freeCodeCamp/guide/english/certifications/javascript-algorithms-and-data-structures/intermediate-algorithm-scripting/dna-pairing/index.md

title
Dna Pairing

Remember to use Read-Search-Ask if you get stuck. Try to pair program and write your own code

Problem Explanation:

• You will get a DNA strand sequence and you need to get the pair and return it as a 2D array of the base pairs. Keep in mind that the provided strand should be first always.

• Another way to interpret the problem: there are four potential characters that exist in DNA: "A", "T", "G", and "C". "A" and "T" are always paired together, and "G" and "C" are always paired together.
  This problem presents you with an input, e.g. "ATCGA". Each of those five characters are missing their pairs.
  E.g. the first character "A" needs to be paired with "T" to give the array element ["A", "T"].
  The second character "T" needs to be paired with "A" to give the array element ["T", "A"].
  The number of elements in the final output equals the number of characters in the input.

This problem does not involve rearranging the input into different combinations or permutations.

Relevant Links

• Array.push()
• String.split()

Hint: 1

• There are two base case, A-T and C-G, these go both way. You can use regular expression, if statements of anything that you can think of.

try to solve the problem now

Hint: 2

• I would recommend using a switch, as it makes things a lot smoother.

try to solve the problem now

Hint: 3

• The result must be an array of arrays, so keep that in mind when pushing things.

try to solve the problem now

Spoiler Alert!

Solution ahead!

Basic Code Solution:

    function pairElement(str) {
      // Return each strand as an array of two elements, the original and the pair.
      var paired = [];

      // Function to check with strand to pair.
      var search = function(char) {
        switch (char) {
          case 'A':
            paired.push(['A', 'T']);
            break;
          case 'T':
            paired.push(['T', 'A']);
            break;
          case 'C':
            paired.push(['C', 'G']);
            break;
          case 'G':
            paired.push(['G', 'C']);
            break;
        }
      };

      // Loops through the input and pair.
      for (var i = 0; i < str.length; i++) {
        search(str[i]);
      }

      return paired;
    }

    // test here
    pairElement("GCG");

Run Code

Code Explanation:

• The program is very simple, the best solution that I have come up with is to use a switch to catch all the possible four elements. Using if statements would take too much code. You could also use Regular Expressions.
• Since we have to the original and the pair, I decided to take all four cases instead of the base two.
• Create an empty array and use the search function to push the right values to the array and return them.

Relevant Links

• Switch Statements

Intermediate Code Solution:

    function pairElement(str) {
    //create object for pair lookup
    var pairs = {
      "A": "T",
      "T": "A",
      "C": "G",
      "G": "C"
    }
    //split string into array of characters
    var arr = str.split("");
    //map character to array of character and matching pair
    return arr.map(x => [x,pairs[x]]);
  }

  //test here
  pairElement("GCG");

Run Code

Code Explanation:

• First define an object with all pair possibilities, this allows us to easily find by key or value.
• Split str into a characters array so we can use each letter to find its pair.
• Use the map function to map each character in the array to an array with the character and it's matching pair, creating a 2D array.

Relevant Links

• Array.prototype.map()
• Property accessors
• Arrow functions

NOTES FOR CONTRIBUTIONS:

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See Wiki Challenge Solution Template for reference.