Solutions and explanations to Lab 4 Handout and additional exercises

Our first JS funtions

function triangle(x, y) {
    return (x * y)/2;
}

console.log(triangle(3,4));

Answer key to number 8 can be found here.

A list of JS arithmetic operators can be found here.

In order to solve number 8 we want to divide any full number by 2 and only if the remainder is equal to zero, should we return true. The way we can achieve this is by using the modulus operator (%).

This is a video that explains the modulus operator with some examples:

Solution

function isEven(n) {
  return n % 2 === 0;
}

Age in years, days hours and minutes

function age(year, month, day) {
  var now = new Date();
  var bday = new Date(year, (month - 1), day);
  var age_in_ms = now - bday;
  var seconds = age_in_ms / 1000;
  var minutes = seconds / 60;
  var hours = minutes / 60;
  var days = hours / 24;
  var years = days / 365; // this line is problematic as the accuracy is not 100 percent

  console.log("You are " + Math.floor(years) + " years, \n or " + Math.floor(days) +
    " days, \n or " + Math.floor(hours) + " hours, \n or " + Math.floor(minutes) + " minutes old." );
}

Fibonacci

function fib(n) {
  var a = 0;
  var b = 1;
  console.log(a);
  console.log(b);
  i = 0;
  while (i < n) {
    var c = a + b;
    console.log(c);
    a = b;
    b = c;
    i ++;
  }
}

There are many alternative ways to solve Fibonacci, and one of the most basic one is perhaps this approach:

function fib(n) {
  var ar = [0, 1];

  for (var i = 0; i < n; i++) {
    ar.push(ar[i + 1] + ar[i])
  }
  console.log(ar);
}

For anyone more interested in the time complexity of Fibonacci in JS, you can read this blog post.

More Advanced challenges

The sort algorithm was supposed to be sorted without the built in .sort function. So the bubble sort method comes to mind. This is a basic implementation in JS:

function bubbleSort(array) {
  for (var i = (array.length - 1); i >= 0; i--) {
    for (var j = (array.length - i); j > 0; j--) {
      if (array[j] < array[j - 1]) {
        var store = array[j];
        array[j] = array[j - 1];
        array[j - 1] = store;
      }
    }
  }
  return array;
}

A good blog post to read regarding this algorithm can be found here.

The count algorithm can be implemented in various ways, but one good solution is here:

function count(ar) {
  res = {};
  ar.forEach(function(element, i) {
      var temp = 1;
      if (!res[element]) {
        res[element] = [i];
      } else {
        res[element].push(i);
      }
  });
  return res;
}

The correct value still needs to be extracted.