← Blog

August, 2020

## Intro 🌐

Problem solving is an important skill, for your career and your life in general.

That’s why I take interesting katas of all levels, customize them and explain how to solve them.

## Understanding the Exercise❗

First, we need to understand the exercise! If you don’t understand it, you can’t solve it!.

My personal method:

1. Input: What do I put in?
2. Output: What do I want to get out?

### Today’s exercise

Today, another `7 kyu` kata, meaning we slightly increase the difficulty.

Source: Codewars

Write a function `missingValues`, that accepts one parameter: `myArray`.

Given an array of number, e.g. `[1, 1, 1, 2, 2, 3]`,

find:

• the number `x`, that appears once, e.g. `3`
• the number `y`, that appears twice, e.g. `2`

and return the product `x` * `x` * `y`, e.g. `18` (=> `3 x 3 x 2`).

Input: an array of numbers.

Output: a number.

## Thinking about the Solution 💭

I think I understand the exercise (= what I put into the function and what I want to get out of it).

Now, I need the specific steps to get from input to output.

I try to do this in small baby steps:

1. Find the number that appears once, `x`
2. Find the number that appears twice, `y`
3. Return the product of `x * x * y`

Example:

• Input: `[1, 1, 1, 2, 2, 3]`
• Find the number that appears once, `x`: `3`
• Find the number that appears twice, `y`: `2`
• Return the product of `x * x * y`: `18` (=> `3 x 3 x 2`)
• Output: `18`

## Implementation ⛑

``````function missingValues(myArray) {
// count amount of each number
const count = myArray.reduce(
(acc, cur) =>
acc.hasOwnProperty(cur)
? { ...acc, [cur]: acc[cur] + 1 }
: { ...acc, [cur]: 1 },
{}
);

// helper function to find the object key (= our number) that appears [amount] times
const appears = (amount) =>
Object.entries(count)
.filter(([key, value]) => value === amount)
.map((entry) => entry[0]);

return appears(1) * appears(1) * appears(2);
}``````

### Result

``````console.log([1, 1, 1, 2, 2, 3]);
// 18 ✅

console.log([6, 5, 4, 100, 6, 5, 4, 100, 6, 5, 4, 200]);
// 4000000 ✅``````

## Playground ⚽

You can play around with the code here

## Next Part ➡️

Great work!

We learned how to use `reduce`, `filter`, `map`, `hasOwnProperty`, `Object.entries`.

I hope you can use your new learnings to solve problems more easily!

Next time, we’ll solve another interesting kata. Stay tuned!

If I should solve a specific kata, shoot me a message here.

If you want to read my latest stuff, get in touch with me!