Basic Data Structures: Access an Array's Contents Using Bracket Notation

Basic Data Structures: Access an Array's Contents Using Bracket Notation
The fundamental feature of any data structure is, of course, the ability to not only store data, but to be able to retrieve that data on command. So, now that we've learned how to create an array, let's begin to think about how we can access that array's information.

When we define a simple array as seen below, there are 3 items in it:

let ourArray = ["a", "b", "c"];
In an array, each array item has an index. This index doubles as the position of that item in the array, and how you reference it. However, it is important to note, that JavaScript arrays are zero-indexed, meaning that the first element of an array is actually at the zeroth position, not the first.

In order to retrieve an element from an array we can enclose an index in brackets and append it to the end of an array, or more commonly, to a variable which references an array object. This is known as bracket notation.

For example, if we want to retrieve the "a" from ourArray and assign it to a variable, we can do so with the following code:

let ourVariable = ourArray[0];
// ourVariable equals "a"
In addition to accessing the value associated with an index, you can also set an index to a value using the same notation:

ourArray[1] = "not b anymore";
// ourArray now equals ["a", "not b anymore", "c"];
Using bracket notation, we have now reset the item at index 1 from "b", to "not b anymore".


In order to complete this challenge, set the 2nd position (index 1) of myArray to anything you want, besides "b".

Regular Expressions: Match Single Characters Not Specified

Regular Expressions: Match Single Characters Not Specified
So far, you have created a set of characters that you want to match, but you could also create a set of characters that you do not want to match. These types of character sets are called negated character sets.

To create a negated character set, you place a caret character (^) after the opening bracket and before the characters you do not want to match.

For example, /[^aeiou]/gi matches all characters that are not a vowel. Note that characters like ., !, [, @, / and white space are matched - the negated vowel character set only excludes the vowel characters.


Create a single regex that matches all characters that are not a number or a vowel. Remember to include the appropriate flags in the regex.

Regular Expressions: Match Single Character with Multiple Possibilities

Regular Expressions: Match Single Character with Multiple Possibilities
You learned how to match literal patterns (/literal/) and wildcard character (/./). Those are the extremes of regular expressions, where one finds exact matches and the other matches everything. There are options that are a balance between the two extremes.

You can search for a literal pattern with some flexibility with character classes. Character classes allow you to define a group of characters you wish to match by placing them inside square ([ and ]) brackets.

For example, you want to match "bag", "big", and "bug" but not "bog". You can create the regex /b[aiu]g/ to do this. The [aiu] is the character class that will only match the characters "a", "i", or "u".

let bigStr = "big";
let bagStr = "bag";
let bugStr = "bug";
let bogStr = "bog";
let bgRegex = /b[aiu]g/;
bigStr.match(bgRegex); // Returns ["big"]
bagStr.match(bgRegex); // Returns ["bag"]
bugStr.match(bgRegex); // Returns ["bug"]
bogStr.match(bgRegex); // Returns null

Use a character class with vowels (a, e, i, o, u) in your regex vowelRegex to find all the vowels in the string quoteSample.

Note
Be sure to match both upper- and lowercase vowels.

RegEx: Extract Matches

Regular Expressions: Extract Matches
So far, you have only been checking if a pattern exists or not within a string. You can also extract the actual matches you found with the .match() method.

To use the .match() method, apply the method on a string and pass in the regex inside the parentheses. Here's an example:

"Hello, World!".match(/Hello/);
// Returns ["Hello"]
let ourStr = "Regular expressions";
let ourRegex = /expressions/;
ourStr.match(ourRegex);
// Returns ["expressions"]

Apply the .match() method to extract the word coding.

ES6: Compare Scopes of the var and let Keywords

When you declare a variable with the var keyword, it is declared globally, or locally if declared inside a function.

The let keyword behaves similarly, but with some extra features. When you declare a variable with the let keyword inside a block, statement, or expression, its scope is limited to that block, statement, or expression.

For example:

var numArray = [];

for (var i = 0; i < 3; i++) {

  numArray.push(i);

}

console.log(numArray);

// returns [0, 1, 2]

console.log(i);

// returns 3

With the var keyword, i is declared globally. So when i++ is executed, it updates the global variable. This code is similar to the following:

var numArray = [];

var i;

for (i = 0; i < 3; i++) {

  numArray.push(i);

}

console.log(numArray);

// returns [0, 1, 2]

console.log(i);

// returns 3

This behavior will cause problems if you were to create a function and store it for later use inside a for loop that uses the i variable. This is because the stored function will always refer to the value of the updated global i variable.

var printNumTwo;

for (var i = 0; i < 3; i++) {

  if(i === 2){

    printNumTwo = function() {

      return i;

    };

  }

}

console.log(printNumTwo());

// returns 3

As you can see, printNumTwo() prints 3 and not 2. This is because the value assigned to i was updated and the printNumTwo() returns the global i and not the value i had when the function was created in the for loop. The let keyword does not follow this behavior:

'use strict';

let printNumTwo;

for (let i = 0; i < 3; i++) {

  if (i === 2) {

    printNumTwo = function() {

      return i;

    };

  }

}

console.log(printNumTwo());

// returns 2

console.log(i);

// returns "i is not defined"

i is not defined because it was not declared in the global scope. It is only declared within the for loop statement. printNumTwo() returned the correct value because three different i variables with unique values (0, 1, and 2) were created by the let keyword within the loop statement.

Fix the code so that i declared in the if statement is a separate variable than i declared in the first line of the function. Be certain not to use the var keyword anywhere in your code.

This exercise is designed to illustrate the difference between how var and let keywords assign scope to the declared variable. When programming a function similar to the one used in this exercise, it is often better to use different variable names to avoid confusion.