Matrix Spiral Algorithm - JavaScript Algorithms

Directions: Write a function that accepts an integer N and returns a NxN spiral matrix.

Examples

matrix(2)[([1, 2], [4, 3])]
matrix(3)[([1, 2, 3], [8, 9, 4], [7, 6, 5])]
matrix(4)[([1, 2, 3, 4], [12, 13, 14, 5], [11, 16, 15, 6], [10, 9, 8, 7])]

Pseudocode:

Create empty array called results.
Create a counter variable called count and set it to 1.
As long as (start column <= end column) AND (start row <= end row)
- loop from start column to end column.
  - at results[start row][i] assign countvariable.
  - increment count by 1.
- increment start row by 1.
- loop from start row to end row.
  - at results[i][end column] assign countvariable.
  - increment count by 1.
- decrement end column by 1.
- ...repeat for other two sides.

This solution relies on variables start column, end column, start row, and end row. We keep track of these variables to know when to stop the loops.

We keep track of the index of character in a row or column.

We use a series of for loops to iterate through some start column and end column. You then insert some value into the row. When you reach the end of the row, you increment the row value.

First 'for loop' determines the top row.

The next 'for loop' determines the right column.

function matrix(n) {
  // create empty array
  const results = []
  // push the number of sub-arrays into results
  for (let i = 0; i < n; i++) {
    results.push([])
  }
  console.log(results)

  // keep track of the number we are pushing into results
  let counter = 1
  // index of the columns
  let startColumn = 0
  let endColumn = n - 1
  //index of rows
  let startRow = 0
  let endRow = n - 1

  while (startColumn <= endColumn && startRow <= endRow) {
    //Top row
    for (let i = startColumn; i <= endColumn; i++) {
      results[startRow][i] = counter
      counter++
    }
    startRow++
    // we are now done row 1

    // right column
    for (let i = startRow; i <= endRow; i++) {
      results[i][endColumn] = counter
      counter++
    }
    endColumn--

    // bottom row
    for (let i = endColumn; i >= startColumn; i--) {
      results[endRow][i] = counter
      counter++
    }
    endRow--

    // start column
    for (let i = endRow; i >= startRow; i--) {
      results[i][startColumn] = counter
      counter++
    }
    startColumn++
  }
  return results
}

console.log(matrix(3))