Published at Nov 17 2018
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Instructions

Test suite

Solution

Calculate the Hamming difference between two DNA strands.

A mutation is simply a mistake that occurs during the creation or copying of a nucleic acid, in particular DNA. Because nucleic acids are vital to cellular functions, mutations tend to cause a ripple effect throughout the cell. Although mutations are technically mistakes, a very rare mutation may equip the cell with a beneficial attribute. In fact, the macro effects of evolution are attributable by the accumulated result of beneficial microscopic mutations over many generations.

The simplest and most common type of nucleic acid mutation is a point mutation, which replaces one base with another at a single nucleotide.

By counting the number of differences between two homologous DNA strands taken from different genomes with a common ancestor, we get a measure of the minimum number of point mutations that could have occurred on the evolutionary path between the two strands.

This is called the 'Hamming distance'.

It is found by comparing two DNA strands and counting how many of the nucleotides are different from their equivalent in the other string.

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

The Hamming distance between these two DNA strands is 7.

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.

Follow the setup instructions for Crystal here:

http://exercism.io/languages/crystal

More help installing can be found here:

http://crystal-lang.org/docs/installation/index.html

Execute the tests with:

```
$ crystal spec
```

In each test suite all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by changing `pending`

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.

```
require "spec"
require "../src/*"
describe "Hamming" do
describe "#compute" do
it "computes no difference for identical single nucleotide strands" do
Hamming.compute("A", "A").should eq 0
end
pending "computes a distance for single nucleotide strands" do
Hamming.compute("A", "G").should eq 1
end
pending "computes a distance for small strands" do
Hamming.compute("AG", "CT").should eq 2
end
pending "computes a distance for medium strands" do
Hamming.compute("GGACG", "GGTCG").should eq 1
end
pending "computes a distance for large strands" do
Hamming.compute("GGACGGATTCTG", "AGGACGGATTCT").should eq 9
end
pending "raises an exception when strands aren't of equal length" do
expect_raises(ArgumentError) { Hamming.compute("GCC", "A") }
end
end
end
```

```
module Hamming
extend self
CHARSET = Set{'A'.bytes[0], 'T'.bytes[0], 'C'.bytes[0], 'G'.bytes[0]}
def compute(nlist_1 : String, nlist_2 : String)
# lengths must match
if nlist_1.bytesize != nlist_2.bytesize
raise ArgumentError.new
end
(0...nlist_1.bytesize).count { |i|
c1 = nlist_1.unsafe_byte_at(i)
c2 = nlist_2.unsafe_byte_at(i)
# err on bad char
unless CHARSET === c1 && CHARSET === c2
raise ArgumentError.new
end
c1 != c2
}
end
end
```

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?

## Community comments