Stacks and Queues

This post shows you how to implement a Stack and a Queue in Java. It might be handy in interviews ;)

Implementing a Stack:

A stack uses LIFO (last-in first-out) ordering. Think of a stack of dinner plates. It provides constant time adds and removes. Sample code:

public class Stack<E> {
  private static class StackItem<E> {
    final E data;
    StackItem<E> next;

    public StackItem(final E data) {
      this.data = data;
    }
  }

  private StackItem<E> top;

  public boolean isEmpty() {
    return top == null;
  }

  public void push(final E e) {
    final StackItem<E> toAdd = new StackItem<>(e);
    toAdd.next = top;
    top = toAdd;
  }

  public E pop() {
    if (isEmpty()) {
      throw new NoSuchElementException("Stack is empty");
    }
    final E data = top.data;
    top = top.next;
    return data;
  }

  public E peek() {
    if (isEmpty()) {
      throw new NoSuchElementException("Stack is empty");
    }
    return top.data;
  }
}

Implementing a Queue:

A queue uses FIFO (first-in first-out) ordering. Sample code:

public class Queue<E> {
  private static class QueueItem<E> {
    final E data;
    QueueItem<E> next;

    public QueueItem(final E data) {
      this.data = data;
    }
  }

  private QueueItem<E> first;
  private QueueItem<E> last;

  public boolean isEmpty() {
    return first == null;
  }

  public void add(final E e) {
    final QueueItem<E> toAdd = new QueueItem<>(e);
    if (last != null) {
      last.next = toAdd;
    }
    last = toAdd;
    if (first == null) {
      first = last;
    }
  }

  public E remove() {
    if (isEmpty()) {
      throw new NoSuchElementException("Queue is empty");
    }
    final E data = first.data;
    first = first.next;
    if (first == null) {
      last = null;
    }
    return data;
  }

  public E peek() {
    if (isEmpty()) {
      throw new NoSuchElementException("Queue is empty");
    }
    return first.data;
  }
}

Shell Scripting: Parsing options using getopt and getopts

This post shows how you can parse shell options using getopts and getopt.

Using getopts:

getopts is a bash built-in command. I find it a lot easier to use than getopt.

Here is an example of using getopts:

# options a and b are followed by a colon because they require arguments
while getopts "ha:b:" opt; do
  case "$opt" in
    h)
      echo "help"
     ;;
    a)
      option_a=$OPTARG
      ;;
    b)
      option_b=$OPTARG
      ;;
  esac
done
shift $((OPTIND-1))

echo "option_a: $option_a"
echo "option_b: $option_b"

# read positional parameters
echo "Param 1: $1"
echo "Param 2: $2"

Running it:

$ myscript.sh -a foo -b bar hello world
option_a: foo
option_b: bar
Param 1: hello
Param 2: world

Using getopt:

getopt supports long options and that's the only time I use it.

Here is an example of using getopt:

options=$(getopt -n "$0" -o ha:b: -l "help,alpha:,bravo:"  -- "$@")
(( $? != 0 )) && echo "Incorrect options provided" >&2 && exit 1
eval set -- "$options"

while true; do
  case "$1" in
    -h|--help)
        echo "help"
        ;;
    -a|--alpha)
        shift
        option_a="$1"
        ;;
    -b|--bravo)
        shift
        option_b="$1"
        ;;
    --)
        shift
        break
        ;;
  esac
  shift
done

echo "option_a: $option_a"
echo "option_b: $option_b"

# read positional parameters
echo "Param 1: $1"
echo "Param 2: $2"

Running it:

$ myscript.sh --alpha foo --bravo bar hello world
option_a: foo
option_b: bar
Param 1: hello
Param 2: world