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Nucleotide Count
Nucleotide Count

Nucleotide Count

Easy

Instructions

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

Running and testing your solutions

From the command line

Simply type make chez if you're using ChezScheme or make guile if you're using GNU Guile. Sometimes the name for the scheme binary on your system will differ from the defaults. When this is the case, you'll need to tell make by running make chez chez=your-chez-binary or make guile guile=your-guile-binary.

From a REPL

  • Enter (load "test.scm") at the repl prompt.
  • Develop your solution in nucleotide-count.scm reloading as you go.
  • Run (test) to check your solution.

Failed Test Cases

If some of the test cases fail, you should see the failing input and the expected output. The failing input is presented as a list because the tests call your solution by (apply nucleotide-count input-list). To learn more about apply see The Scheme Programming Language -- Chapter 5

Last updated 28 July 2022
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