//for loops

// Basic JavaScript: Iterate with JavaScript For Loops

// You can run the same code multiple times by using a loop

// The most common type of JavaScript loop is called a for loop because it runs "for" a specific number of times.

// For loops are declared with three optional expressions separated by semicolons:

// for ([initialization]; [condition]; [final-expression])

// The initialization statement is executed one time only before the loop starts. It is typically used to define and setup your loop variable.

// The condition statement is evaluated at the beginning of every loop iteration and will continue as long as it evaluates to true. When condition is false at the start of the iteration, the loop will stop executing. This means if condition starts as false, your loop will never execute.

// The final-expression is executed at the end of each loop iteration, prior to the next condition check and is usually used to increment or decrement your loop counter.

// In the following example we initialize with i = 0 and iterate while our condition i < 5 is true. We'll increment i by 1 in each loop iteration with i++ as our final-expression.

// Example

var ourArray = [];

for (var i = 0; i < 5; i++) {

ourArray.push(i);

}

// Setup

var myArray = [];

// Only change code below this line.

for (var i = 1; i < 6; i++){

myArray.push(i);

}

// Iterate Odd Numbers With a For Loop

// For loops don't have to iterate one at a time. By changing our final-expression, we can count by even numbers.

// We'll start at i = 0 and loop while i < 10. We'll increment i by 2 each loop with i += 2.

var ourArray = [];

for (var i = 0; i < 10; i += 2) {

ourArray.push(i);

}

//ourArray will now contain [0,2,4,6,8]. Let's change our initialization so we can count by odd numbers.

// Count Backwards With a For Loop

// A for loop can also count backwards, so long as we can define the right conditions.

// In order to count backwards by twos, we'll need to change our initialization, condition, and final-expression.

// We'll start at i = 10 and loop while i > 0. We'll decrement i by 2 each loop with i -= 2.

var ourArray = [];

for (var i=10; i > 0; i-=2) {

ourArray.push(i);

}

// ourArray will now contain [10,8,6,4,2]. Let's change our initialization and final-expression so we can count backward by twos by odd numbers.

//Iterate Through an Array with a For Loop

//A common task in JavaScript is to iterate through the contents of an array. One way to do that is with a for loop. This code will output each element of the array arr to the console:

var arr = [10, 9, 8, 7, 6];

for (var i = 0; i < arr.length; i++) {

console.log(arr[i]);

}

//Remember that arrays have zero-based indexing, which means the last index of the array is length - 1. Our condition for this loop is i < arr.length, which stops the loop when i is equal to length.

// In this case the last iteration is i === 4 i.e. when i becomes equal to arr.length and outputs 6 to the console.

//Nesting For Loops

// If you have a multi-dimensional array, you can use the same logic as the prior waypoint to loop through both the array and any sub-arrays. Here is an example:

var arr = [

[1,2], [3,4], [5,6]

];

for (var i=0; i < arr.length; i++) {

for (var j=0; j < arr[i].length; j++) {

console.log(arr[i][j]);

}

}

//This outputs each sub-element in arr one at a time. Note that for the inner loop, we are checking the .length of arr[i], since arr[i] is itself an array.

// Iterate with JavaScript Do...While Loops

// The next type of loop you will learn is called a do...while loop. It is called a do...while loop because it will first do one pass of the code inside the loop no matter what, and then continue to run

// the loop while the specified condition evaluates to true.

var ourArray = [];

var i = 0;

do {

ourArray.push(i);

i++;

} while (i < 5);

// The example above behaves similar to other types of loops, and the resulting array will look like [0, 1, 2, 3, 4]. However, what makes the do...while different from other loops is how it behaves

// when the condition fails on the first check. Let's see this in action: Here is a regular while loop that will run the code in the loop as long as i < 5:

var ourArray = [];

var i = 5;

while (i < 5) {

ourArray.push(i);

i++;

}

// In this example, we initialize the value of ourArray to an empty array and the value of i to 5. When we execute the while loop, the condition evaluates to false because i is not less than 5, so we do

// not execute the code inside the loop. The result is that ourArray will end up with no values added to it, and it will still look like [] when all of the code in the example above has completed running.

// Now, take a look at a do...while loop:

var ourArray = [];

var i = 5;

do {

ourArray.push(i);

i++;

} while (i < 5);

// In this case, we initialize the value of i to 5, just like we did with the while loop. When we get to the next line, there is no condition to evaluate, so we go to the code inside the curly braces and execute it.

// We will add a single element to the array and then increment i before we get to the condition check. When we finally evaluate the condition i < 5 on the last line, we see that i is now 6, which fails the

// conditional check, so we exit the loop and are done. At the end of the above example, the value of ourArray is [5]. Essentially, a do...while loop ensures that the code inside the loop will run at least once.

// Let's try getting a do...while loop to work by pushing values to an array.

// Setup

var myArray = [];

var i = 10;

// Only change code below this line

do{

myArray.push(i);

i++;

} while (i < 5)

// Replace Loops using Recursion

// Recursion is the concept that a function can be expressed in terms of itself. To help understand this, start by thinking about the following task: multiply the elements from 0 to n inclusive in an array to

// create the product of those elements. Using a for loop, you could do this:

function multiply(arr, n) {

var product = arr[0];

for (var i = 1; i <= n; i++) {

product *= arr[i];

}

return product;

}

// However, notice that multiply(arr, n) == multiply(arr, n - 1) * arr[n]. That means you can rewrite multiply in terms of itself and never need to use a loop.

function multiply(arr, n) {

if (n <= 0) {

return arr[0];

} else {

return multiply(arr, n - 1) * arr[n];

}

}

// The recursive version of multiply breaks down like this. In the base case, where n <= 0, it returns the result, arr[0]. For larger values of n, it calls itself, but with n - 1.

// That function call is evaluated in the same way, calling multiply again until n = 0. At this point, all the functions can return and the original multiply returns the answer.

// Note: Recursive functions must have a base case when they return without calling the function again (in this example, when n <= 0), otherwise they can never finish executing.

// Profile Lookup

// We have an array of objects representing different people in our contacts lists.

// A lookUpProfile function that takes name and a property (prop) as arguments has been pre-written for you.

// The function should check if name is an actual contact's firstName and the given property (prop) is a property of that contact.

// If both are true, then return the "value" of that property.

// If name does not correspond to any contacts then return "No such contact".

// If prop does not correspond to any valid properties of a contact found to match name then return "No such property".