Published at Sep 18 2019
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Instructions

Test suite

Solution

Calculate the Hamming Distance between two DNA strands.

Your body is made up of cells that contain DNA. Those cells regularly wear out and need replacing, which they achieve by dividing into daughter cells. In fact, the average human body experiences about 10 quadrillion cell divisions in a lifetime!

When cells divide, their DNA replicates too. Sometimes during this process mistakes happen and single pieces of DNA get encoded with the incorrect information. If we compare two strands of DNA and count the differences between them we can see how many mistakes occurred. This is known as the "Hamming Distance".

We read DNA using the letters C,A,G and T. Two strands might look like this:

```
GAGCCTACTAACGGGAT
CATCGTAATGACGGCCT
^ ^ ^ ^ ^ ^^
```

They have 7 differences, and therefore the Hamming Distance is 7.

The Hamming Distance is useful for lots of things in science, not just biology, so it's a nice phrase to be familiar with :)

The Hamming distance is only defined for sequences of equal length, so an attempt to calculate it between sequences of different lengths should not work. The general handling of this situation (e.g., raising an exception vs returning a special value) may differ between languages.

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

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

Install assignment dependencies:

```
$ yarn install
```

Execute the tests with:

```
$ yarn 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 `xit`

to
`it`

.

The Calculating Point Mutations problem at Rosalind http://rosalind.info/problems/hamm/

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

```
import Hamming from './hamming'
describe('Hamming', () => {
const hamming = new Hamming()
it('no difference between identical strands', () => {
expect(hamming.compute('A', 'A')).toEqual(0)
})
xit('complete hamming distance for single nucleotide strand', () => {
expect(hamming.compute('A', 'G')).toEqual(1)
})
xit('complete hamming distance for small strand', () => {
expect(hamming.compute('AG', 'CT')).toEqual(2)
})
xit('small hamming distance', () => {
expect(hamming.compute('AT', 'CT')).toEqual(1)
})
xit('small hamming distance in longer strand', () => {
expect(hamming.compute('GGACG', 'GGTCG')).toEqual(1)
})
xit('large hamming distance', () => {
expect(hamming.compute('GATACA', 'GCATAA')).toEqual(4)
})
xit('hamming distance in very long strand', () => {
expect(hamming.compute('GGACGGATTCTG', 'AGGACGGATTCT')).toEqual(9)
})
xit('throws error when strands are not equal length', () => {
expect(() => { hamming.compute('GGACGGATTCTG', 'AGGAC') }
).toThrowError('DNA strands must be of equal length.' )
})
})
```

```
class Hamming {
compute(strand1: string, strand2: string): number {
if (strand1.length != strand2.length) {
throw new Error("DNA strands must be of equal length.");
}
let count = 0;
for (let i = 0; i < strand1.length; i++) {
if (strand1[i] !== strand2[i]) {
count += 1;
}
}
return count;
}
};
export default Hamming;
```

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?

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