Monday, May 29, 2017

Stack Overflow - 150,000 rep reached!

I've been a bit quiet on Stack Overflow lately, but just managed to reach 150,000 reputation!

For me, Stack Overflow has not simply been a quest for reputation, but more about learning and helping fellow programmers in need.

Sunday, May 21, 2017

Shell Scripting: <, << and <<<

This post shows the difference between the three standard input redirection operators: <, << and <<<.

1. <

< allows you to pass data contained in a file to the standard input of a command. The format is:

command < file


# replace comma with new-lines:
tr ',' '\n' < file

# read a file line-by-line and do something with each line:
while IFS= read -r line; do
  # do something with each line
  ssh "$line" who -b
done < hosts.txt

2. <<

<< allows you to pass multi-line text (called a here-document) to a command until a specific "token" word is reached. The format is:

command << TOKEN
line 1
line 2

For example:

# query a database
sqsh -S server -D db << END
select * from table
where foo='bar'

3. <<<

<<< allows you to pass a string of text (called a here-string) to a command. If you find yourself echo-ing text and piping it to a command, you should use a here-string instead. (See my previous post on Useless Use of Echo.) The format is:

command <<< text


tr ',' '\n' <<< "foo,bar,baz"

grep "foo" <<< "foo,bar,baz"

mailx -s subject $USER <<< "email body"

Saturday, March 11, 2017

Java 8: Tree Traversal Using Streams

It's quite common to traverse a tree using recursion, but with Java 8 it is now possible to lazily traverse a tree using streams.

The class below represents a tree. The stream() method streams the nodes contained in the tree. You can then do all the cool things that you can do with other streams, such as filtering, mapping and collecting!

public class TreeNode<E> {

  private final E data;
  private final List<TreeNode<E>> children;

   * Creates the tree node with the specified data and no children.
   * @param data
  public TreeNode(final E data) { = data;
    this.children = new ArrayList<>();

   * @return the data contained in this node
  public E getData() {
    return data;

   * Adds a child to this tree node.
   * @param data the data to add
  public TreeNode<E> addChild(final E data) {
    final TreeNode<E> toAdd = new TreeNode<>(data);
    return toAdd;

   * @return a stream of nodes in this tree in depth first order
  public Stream<TreeNode<E>> stream() {
    return Stream.concat(Stream.of(this),;

Example usage:

TreeNode<String> root = new TreeNode<>("Root");
TreeNode<String> a = root.addChild("A");
TreeNode<String> c = root.addChild("C");

int count =; // 7

String tree =","));
// Root,A,A1,A2,B,C,C1

Saturday, January 28, 2017

Power Set Algorithm - Iterative

The power set of a set S is the set of all subsets of S including the empty set and S itself. For example, the power set of {1, 2, 3} is {{}, {1}, {2}, {3}, {1, 2}, {1, 3}, {2, 3}, {1, 2, 3}}.

Previously, I wrote about how you can generate the power set using a recursive algorithm. This time I'll show you how to use iteration.

Recall, that when we are generating a set for the power set, each element can either be in the set or out of the set. In other words, each set can be represented in binary form, where 1 means that the element is in the set and 0 means that it is not. For example, given a set {a, b, c}, the binary string 101 would represent {a, c}.

Generating the power set just comes down to generating all numbers from 0 to 2^n (since there are 2^n possible subsets) and converting the binary representation of the number into the set!

Here is the algorithm:

public static <E> List<List<E>> powerSet(final List<E> list) {
  final List<List<E>> result = new ArrayList<>();
  final int numSubSets = 1 << list.size(); // 2^n
  for (int i = 0; i < numSubSets; i++) {
    final List<E> subSet = new ArrayList<>();
    int index = 0;
    for (int j = i; j > 0; j >>= 1) { // keep shifting right
      if ((j & 1) == 1) { // check last bit
  return result;

Sunday, January 22, 2017

Java 8: Top N elements from a stream

This post shows how you can sort a stream in reverse order and then select the top N elements.

This is quite common in financial systems.

For example, let's say that you have a list of currency exchange rate movements and you want to see the top 5 largest movements. You can do this using Java 8 Streams as shown below:

import static java.util.Comparator.*;
import static*;

// list of currency exchange rate percentage moves
final List<Currency> currencies = Arrays.asList(
    new Currency("EUR/USD", 0.37),
    new Currency("USD/JPY", -0.21),
    new Currency("GBP/USD", 0.27),
    new Currency("AUD/USD", -0.08),
    new Currency("USD/CAD", 0.02),
    new Currency("USD/CHF", -0.46),
    new Currency("EUR/JPY", 0.16),
    new Currency("EUR/GBP", 0.13),
    new Currency("USD/HKD", 0.0),
    new Currency("EUR/CHF", 0.05),
    new Currency("USD/KRW", -0.71)
  .sorted(comparing(Currency::getMove, comparing(Math::abs)).reversed())

The result is:

Currency [ccy=USD/KRW, move=-0.71]
Currency [ccy=USD/CHF, move=-0.46]
Currency [ccy=EUR/USD, move=0.37]
Currency [ccy=GBP/USD, move=0.27]
Currency [ccy=USD/JPY, move=-0.21]

The two argument Comparator.comparing method easily allows us to compare currency moves on absolute value.