Published at Nov 24 2018
·
0 comments

Instructions

Test suite

Solution

Detect palindrome products in a given range.

A palindromic number is a number that remains the same when its digits are
reversed. For example, `121`

is a palindromic number but `112`

is not.

Given a range of numbers, find the largest and smallest palindromes which are products of numbers within that range.

Your solution should return the largest and smallest palindromes, along with the factors of each within the range. If the largest or smallest palindrome has more than one pair of factors within the range, then return all the pairs.

Given the range `[1, 9]`

(both inclusive)...

And given the list of all possible products within this range:
`[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 15, 21, 24, 27, 20, 28, 32, 36, 25, 30, 35, 40, 45, 42, 48, 54, 49, 56, 63, 64, 72, 81]`

The palindrome products are all single digit numbers (in this case):
`[1, 2, 3, 4, 5, 6, 7, 8, 9]`

The smallest palindrome product is `1`

. Its factors are `(1, 1)`

.
The largest palindrome product is `9`

. Its factors are `(1, 9)`

and `(3, 3)`

.

Given the range `[10, 99]`

(both inclusive)...

The smallest palindrome product is `121`

. Its factors are `(11, 11)`

.
The largest palindrome product is `9009`

. Its factors are `(91, 99)`

.

Go through the setup instructions for Javascript to install the necessary dependencies:

https://exercism.io/tracks/javascript/installation

Install assignment dependencies:

```
$ npm install
```

Execute the tests with:

```
$ npm test
```

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by
changing `xtest`

to `test`

.

Problem 4 at Project Euler http://projecteuler.net/problem=4

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

```
import generate from './palindrome-products';
describe('Palindrome', () => {
test('largest palindrome from single digit factors', () => {
const palindromes = generate({ maxFactor: 9 });
const largest = palindromes.largest;
expect(largest.value).toEqual(9);
const containsMatch = [[3, 3], [1, 9]].filter(el => numericalArraysMatch(el, [3, 3])).length > 0;
expect(containsMatch).toBe(true);
});
xtest('largest palindrome from double digit factors', () => {
const palindromes = generate({ maxFactor: 99, minFactor: 10 });
const largest = palindromes.largest;
expect(largest.value).toEqual(9009);
expect(largest.factors).toEqual([91, 99]);
});
xtest('smallest palindrome from double digit factors', () => {
const palindromes = generate({ maxFactor: 99, minFactor: 10 });
const smallest = palindromes.smallest;
expect(smallest.value).toEqual(121);
expect(smallest.factors).toEqual([11, 11]);
});
xtest('largest palindrome from triple digit factors', () => {
const palindromes = generate({ maxFactor: 999, minFactor: 100 });
const largest = palindromes.largest;
expect(largest.value).toEqual(906609);
expect(largest.factors).toEqual([913, 993]);
});
xtest('smallest palindrome from triple digit factors', () => {
const palindromes = generate({ maxFactor: 999, minFactor: 100 });
const smallest = palindromes.smallest;
expect(smallest.value).toEqual(10201);
expect(smallest.factors).toEqual([101, 101]);
});
});
function numericalArraysMatch(a, b) {
if (a.length !== b.length) {
return false;
}
const one = [...a].sort(numericalSort);
const two = [...b].sort(numericalSort);
let result = true;
let index = 0;
while (index < one.length) {
result = result && one[index] === two[index];
index++;
}
return result;
}
function numericalSort(x, y) {
if (x < y) {
return -1;
}
if (x > y) {
return 1;
}
return 0;
}
```

```
function generate(args) {
const smallest={value:0};
const largest={value:0};
for (let x=(args.minFactor || 1); x<=args.maxFactor; ++x) {
for (let y=x; y<=args.maxFactor; ++y) {
let prod=x*y;
let prodStr=prod.toString();
if (prodStr === prodStr.split('').reverse().join('')) {
if ((smallest.value===0) || (smallest.value===prod)) {
smallest.value=prod;
smallest.factors=[x,y]
} else if (prod > largest.value) {
largest.value=prod;
largest.factors=[x,y];
}
}
}
}
return {largest, smallest};
}
module.exports=generate;
```

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

Level up your programming skills with 3,373 exercises across 50 languages, and insightful discussion with our volunteer team of welcoming mentors.
Exercism is
**100% free forever**.

## Community comments