hofs-part-3.md

Map

Lesson

Slides - Map

Slides - Map Objects

Exercises - Map

function each(coll, f) {
  if (Array.isArray(coll)) {
    for (var i = 0; i < coll.length; i++) {
      f(coll[i], i);
    }
  } else {
    for (var key in coll) {
      f(coll[key], key);
    }
  }
}

function map(array, f) {
  var acc = [];
  each(array, function(element, i) {
    acc.push(f(element, i));
  });
  return acc;
}

Basic Requirements

1. We can rewrite the ages function from the warmup using map like this:

   var people = [
     {name: {first: "Alyssa", middle: "P.", last: "Hacker"}, age: 26},
     {name: {first: "Ben", last: "Bitdiddle"}, age: 34},
     {name: {first: "Eva", middle: "Lu", last: "Ator"}, age: 40},
     {name: {first: "Lem", middle: "E.", last: "Tweakit"}, age: 45},
     {name: {first: "Louis", last: "Reasoner"}, age: 21}
   ];
   
   function ages(people) {
     return map(people, function(person) {
       return person.age;
     });
   }

   Rewrite the functions firstNames, lastNames and fullNames using map as shown above.

2. Given the function abs that computes the absolute value, finish the invocation of map that should compute the absolute value of each number in its array argument.

   function abs(x) {
     if (x >= 0) {
       return x;
     }
     return -x;
   }
   
   map([1, -2, 37, -100, -8, 5], ???);
   // => [1, 2, 37, 100, 8, 5]

   HINT: To test this, it may be easiest to paste all of the above into a console.

3. We're going to write a function maximums that, given an array of arrays of numbers as an argument, returns a new array containing the maximum number of each inner array. That's a bit of a mouthful, so let's break it down:

   // our argument would look like this:
   var sampleInput = [ // it's an array
     [1, 3, 2], // of arrays of numbers
     [4, 23, 100],
     [7, 6, 3, -2]
   ];
   
   // and we want to be able to use maximums to do this:
   maximums(sampleInput); // => [3, 100, 7]

   Part One: Let's start by writing a function max that, when given an array of numbers computes the maximum number in that array. You will want to use each for this.

   function max(numbers) {
     // YOUR CODE HERE
   }
   max([1, 3, 2]); // => 3
   max([4, 23, 100]); // => 100

   Part Two: Now that you have a function that computes the the maximum number in an array of numbers, use map to transform each array in the sampleInput into its maximum by completing the maximums function:

   function maximums(arrays) {
     return // YOUR CODE HERE
   }

More Practice

1. Complete the function exponentials that accepts an array of numbers as a parameter, and raises each number n to the nth power, e.g:

   function exponentials(numbers) {
     // YOUR CODE HERE
   }
   exponentials([1, 2, 3, 4]); // => [1, 2*2, 3*3*3, 4*4*4*4] => [1, 4, 27, 256]
   exponentials([3, 2, 5]); // => [27, 4, 3125]

2. First, write a function reverse that accepts a string as a parameter and returns a reversed version of that string (you'll want to use a for loop for this). Then, use reverse to write a function called reverseWords that accepts a string as an argument, and returns a string with all of its words reversed. You'll want to use split and join in both of these functions.

   "hello".split(""); // => ["h", "e", "l", "l", "o"]
   ["h", "e", "l", "l", "o"].join(""); // => "hello"
   
   "hello world".split(" "); // => ["hello", "world"]
   ["hello", "world"].join(" "); // => "hello world"
   
   reverseWords("hello world"); // => "olleh dlrow"

   Note that reverseWords should reverse each word individually, not the entire string.

