Accumulator Pattern Instructor Notes

accumulator-pattern.md

Accumulator Pattern Instructor Notes

Objectives

• Identify the parts of the Accumulator Pattern
• Use the Accumulator Pattern to solve common problems

What is the Accumulator Pattern

Here is an example of the Accumulator Pattern in action:

function joinWords (words) {
  let result = ""
  for (var i = 0; i < words.length; i++) {
    result += words[i]
  }
  return result;
}

if var words = ['delicious', 'lobster', 'bisque']

then joinWords(words) -> "deliciouslobsterbisque"

further explanation here: https://learn.galvanize.com/content/gSchool/javascript-curriculum/master/20_Functional_Patterns/04_Accumulator_Pattern/README.md

Steps

• Define the functions
• Declare and return the result variable
• Set up the iteration
• Alter the accumulator as necessary

Question

Turn to your neighbor and discuss what the Accumulator Pattern is in your own words. Try to come up with a real-world example of the "Accumulator Pattern".

1) Identify the inputs and outputs

Take a minute to identify the input and output in the joinWords example above. What are their data types?

When finished, identify the input and output of the findMax function:

function findMax(nums) {

}

2) Choose an initial result value

Looking at the findMax function, identify the type and value of the result.

When finished, identify the result type and value for a joinWords function.

How about an allEven function?

allEven([2, 4, 6]) // returns true

3) Choose your iteration

Looking at the findMax function, what iteration style would you choose?

How about this function:

 flipFlop({ name: 'Sandy', hair: 'blue' }) // returns { Sandy: 'name', blue: 'hair' }

4) Alter the accumulator (result)?

Looking at the findMax function, how would you alter the accumulator (result variable) inside the loop?

How about this function:

doubleNumbers([1, 3]) // returns [2, 6]

More practice!

Try implementing the following functions using the accumulator pattern:

• countLetters(word)
• sum(nums)
• addTo(nums, value)
  • addTo([1, 2, 3], 3) -> [4, 5, 6]
  • addTo([], 3) -> []
• diff(array1, array2)
  • diff([1, 2, 3], [2, 3, 4]) -> [1, 1, 1]
  • diff([1, 2, 3], [2, 2, 2]) -> [1, 0, -1]
  • diff([1, 2, 3], [1, 2, -1]) -> [0, 0, -4]

Challenge

Given this input:

var medals = [
  {country: 'South Korea', type: 'gold'},
  {country: 'Russia', type: 'silver'},
  {country: 'Norway', type: 'gold'},
  {country: 'Norway', type: 'silver'},
  {country: 'Sweden', type: 'bronze'},
  {country: 'United States', type: 'gold'},
]

Write a goldCountries function that returns a list of all countries that won gold medals.

goldCountries(medals) -> ['South Korea', 'Norway', 'United States']