## 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:

- Input: What do I put in?
- 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:

- Find the number that appears once,
`x`

- Find the number that appears twice,
`y`

- 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!

## Further Reading 📖

## Questions ❔

- How often do you do katas?
- Which implementation do you like more? Why?
- Any alternative solution?

## Discussion

This doesn't look like a very reasonable use of reduce.

I'd suggest starting with something like this.