hofs-part-4.md

This lesson has slides and exercises for "Filter", "Reduce", and "Reduce Optional"

Filter

Lesson

Filter Slides

Exercises

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 filter(array, predicate) {
  var acc = [];
  each(array, function(element, i) {
    if (predicate(element, i)) {
      acc.push(element);
    }
  });
  return acc;
}

Basic Requirements

Use filter to write the following functions:

1. Write a function called evens that takes an array of numbers as a parameter, and returns an array of only the even numbers in the parameter.

2. Write a function called multiplesOfThree that takes an array of numbers as a parameter, and returns an array of only the numbers that are multiples of three.

3. Write a function called positives that takes an array of numbers as a parameter, and returns an array of only the numbers that are positive.

4. Write a function called evenLength that takes an array of strings and returns an array of only the strings with an even length.

More Practice

1. Use filter to write the following functions:

   • odds: Accepts an array of numbers, and returns the odd ones.
   • negatives: Like positives, but with negatives!
   • largerThanSix: given an array of numbers, returns those larger than 6.

2. Using filter, write a function startsWithChar that accepts two parameters: an array of strings, and a character (e.g. "a"), and returns an array of only the strings that start with that character:

   function startsWithChar(strings, character) {
     // ...
   }
   var words = "the quick brown fox jumps over the lazy dog".split(" ");
   startsWithChar(words, "q"); // => ["quick"]
   startsWithChar(words, "t"); // => ["the", "the"]

3. Extend the filter function to pass the index of each element to the predicate; then, complete the following:

   Write a function called evenIndexedEvenLengths (yeah, a bit of a mouthful) that accepts an array of strings as a parameter, and returns only the strings that are found at an even index that also have an even length, e.g.:

   function evenIndexedEvenLengths(strings) {
     // TODO: your code here
   }
   evenIndexedEvenLengths(["lion", "monkey", "aardvaark", "cat", "doge"]);
   // => ["lion", "doge"]
   evenIndexedEvenLengths(["red", "green", "purple", "blue", "yellow"]);
   // => ["purple", "yellow"]

   HINT: You'll need to ensure that your version of filter also passes each elements' index (or key in the case of objects) to the predicate.

4. Write a version of filter that works on arrays and objects; then, use your updated version of filter to filter an object with values of mixed types to an object with only numeric values. You should be able to use your modified version of filter as follows:

   filter({a: 1, b: "dog", c: true}, function(value) {
     return typeof value === "number";
   });
   // => {a: 1}

   Reduce

Lesson

Reduce Slides

Exercises

function reduce(array, f, start) {
  var acc = start;
  each(array, function(element) {
    acc = f(acc, element);
  });
  return acc;
}

Basic Requirements

1. We've shown that we can write the function sum -- which sums an array of numbers -- using reduce like this (see slides for more):

   function sum(numbers) {
     return reduce(numbers, function(total, number) {
       return total + number;
     }, 0);
   }

   Replace the ??? below to complete the implementation of product, that returns the product of an array of numbers.

   function product(numbers) {
     return reduce(???, function(???, number) {
       return ??? * number;
     }, ???);
   }

2. Rewrite the averageAge function from the warmup using reduce. The function signature and people array are copied below for your convenience:

   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 averageAge(people) {
     // First you'll need to compute the sum of all the ages
     // Second, divide the sum by the number of people (people.length)
   
     // YOUR CODE HERE
   }

3. The range function from yesterday looks like this:

   function range(start, end) {
     var acc = [];
     for (var i = start; i < end; i++) {
       acc.push(i);
     }
     return acc;
   }

   Using range and reduce, complete the factorial function below that should compute the factorial of n.

   function factorial(n) {
     // YOUR CODE HERE
   }

   HINT: If you cannot think of how to do this, it may help to first try writing the function using each, and then refactor it to use range & reduce.

More Practice

1. The sumBy function from previous lessons can be implemented using each like this:

   function sumBy(numbers, f) {
     var sum = 0;
     each(numbers, function(number) {
       sum = sum + f(number);
     });
     return sum;
   }

   Rewrite sumBy using reduce.

