Nested Skips (UPenn)

Question

Exercise 1: skips :: [a] -> [[a]]

The nth list in the output should contain every nth element from the input list. For example, skips "hello!" == ["hello!", "el!", "l!", "l", "o", "!"].

The question UPenn Homework 3: skips function and it's associated answers inspired me to write a solution in Javascript. I used the opportunity to (finally) better understand high order array functions. If you would be so kind to critique:

1. Code Layout, specifically if nesting function skips() makes any sense and why.
2. Do higher order functions provide more than just readability, are they more efficient?
3. Can I make the nested function skips() better (readable/faster)?
4. Anything at all.

function skip(str, cb) {
  var input = str.split('');

  cb(input.map(skips));

  function skips(val, index, arr) {
    var res = [];
    var i = index;
    var iterator = i + 1;
    for (i; i < arr.length; i += iterator){
      res.push(arr[i]);
    }
    return res;
  }
}

skip('hello!', console.log);

Answer 1

I agree with most of what @Dair said, but disagree with polluting the global namespace with a skipHelper() that is of little value to anyone else.

I'd also make a few tweaks to the helper:

• It can be an anonymous function.
• I'd call the first parameter _ to indicate that it is unused.
• var i can be declared in the for loop header.
• extracted would be a slightly more descriptive name than res.

function skips(str) {
    return str.split('').map(function(_, index, array) {
        var extracted = [];
        for (var i = index; i < array.length; i += 1 + index) {
            extracted.push(array[i]);
        }
        return extracted.join('');
    });
}

Answer 2

Ok, first off: Passing in console.log as a paramater seems unnecessary. The function should have one task:

Generate the skips

Then once you have the skips you can do whatever you want with it. So:

function skip(str, cb) -> function skip(str)

and:

skip('hello!', console.log); -> console.log(skip('hello'));

I would also not use a function within a function. I would try keeping the functions relatively small. Split the functions:

function skipHelper(val, index, arr) {
  var res = [];
  var i = index;
  var iterator = i + 1;
  for (i; i < arr.length; i += iterator){
    res.push(arr[i]);
  }
  return res;
}

function skip(str) {
  var input = str.split('')
  return input.map(skips);
}

Also, given that str.split('') isn't too much to explicitly write out, change:

  var input = str.split('')
  return input.map(skips);

To just:

  return str.split('').map(skips);

Also, your code doesn't run exactly as described. Add a .join('') to return res; and this should fix it:

return res; -> return res.join('');

All in all:

function skipHelper(val, index, arr) {
  var res = [];
  var i = index;
  var iterator = i + 1;
  for (i; i < arr.length; i += iterator){
    res.push(arr[i]);
  }
  return res.join('');
}

function skip(str) {
  return str.split('').map(skipHelper);
}

console.log(skip('hello!'));

There are probably some Javascript language specific stylistic choices I am missing, but this is for general code structure.