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Challenge: Number Range Using Recursion

Instructions

Write a function called numberRange that takes in a startNum and an endNum and returns an array of numbers from startNum to endNum, inclusive. Be sure to use recursion in your solution.

Function Signature

/**
 * Returns an array of numbers from startNum to endNum, inclusive.
 * @param {number} startNum - The starting number.
 * @param {number} endNum - The ending number.
 * @returns {number[]} - An array of numbers.
 */
function numberRange(startNum: number, endNum: number): number[];

Examples

numberRange(1, 5); // should return [1, 2, 3, 4, 5]
numberRange(3, 10); // should return [3, 4, 5, 6, 7, 8, 9, 10]
numberRange(7, 7); // should return [7] (only one number)

Hints

• You can construct the array by first calling numberRange on a smaller range and then adding the endNum to the array.

Solutions

Click For Solution

function numberRange(startNum, endNum) {
  if (startNum === endNum) {
    return [startNum];
  }

  const numbers = numberRange(startNum, endNum - 1);
  numbers.push(endNum);
  return numbers;
}

Explanation

• We first add our base case and check if the startNum is equal to the endNum. If so, we return the startNum in an array.
• For the recursive case, we set the variable numbers to the function with the startNum and one less than the endNum.
• Then we add the endNum to the numbers array and return it.

That is the gist of it, but let's go step by step for numberRange(1, 5)

Step 1: Since startNum(1) is not equal to endNum(5), we move to the recursive block.

Step 2: We make a recursive call to numberRange(1, 4), which will further call numberRange(1, 3) and so on, until we reach the base case.

Step 3: The base case is reached when startNum(1) becomes equal to endNum (1). At this point, the function returns an array containing just the number 1.

Step 4: The unwinding begins. The result of numberRange(1, 2) is now an array [1, 2]. Then, the result of numberRange(1, 3) is [1, 2, 3], and so on, until we finally get the result of numberRange(1, 5) as [1, 2, 3, 4, 5].

The function follows the same recursive process to build the array of numbers, starting with the base case and adding numbers one by one as it unwinds the recursive calls.

Test Cases

test('Calculating the range of numbers', () => {
  expect(numberRange(1, 5)).toEqual([1, 2, 3, 4, 5]);
  expect(numberRange(3, 10)).toEqual([3, 4, 5, 6, 7, 8, 9, 10]);
  expect(numberRange(7, 7)).toEqual([7]);
});