Session Notes 2016.05.10

TLC 5.10.2015 Session: Command Line, Node Programs, and Arrays!

Command Line

Useful commands:

pwd
cd
cd ../
cp
mv
mkdir
ls
ls -al

REPL

• Stands for Read-Evaluate-Print Loop
• There's one in your browser! (Developer Tools / Console)
• Or use node from the command line:

  • Install the package manager homebrew:

    /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

  • Install node:

    brew isntall node

  • Run node:

    node
    > console.log("hello world!")
    

Arrays

Useful Resources

• MDN
• Stack Overflow

Covered:

• printing stuff to the REPL:

  console.log("hello world!");

• Building an array:

  var fruits = ["Apple", "Orange", "Banana"]

• finding head and tail with slice:

  fruits[0] // => "Apple"
  fruits.slice(1) // => ["Orange", "Banana"]
  // shortcut for:
  fruits.slice(1, fruits.length) // who wants to type that all the time!?

• adding stuff to an array:

  fruits.concat("Kiwi", "Strawberry") // => ["Apple", "Orange", "Banana", "Kiwi", "Strawberry"]

• Array.pop(), and Array.push() work, but are evil!

  • Use Array[-1] and Array.concat() instead!

  • Pop quiz: why are they evil?

  • Hint: what happens if you try to substitute the value of x for every occurrence of x below:

    var fruit = ["Apple", "Orange", "Banana"];
    var x = fruit.push("Kiwi");
    var y = x.push("Strawberry");
    y // => ["Apple", "Orange", "Banana", "Kiwi", "Strawberry"]

    Ie: if x is equivalent to fruit.push("Kiwi""), then I should be able to substitute fruit.push("Wiki") everywhere I see x and get the same result for y, but do I? Try it and see:

    var fruit = ["Apple", "Orange", "Banana"];
    var x = fruit.push("Kiwi");
    var y = fruit.push("Kiwi").push("Strawberry");
    y // => ???

Running programs with node

Given the file fruits.js stored at ~/myProjects/fruit.js:

var fruit = ["Apple", "Orange", "Banana"];
console.log(fruit);

Running this from the command line should print the contents of the fruits array to the REPL:

$ cd ~/myProjects
$ node fruits.js

Testing

It's sort of annoying to run everything from the CLI and console-log every time we want to check something. Wouldn't it be nice to have tests like we did in TLC.js? We didn't get there yet... but we will!

Sneak preview: mocha is great!

Example Problems for next couple of weeks:

• Given as input the array [1,2,3,4,5,6,7,8]:

  1. Write a function sum that adds up every number in the array
  2. Write a function countEvens that counts the number of even numbers in the array

• Given as input the array:

  [
    "How", "much", "wood", "would", "a",
    "wood", "chuck", "chuck", "if", "a",
    "wood", "chuck", "could", "chuck", "wood", "?"
  ]

  1. Write a function countChucks that counts the number of times the word chuck appears in the array
  2. Write a function countAll that returns an object whose keys are each word in the array, and whose values are the number of times each word appears in the array

• HARD: Given as input the first section of Gravity's Rainbow:

  • Generate a word frequency concordance of the section
  • Hints: you will likely find String#split and Object#assign