2. The max function below takes an array of numbers as a parameter, and returns the maximum number in the array:

   function max(numbers) {
     var max = numbers[0];
     each(numbers, function(number) {
       if (number > max) {
         max = number;
       }
     });
     return max;
   }

   Rewrite max using reduce.

3. Write a function called countOccurrences that, given a string and a character (a string with length one) as parameters, returns the total number of times that character occurs in the string. For example:

   countOccurrences("hello", "l"); // => 2
   countOccurrences("the", "z"); // => 0
   countOccurrences("hello, world!", "l"); // => 3

4. In the lecture (see slides) we ran through a function called everyNumberEven -- write functions like it that do the following:

   • Given an array of numbers, determine if every one of them is odd
   • Given an array of numbers, determine if every one of them is positive
   • Given an array of strings, determine if all of them have a length greather than 3.
   • Given an array of strings, determines if all of them contain the letter "e".

5. Write a function every that takes two parameters: an array and a predicate (a function that returns true or false). every should return true if the predicate returns true for every element in the array. You should be able to use it to write everyNumberEven like this:

   function everyNumberEven(numbers) {
     return every(numbers, function(number) {
       return number % 2 === 0;
     });
   }

   Test that it works by rewriting the functions in exercise (4) above using every.

Advanced

1. Let's write a function called join that works just like the built-in join, but using reduce! If you're unfamiliar with the built-in version of join, this is how it works:

   ["the", "quick", "brown", "fox"].join(" "); // => "the quick brown fox"
   ["one", "two", "three"].join(":"); // => "one:two:three"

   Part One: First, write a function called joinWith that takes three arguments: the string to join onto, the string that will be joined, and a separator. That is, it should work like this:

   function joinWith(onto, next, separator) {
    // YOUR CODE HERE
   }
   joinWith("the quick", "brown", " "); // => "the quick brown"
   joinWith("one:two", "three", ":"); // => "one:two:three"

   Part Two: Now, using joinWith and reduce, write join:

   function join(array, separator) {
     // YOUR CODE HERE
   }
   join(["the", "quick", "brown", "fox"], " "); // => "the quick brown fox"
   join(["one", "two", "three"], ":"); // => "one:two:three"

2. Uses of reduce are not restricted to turning arrays into numbers -- use reduce to rewrite map (instead of using each).

3. We previously covered the function countWords that worked like this:

   function countWords(s) {
     var acc = {};
     var words = s.split(" ");
     for (var i = 0; i < words.length; i = i + 1) {
       var word = words[i];
       if (acc[word] === undefined) {
         acc[word] = 1;
       } else {
         acc[word]++;
       }
     }
     return acc;
   }

   • Rewrite countWords using reduce.

   • Write the function countChars that works like countWords, but counts characters instead of words (using reduce of course).

     Improved reduce: Optional Starting Point & Optional Parameters

Lesson

Reduce Optional Slides

Exercises

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 reduce(array, f, acc) {
  if (acc === undefined) {
    acc = array[0];
    array = array.slice(1);
  }
  each(array, function(element, i) {
    acc = f(acc, element, i);
  });
  return acc;
}

Basic Requirements

1. The factorial function can be written using reduce and range; and furthermore, this is a case where the initial value can be omitted. Write factorial using our updated version of reduce and omit the third argument.

   HINT: Remember that the factorial of a number n is the product of all of the numbers from 1 to n, and the range function (shown below in problem 3) produces an array of numbers from start to end...

2. Use the updated version of reduce to write a function max that computes the maximum number in an array of numbers. Be sure to not include the third argument (the starting point).

3. The range function that we've explored so far looks like this:

   function range(start, end) {
     var acc = [];
     for (var i = start; i < end; i++) {
       acc.push(i);
     }
     return acc;
   }

   Modify range so that the end parameter is optional, and in the event that it is not supplied, range computes a range from 0 to start.

Advanced

There is a way to implement reduce such that it works on both arrays and objects. You'll likely want to make use of Object.keys function.