Algorithm Pairwise

🚩 Remember to use Read-Search-Ask if you get stuck. Try to pair program πŸ‘₯ and write your own code πŸ“

🏁 Problem Explanation:

The program should look for pairs of numbers in the array whose sum equal the second argument arg. Then instead of adding those numbers up, their indices are to be added.

Remember that arrays start at index 0 and go from there so from [1,4,2,3,0,5] if we switch to their indices it would be [0,1,2,3,4,5]. Then, we add indices 1 + 2 + 3 + 5 and we get 11. That is what we need to return.

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πŸ’¬ Hint: 1

Remember to return the smaller sum if multiple are possible. This means ([1,1,1], 1) should use 0 + 1 instead of 0 + 1 & 1 + 1.

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πŸ’¬ Hint: 2

Try using an array of indices to track whether an index has been used or not.

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πŸ’¬ Hint: 3

It is easy to confuse indices as being numbers, but since you will be interacting with them, make sure to work with them as integers to prevent the code from behaving erratically.

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Spoiler Alert!

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Solution ahead!

πŸ”° Basic Code Solution:

function pairwise(arr, arg) {
 // Set sum of indices to zero
 var sum = 0;
 // make a local copy of the arguments object so we don't modify it directly
 var pairArr = arr.slice();
 // looping from first element
 for(i = 0; i < pairArr.length; i++) {
   //Looping from second element by setting first element  constant
   for(j = i + 1; j < pairArr.length; j++) {
     // Check whether the sum is equal to arg
     if(pairArr[i] + pairArr[j] == arg) {
       //Add the indices
       sum += i + j;
       //Set the indices to NaN so that they can't be used in next iteration
       pairArr[i] = pairArr[j] = NaN;
     }
   }
 }
 return sum;
}

// test here
pairwise([1,4,2,3,0,5], 7);

πŸš€ Run Code

Code Explanation:

  • The variable sum holds the sum of indices.
  • The outer for loop starts from the first element of arr.
  • The inner for loop starts from the second element of arr.
  • If the sum of an element and the element succeeding it is equal to arg:
    • The sum of the indices of these elements is added to sum.
    • These elements are set to NaN so that they’re not used in the next iteration.
  • After the loops are completed, the sum is returned.

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🌻 Intermediate Code Solution:

function pairwise(arr, arg) {
  // Create empty array to keep the arrays we will add.
  var index = [];

  // Loop to check the first number.
  for (var a in arr) {
    // temporal first number.
    var temp = arr[a];

    // Second loop to check against the first number.
    for (var i = 1; i < arr.length; i++) {
      // temporal second number.
      var temp2 = arr[i];

      // Key element, this check to make sure that the numbers add to arg
      // also that the second index is greater than the first, and that neither
      // of those indices are already on the array.
      if (temp + temp2 === arg && i > a && index.indexOf(+a) === -1 && index.indexOf(+i) === -1) {
        // if true then add both indices as integers then stop checking to avoid repeats.
        index.push(+a, +i);
        break;
      }
    }
  }

  // After the two loops are done, check if index is empty to return 0
  // or if it is not, then use Array.reduce(callbackFunc) to return the sum
  // of the numbers.
  if (index.length >= 1) {
    var addAll = function(a, b) {
      return a + b;
    };

    return index.reduce(addAll);
  } else
      return 0;
}

// test here
pairwise([1,4,2,3,0,5], 7);

πŸš€ Run Code

Code Explanation:

  • First, an empty array index is created to store the indices that will be added.
  • The outer loop gets the first number.
  • The inner loop gets the second number.
  • The following has to be made sure:
    • The two numbers add to arg that was passed as a parameter to the function.
    • The index from the second loop is greater than the one from the first loop. This avoids adding wrong indices.
    • The indices are not already part of the index array.
  • If all the conditions are true, the two indices are added as integers using + or parseInt(). The inner loop is hence stopped; everything else would be redundant.
  • After all the loops are over, it is checked if index is empty:
    • If it is empty, then 0 is returned.
    • Otherwise, the sum of all the integers in it is returned. This is done using the reduce() method.

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🚨 Advanced Code Solution:

function pairwise(arr, arg) {
  // search array for elements that when paired, equal the second argument, then sum their indices
  // make a local copy of the arguments object so we don't modify it directly
  var pairArr = arr.slice();
  return pairArr.reduce( function (a,b,index){ // use native reduce to collect running total of summed indices
      var search = arg - b; // get difference of current item so we know what value will sum to arg

      // check if search value in rest of the array, but also make sure it doesn't match current search index
      if ( pairArr.indexOf(search) != -1 && pairArr.indexOf(search) != index ){ 
         var total = index + pairArr.indexOf(search);  // if found, add to the runnning total
         pairArr.splice(index,1,NaN); // remove current index from the array
         pairArr.splice(pairArr.indexOf(search),1,NaN); // remove the other matched element from the array
         return a + total; //return the running total back to reduce for next item
      }
      return a; // simply return previous total if no operations needed
  },0);
  
}

// test here
pairwise([1,4,2,3,0,5], 7);

πŸš€ Run Code

Code Explanation:

See comments in code This code takes advantage of the fact that the native Array.prototype.indexOf() method will return the lowest index of the value it finds, a requirement of the challenge. Given that you start with the first item in the array (automatically the lowest of it’s value), you’re guaranteed to always find the lowest pairs, before removing them from the search space.

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πŸ† Credits:

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Thanks @Rafase282 @coded9 @SaintPeter @geligelu for your help with Algorithm: Pairwise

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