows us to inject code in different places of the generated parser. We need to know when a class/method scope is entered and the end of any scope is reached. Below is some grammar code from the project : classDecl @init {visitor.beforeClass(); System.out.p
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A Code Analyzer for a Subset of Java Language (J-) Using ANTLR - PART 1

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Our goal is to write a code analyzer for a language that is called "J minus". Source code of the whole project can be found here: https://code.google.com/p/j-minus/ Lets start with the language definition: Lexical Rules: Identifiers: Sequence of letters. Integer literals: A sequence of decimal digits. Binary operators: && < + - * Grammar Rules: Program -> MainClass ClassDecl* MainClass -> class id { public static void main (String [] id) { Statement* }} ClassDecl -> class id { VarDecl* MethodDecl* } VarDecl -> Type id ; MethodDecl -> public Type id ( ( formalParameter (, formalParameter)* )? ) { varDecl* statement* return exp; } FormalParameter -> Type id Statement -> id = Exp; Exp -> AdditionExp | SimpleExp SimpleExp -> id | int AdditionExp -> SimpleExp + SimpleExp Type: 'int' | 'boolean' ; We must pass following steps to reach our goal: 1) Implement the lexer and parser for the language. (use ANTLR) 2) Bu
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"Parsing" Konusu ile ilgili Temel Kavramlar

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--Halen yazım aşamasında-- Regular expressions yardımıyla bir programın "lexical" yapısını ifade edebildiğimizi görmüştük. Aynı şeyi dildeki cümleler için yapmak mümkün mü? Dildeki cümlelerin yapısını ifade etmek için "context free grammar" kavramı kullanılmakta. Dildeki karmaşık yapıları oluşturmak için "production" yaparak ilerlemek gerekiyor. Aşağıda 5 tane production ile ifade edilen bir dili inceleyelim: S -> S ; S S -> ID = E E -> E + E E -> ID E -> NUM Bu kuralları kullanarak bir cümle üretmeye çalışalım: S ; S ID = E ; ID = E ID = E + E; ID = E ID = ID + NUM; ID = NUM Burada ürettiğimiz cümlenin kaynak kodu (lexer ele almadan önce) şöyle olabilirdi: var = i + 4; j = 5 Burada tokenlerin değerleri: var, i, j, 4, 5 Token türleri: ID, NUM, "+", ";", "=" Gramer ifadesinde sağ tarafta bulunamayan sonlanmış semboller dikkati çekiyor. Bunlara "terminal symbol" deniliyor. Terminal semboller, token tür
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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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Virtual Memory

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The main memory can act as a “cache” for the secondary storage, usually implemented with magnetic disks. This technique is called virtual memory. There were two major motivations for virtual memory: to allow efficient and safe sharing of memory among multiple programs, and to remove the programming burdens of a small, limited amount of main memory. Lets consider a collection of programs running all at once on a computer. Of course, to allow multiple programs to share the same memory, we must be able to protect the programs from each other, ensuring that a program can only read and write the portions of main memory that have been assigned to it. Main memory need contain only the active portions of the many programs, just as a cache contains only the active portion of one program. Virtual memory allows us to efficiently share the processor as well as the main memory. We cannot know which programs will share the memory with other programs when we compile them. In fact, the programs s
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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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