Nucleotide Count

Intro

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.

Task

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:

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"GATTACA" -> 'A': 3, 'C': 1, 'G': 1, 'T': 2
"INVALID" -> error

The Code

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#include "nucleotide_count.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// A C G T

char *count(const char *dna_strand) {
  char *var=malloc(50);
  memset(var, 0, 50);

  int a = 0;
  int c = 0;
  int g = 0;
  int t = 0;
  int i;
  for (i = 0; i < (int)strlen(dna_strand); i++) {
    switch (dna_strand[i]) {
    case 'A':
      a++;
      break;
    case 'C':
      c++;
      break;
    case 'G':
      g++;
      break;
    case 'T':
      t++;
      break;
    default:
      strcpy(var, "");
      return var;
    }
  }

  sprintf(var, "A:%d C:%d G:%d T:%d", a, c, g, t);

  return var;
}

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#ifndef _NUCLEOTIDE_COUNT_H
#define _NUCLEOTIDE_COUNT_H

char *count(const char *dna_strand);

#endif

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