3. We often want to transform an array of objects by looking up the value found under a specific key in each object; for instance, consider the problem of finding the ages of all the people in the people array like we did in the first exercise:

   var people = [
     {name: {first: "Alyssa", middle: "P.", last: "Hacker"}, age: 26},
     {name: {first: "Ben", last: "Bitdiddle"}, age: 34},
     {name: {first: "Eva", middle: "Lu", last: "Ator"}, age: 40},
     {name: {first: "Lem", middle: "E.", last: "Tweakit"}, age: 45},
     {name: {first: "Louis", last: "Reasoner"}, age: 21}
   ];
   
   function ages(people) {
     return map(people, function(person) {
       return person.age;
     });
   }

   Write a function called pluck that takes an array of objects and a string representing a key as parameters, and returns an array of all the values found under each object using the key, e.g.:

   pluck(people, "age"); // => [26, 34, 40, 45, 21]

   Your implementation should use map.

4. Let's say that we have a string representing a CSV (comma-separated values) file that looks like this:

   "Alyssa,P.,Hacker,26\nBen,,Bitdiddle,34\nEva,Lu,Ator,40\nLem,E.,Tweakit,45\nLouis,,Reasoner,21"

   And we want to parse it into an array people objects like we showed in the warmup, e.g.:

   var people = [
     {name: {first: "Alyssa", middle: "P.", last: "Hacker"}, age: 26},
     {name: {first: "Ben", last: "Bitdiddle"}, age: 34},
     {name: {first: "Eva", middle: "Lu", last: "Ator"}, age: 40},
     {name: {first: "Lem", middle: "E.", last: "Tweakit"}, age: 45},
     {name: {first: "Louis", last: "Reasoner"}, age: 21}
   ];

   Using map and split, write a function parseCSV that accepts a CSV string as a parameter and outputs an array of people objects with the exact format as shown above.

Advanced

1. Disclaimer: This problem is very difficult! Write a function called map2 that accepts two arrays and a function as arguments, and constructs a new array by invoking its function argument on the elements of both arrays, e.g.:

   map2([1, 2, 3], [4, 5, 6], function(a, b) {
     return a * b;
   });
   // => [4, 10, 18]

   Now, write a function called mapN that accepts an arbitrary number of arrays and a n-ary function as arguments, and constructs a new array by combining the elements of all the arrays, e.g.:

   mapN([1, 2, 3], [4, 5, 6], [2, 2, 2], function(a, b, c) {
     return (a * b) + c;
   });
   // => [6, 12, 20]
   
   mapN([1, 2, 3], function(x) { return x * x; }) // => [1, 4, 9]

   You'll need to read about the arguments keyword and apply to complete this function.

   Exercises - Map Objects

function each(coll, f) {
  if (Array.isArray(coll)) {
    for (var i = 0; i < coll.length; i++) {
      f(coll[i], i);
    }
  } else {
    for (var key in coll) {
      f(coll[key], key);
    }
  }
}

function map(coll, f) {
  var acc = [];
  if (!Array.isArray(coll)) {
    acc = {};
  }
  each(coll, function(element, key) {
    acc[key] = f(element, key);
  });
  return acc;
}

Basic Requirements

1. The incrementValues function from yesterday can be written much more elegantly using map. Rewrite that function using our new and improved version of map.

   For convenience, the prompt is reproduced below:

   Write a function called incrementValues that accepts an object as a parameter, and outputs an object with all of its numeric values incremented by one. You'll want to use the updated version of map for this, and you will need to check the type of each value to determine whether or not it should be incremented.

2. Write a function called uppercaseValues that, given an object as a parameter, returns a new object with all of its string values converted to uppercase. You'll want to make use of .toUpperCase() for this:

   "hello".toUpperCase(); // => "HELLO"

   Also, ensure that you only attempt to convert strings to uppercase -- you may want to use typeof for this.

3. Write a function called countNestedKeys that, given an object where all values are also objects, returns an object that contains the count of keys in each nested object, e.g.

   function countNestedKeys(object) {
     // TODO: Your code here
   }
   countNestedKeys({a: {b: 1, c: 7}, f: {h: 22, g: 12, i: 24}});
   // => {a: 2, b: 3}

I believe there's a typo on the last line of this markdown: The expected output should be {a: 2, f: 3}