Recursion Practice

recursion-practice.markdown

Recursion Practice

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script.js

// Clear console messages.
console.clear();

// Exercise 1: Write a recusrive sum function.
// sum will take an array of numbers and add every
// number in the array.
// i.e. sum([1,2,3,4]) returns 10 (1 + 2 + 3 + 4).
function sum(arr) {
  if (arr.length === 0 ) {
    return 0;
  }
  const secondToLastValue = arr.pop();
  const total = secondToLastValue + sum(arr);
  return total;
}

test('Sum', function(assert) {
  assert.equal(sum([1, 2, 3, 4, 5, 6]), 21);
  assert.equal(sum([]), 0);
  assert.equal(sum([0,0,0]), 0);
});

// Exercise 2: Write a recursive factorial function.
// factorial takes a single number and then computes
// the factorial of that number.
// i.e. factorial(4) returns 24 (1 * 2 * 3 * 4).
function factorial(n) {
  // needs handle negative cases
  if (n === 1 || n === 0) {
    return 1;
  }
  return n * factorial(n-1);
}

test('Factorial', function(assert) {
  assert.equal(factorial(0), 1);
  assert.equal(factorial(3), 6);
  assert.equal(factorial(5), 120);
  assert.equal(factorial(10), 3628800);
});

// Exercise 3: Write a recursive function that reverses a // string.
// reverseString takes in a string and returns the reverse
// of that string. i.e. reverseString('abc') returns
// 'cda'.
function reverseString(str) {
  let reversedStr ='';
  for (let i = str.length; i >= 0; i--) {
    reversedStr += str.charAt(i);
  }
  return reversedStr;
}

test('Reverse string', function(assert) {
  assert.equal(reverseString('abcdef'), 'fedcba');
  assert.equal(reverseString('racecar'), 'racecar');
  assert.equal(reverseString(''), '');
});

// Exercise 4: Write a recursive isEven function.
// isEven takes a number and returns true if it is even,
// false if it is odd. i.e. isEven(4) returns true,
// isEven(5) returns false.
function isEven(num) {
  if (num % 2 === 0) {
    return true;
  }
  return false;
}

test('Even', function(assert) {
  assert.isTrue(isEven(0));
  assert.isFalse(isEven(1));
  assert.isTrue(isEven(4));
  assert.isFalse(isEven(7));
  assert.isFalse(isEven(11));
  assert.isTrue(isEven(42));
  assert.isFalse(isEven(43));
});

// Exercise 5: Write a recursive range function with exclusive bounds.
// range takes a start and ending number. It will return
// and array with all numbers inbetween this start and
// end number, exluding them. i.e. range(2, 5) returns 
// [3, 4], range(5, 6) returns [].
function range(start, end) {
  const rangeArr = [];
  if (start > end) {
    return 'Sorry, starting value needs to be less than the end value.';
  }
  if(start === end || start + 1 === end) {
    return rangeArr;
  }
  for (var i = start+1; i< end; i++) {
    rangeArr.push(i);
  }
  return rangeArr;
}

test('Range', function(assert) {
  assert.deepEqual(range(3, 9), [4, 5, 6, 7, 8]);
  assert.deepEqual(range(2,2), []);
  assert.deepEqual(range(2,3), []);
  assert.deepEqual(range(2,4), [3]);
  // assert.deepEqual(range(4,2), []);
});

scripts

<script src="https://cdnjs.cloudflare.com/ajax/libs/chai/3.5.0/chai.min.js"></script>
<script src="https://rawgit.com/efuquen/798c7718f54e7204289fd307845f3e98/raw/d9fcf59f9202ed00e56b2ca97d5c0d7a238a407b/chai-assert-exercise-helper.js"></script>