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# Nucleotide Count
Each of us inherits from our biological parents a set of chemical instructions known as DNA that influence how our bodies are constructed. All known life depends on DNA!
> Note: You do not need to understand anything about nucleotides or DNA to complete this exercise.
DNA is a long chain of other chemicals and the most important are the four nucleotides, adenine, cytosine, guanine and thymine. A single DNA chain can contain billions of these four nucleotides and the order in which they occur is important!
We call the order of these nucleotides in a bit of DNA a "DNA sequence".
We represent a DNA sequence as an ordered collection of these four nucleotides and a common way to do that is with a string of characters such as "ATTACG" for a DNA sequence of 6 nucleotides.
'A' for adenine, 'C' for cytosine, 'G' for guanine, and 'T' for thymine.
Given a string representing a DNA sequence, count how many of each nucleotide is present.
If the string contains characters that aren't A, C, G, or T then it is invalid and you should signal an error.
For example:
```
"GATTACA" -> 'A': 3, 'C': 1, 'G': 1, 'T': 2
"INVALID" -> error
```
## Getting Started
Make sure you have read the "Guides" section of the
[C track][c-track] on the Exercism site. This covers
the basic information on setting up the development environment expected
by the exercises.
## Passing the Tests
Get the first test compiling, linking and passing by following the [three
rules of test-driven development][3-tdd-rules].
The included makefile can be used to create and run the tests using the `test`
task.
make test
Create just the functions you need to satisfy any compiler errors and get the
test to fail. Then write just enough code to get the test to pass. Once you've
done that, move onto the next test.
As you progress through the tests, take the time to refactor your
implementation for readability and expressiveness and then go on to the next
test.
Try to use standard C99 facilities in preference to writing your own
low-level algorithms or facilities by hand.
## Source
The Calculating DNA Nucleotides_problem at Rosalind [http://rosalind.info/problems/dna/](http://rosalind.info/problems/dna/)
## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
[c-track]: https://exercism.io/my/tracks/c
[3-tdd-rules]: http://butunclebob.com/ArticleS.UncleBob.TheThreeRulesOfTdd
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