Inserting null values at the original array indices - sorting

Question

I am sorting an array for an *ngFor on my Angular application. So far my array is being sorted on sortId value in ascending order. It then sorts the objects with sordId: null back into the array in its original indices position, result below:

Row 1 - Sort ID: null - original array position: 1
Row 2 - Sort ID: null - original array position: 2
Row 3 - Sort ID: null - original array position: 3
Row 4 - Sort ID: 1 - original array position: 8
Row 5 - Sort ID: null - original array position: 5
Row 6 - Sort ID: null - original array position: 6
Row 7 - Sort ID: 4 - original array position: 7
Row 8 - Sort ID: 5 - original array position: 4

.ts method

this.fieldsList = this.users
  .filter(x => x.sortId)
  .sort((a, b) => a.sortId - b.sortId);

this.users.forEach((x, i) => {
  if (x.sortId === null) this.fieldsList.splice(i, 0, x);
});

What I need it to do is for sortID to take priority over the null values. But the null values keep their original indice number/order unless there is a sortId that takes its place. So the example above will be the expected results below:

Row 1 - Sort ID: 1 - original array position: 8
Row 2 - Sort ID: null - original array position: 1
Row 3 - Sort ID: null - original array position: 2
Row 4 - Sort ID: 4 - original array position: 7
Row 5 - Sort ID: 5 - original array position: 4
Row 6 - Sort ID: null - original array position: 3
Row 7 - Sort ID: null - original array position: 4
Row 8 - Sort ID: null - original array position: 5

Do you want Sort ID to be equal to index + 1 in the output? Like, Sort ID: 1 is at index = 0, Sort ID: 4 is at 3 etc. — adiga
– adiga, Commented Jul 30, 2021 at 10:33
What I need it to do is for sortID to take priority over the null values. But the null values keep their original indice number/order unless there is a sortId that takes its place. — Tom Rudge
– Tom Rudge, Commented Jul 30, 2021 at 11:41
null - 1? the - is just a border in the row to signify another field — Tom Rudge
– Tom Rudge, Commented Jul 30, 2021 at 11:43

Harsh Mittal · Accepted Answer · 2021-07-30 21:54:16Z

1

This does the job the way you wanted:

ngOnInit(): void {
    this.fieldsList = [...this.users];
    Array(this.fieldsList.length)
      .fill('1')
      .forEach((_, index) => {
        let user;
        let oldIndex;
        this.fieldsList.some((u, i) => {
          if(+u.sortId === index + 1) {
            user = u;
            oldIndex = i;
            return true;
          }
          return false;
        })
        const newIndex = +user?.sortId - 1;
        this.fieldsList.splice(newIndex, 0, user);
        this.fieldsList.splice(oldIndex + 1, 1);
      });
  }

answered Jul 30, 2021 at 21:54

Harsh Mittal

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Comments

Bergi · Accepted Answer · 2021-07-30 23:27:38Z

You don't want a sorting algorithm. Your sortId simply specifies the index at which to place the item in the new array. Then fill up the rest with the null values. This can be done in linear time, without any .sort().

function reorder(items) {
  const result = new Array(items.length).fill(null);
  for (const item of items)
    if (item.sortId != null)
      result[item.sortId-1] = item;
  for (let i=0, j=0; i<result.length; i++) {
    if (result[i] == null) {
      while (items[j].sortId != null) j++
      result[i] = items[j++];
    }
  }
  return result;
}

const users = [
  {
    arrayPosition: 1,
    sortId: null
  },
  {
    arrayPosition: 2,
    sortId: null
  },
  {
    arrayPosition: 3,
    sortId: null
  },
  {
    arrayPosition: 4,
    sortId: '5'
  },
  {
    arrayPosition: 5,
    sortId: null
  },
  {
    arrayPosition: 6,
    sortId: null
  },
  {
    arrayPosition: 7,
    sortId: '4'
  },
  {
    arrayPosition: 8,
    sortId: '1'
  }
];

for (const [index, user] of reorder(users).entries())
  console.log(`Row ${index+1} - Sort ID: ${user.sortId} - original array position: ${user.arrayPosition}`);

Aviad P. · Accepted Answer · 2021-07-31 10:47:17Z

0

Here's another alternative using generators:

ngOnInit() {
  this.fieldsList = [...this.generator(this.users)];
}

*generator(users: any[]) {
  const sorted = users
    .filter(x => x.sortId)
    .sort((a, b) => +a.sortId - +b.sortId);

  let j = 0;
  for (let i = 0; i < users.length; i++) {
    if (users[i].sortId === null) {
      yield users[i];
    } else {
      yield sorted[j++];
    }
  }
}

edited Jul 31, 2021 at 10:47

answered Jul 31, 2021 at 10:33

Aviad P.

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