2. Storing data - Variables, Arrays, and Dictionaries.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Variables\n",
    "\n",
    "Variables are one way of storing data so that we can manipulate it and access it later. You can declare a variable in Julia by assigning a character of set of characters to a string or number (for example) with an equal sign."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "a = 3"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "b = 3.0"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "myname = \"Jane\""
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "myaliens = \"👽👽👽\""
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Notice that when we declared these variables, we didn't have to tell Julia what their types were. If we query Julia, however, we see that it has figured them out via type inference."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "typeof(a)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "typeof(b)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "typeof(myname)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "typeof(myaliens)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "While you don't have to give type information, note that you *can* give type information for *local* variables. One way to transition from working with *global* variables to working with *local* variables in Julia is to work inside a let block.\n",
    "\n",
    "Below, we declare a variable `x` as a `Float64` and set it equal to `3`. Unlike variable `a` from above, we see that `x` is now equal to `3.0`."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "let j::Float64 = 3\n",
    "    println(typeof(j))\n",
    "    println(\"j = $j\")\n",
    "end"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Arrays\n",
    "\n",
    "Once we start working with sets of data rather than individual pieces of it, it will be convenient for us to store data in structures like arrays or dictionaries.\n",
    "\n",
    "For example, we might create an array to keep track of my friends"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "myfriends = [\"Ted\", \"Robyn\", \"Barney\", \"Lily\", \"Marshall\"]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "or to store a sequence of numbers"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "fibonacci = [1, 1, 2, 3, 5, 8, 13]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Once we have an array, we can grab individual pieces of data from inside that array by indexing into the array. For example, if we want the third friend listed in `myfriends`, we write"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "myfriends[3]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "If we'd like to edit an existing array, we can use the `push!` and `pop!` functions. `push!` adds an element to the end of an array and `pop!` removes the last element of an array.\n",
    "\n",
    "We can add another number to our fibonnaci sequence"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "push!(fibonacci, 21)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "and then remove it"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "pop!(fibonacci)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "fibonacci"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Dictionaries\n",
    "\n",
    "If we have sets of data related to one another, we may choose to store that data in a dictionary. A good example is a contacts list, where we associate names with phone numbers."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "myphonebook = Dict(\"Ted\" => 8675309, \"help!\" => 911)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "collapsed": true
   },
   "source": [
    "In this example, each name and number is a \"key\" and \"value\" pair. We can grab Ted's number (a value) using the associated key"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "myphonebook[\"Ted\"]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "We can also get Ted's number - and simultaneously delete him from our contact list - by using `pop!`"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "pop!(myphonebook, \"Ted\")"
   ]
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Julia 0.6.0",
   "language": "julia",
   "name": "julia-0.6"
  },
  "language_info": {
   "file_extension": ".jl",
   "mimetype": "application/julia",
   "name": "julia",
   "version": "0.6.0"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}