Loop through an array to create a 2D array - loop not working?

Question

The following should take the input and create a 2D array of letter pairs - [["A", "T"], ["T", "A"], ["C","G"]] but it is currently only returning [["A", "T"], ["T", "A"]], which makes me suspect there is something wrong with my loop logic.

function pair(str) {
var paired = [];
var str = str.split("");

for (i=0; i<str.length; i++) {

var newStr = str.splice(0,1);

  if (newStr[0] === "A") {
   newStr.push("T");
  }
  if (newStr[0] === "T") {
   newStr.push("A");
  }
  if (newStr[0] === "C") {
   newStr.push("G")
  }
  if (newStr[0] === "G") {
   newStr.push("C")
  }

paired.push(newStr);
}
   return paired;
}

pair("ATC");

when you splice , the array length decreases

gaurav5430
– gaurav5430

2015-10-15 18:19:53 +00:00
Commented Oct 15, 2015 at 18:19 — gaurav5430
– gaurav5430, Commented Oct 15, 2015 at 18:19

Fabio Beltramini · Accepted Answer · 2015-10-15 18:34:12Z

1

Just to promote thinking functionally for more readable code, consider this way of writing it:

pair={"A":"T","T":"A","C":"G","G":"C"};

"ATC".split("").map(function(letter){return [letter,pair[letter]];})

split makes the string an array (as you already know), and then the array's map method lets you say, for each item in this array, use the function provided on that item to produce a new array.

answered Oct 15, 2015 at 18:34

Fabio Beltramini

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Comments

Bartek Banachewicz · Accepted Answer · 2015-10-15 18:23:19Z

0

The splice is the problem. What you want is:

var newStr = [str[i]];

You need to index it at the character pointed by the index, and then wrap into [] because you want to treat it as an array.

In your original code, you didn't move forward with your loop at all. Worse, because splice modified the original string, its length changed as well, making the loop run less times than it should.

edited Oct 15, 2015 at 18:23

answered Oct 15, 2015 at 18:15

Bartek Banachewicz

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