In JavaScript, given an input string, create a function that returns an array containing an n number of 2d arrays

Question

I'm trying to get the console.log output at the bottom of the sample code. Currently, my code is returning undefined. I would like to know which part I did wrong.

I would also like to know if there are other, more efficient approaches in solving the problem.

function pairElement(str) {

  return str.split('')
    .forEach(function(element){
      return element.split('').map(function(pair){
        if (pair == "G") {return ["G","C"];}
        else if (pair == "C") {return ["C","G"];}
        else if (pair == "T") {return ["T","A"];}
        else {return ["A","T"];}
      });
    });
}

console.log(pairElement("ATCG"));
// -> Should be [["A","T"], ["T","A"], ["C","G"], ["G","C"]]
// -> But currently, it is returning undefined

I think the first split makes the "ATCG" input into ["ATCG"]. The second split then makes it into [["A"], ["T"], ["C"], ["G"]], creating the 2-layers deep array that I need. Then I think need to push or return based on the index of the layer-1 array. — jpls93
– jpls93, Commented Jul 30, 2016 at 22:45

Derek 朕會功夫 · Accepted Answer · 2016-07-30 22:53:28Z

2

You don't need to do .split twice. Here's a possible solution:

function pairElement(input){
    var pairs = { A: "T", T: "A", C: "G", G: "C" };
    return input.split("").map(char => [char, pairs[char]]);  // ES6 arrow function
}

console.log(pairElement("ATCG"));

Fiddle

"ATCG".split("") turns the string into an array ["A", "T", "C", "G"]. Then by applying a mapping function we can transform the output, character by character, into predefined pairs [["A","T"], ["T","A"], ["C","G"], ["G","C"]].

edited Jul 30, 2016 at 22:53

answered Jul 30, 2016 at 22:42

Derek 朕會功夫

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2 Comments

jpls93 Over a year ago

Accepting this as the correct answer since it demonstrates how key/value pairs can be used instead of if/else statements. Thanks. I learned a new technique because of this.

Tibrogargan Over a year ago

Trivial optimization: Since it could be static, move the pairs var outside the function so you're not creating it every time the function is called.

Whothehellisthat · Accepted Answer · 2016-07-30 22:40:59Z

1

What's returned is str.split('').forEach(...). The forEach iterator method does not return anything; it simply runs for every item in the split array.

Perhaps you meant to .map(...) instead? This would take the returned values from the callback function and create a new array with them.

answered Jul 30, 2016 at 22:40

Whothehellisthat

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Comments

Konstantin K. · Accepted Answer · 2016-07-30 22:44:48Z

1

Use .map instead .forEach. The .forEach() executes the callback function once for each array element; unlike map() or reduce() it always returns the value undefined and is not chainable. The typical use case is to execute side effects at the end of a chain.

answered Jul 30, 2016 at 22:44

Konstantin K.

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Comments

Tibrogargan · Accepted Answer · 2016-07-30 22:49:07Z

1

You're splitting too many times and you don't need the foreach. The split creates an array of from the string, (i.e. ["A","T","C","G"]). The map then creates a new array with each element modified according to your function (i.e. "A" becomes ["A","T"])

function pairElement(str) {
    return str.split('').map(function(element){
        if (element == "G") {return ["G","C"];}
        else if (element == "C") {return ["C","G"];}
        else if (element == "T") {return ["T","A"];}
        else {return ["A","T"];}
    });
}

edited Jul 30, 2016 at 22:49

answered Jul 30, 2016 at 22:43

Tibrogargan

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