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Problem Set 2

  1. Write a method that takes an array of numbers. If a pair of numbers in the array sums to zero, return the positions of those two numbers. If no pair of numbers sums to zero, return nil.
puts("\nTests for #two_sum")
puts("===============================================")
    puts(
      'two_sum([1, 3, 5, -3]) == [1, 3]: ' + (two_sum([1, 3, 5, -3]) == [1, 3]).to_s
    )
    puts(
      'two_sum([1, 3, 5]) == nil: ' + (two_sum([1, 3, 5]) == nil).to_s
    )
puts("===============================================")
  1. Write a method that takes an array of numbers in. Your method should return the third greatest number in the array. You may assume that the array has at least three numbers in it.
puts("\nTests for #third_greatest")
puts("===============================================")
    puts(
      'third_greatest([5, 3, 7]) == 3: ' +
      (third_greatest([5, 3, 7]) == 3).to_s
    )
    puts(
      'third_greatest([5, 3, 7, 4]) == 4: ' +
      (third_greatest([5, 3, 7, 4]) == 4).to_s
    )
    puts(
      'third_greatest([2, 3, 7, 4]) == 3: ' +
      (third_greatest([2, 3, 7, 4]) == 3).to_s
    )
puts("===============================================")
  1. Write a method that takes in a array of letters. Your method should return the most common letter in the array, and a count of how many times it appears.
puts("\nTests for #most_common_letter")
puts("===============================================")
    puts(
      'most_common_letter("abca".split("")) == ["a", 2]: ' +
      (most_common_letter('abca'.split("")) == ['a', 2]).to_s
    )
    puts(
      'most_common_letter("abbab".split("")) == ["b", 3]: ' +
      (most_common_letter('abbab'.split("")) == ['b', 3]).to_s
    )
puts("===============================================")
  1. Write a method that takes in an integer (greater than one) and returns true if it is prime; otherwise return false.

You may want to use the % modulo operation. 5 % 2 returns the remainder when dividing 5 by 2; therefore, 5 % 2 == 1. In the case of 6 % 2, since 2 evenly divides 6 with no remainder, 6 % 2 == 0. More generally, if m and n are integers, m % n == 0 if and only if n divides m evenly.

puts("\nTests for #is_prime?")
puts("===============================================")
    puts('is_prime?(2) == true: ' + (is_prime?(2) == true).to_s)
    puts('is_prime?(3) == true: ' + (is_prime?(3) == true).to_s)
    puts('is_prime?(4) == false: ' + (is_prime?(4) == false).to_s)
    puts('is_prime?(9) == false: ' + (is_prime?(9) == false).to_s)
puts("===============================================")
  1. Write a method that returns the nth prime number. Recall that only numbers greater than 1 can be prime.
puts("\nTests for #nth_prime")
puts("===============================================")
    puts('nth_prime(1) == 2: ' + (nth_prime(1) == 2).to_s)
    puts('nth_prime(2) == 3: ' + (nth_prime(2) == 3).to_s)
    puts('nth_prime(3) == 5: ' + (nth_prime(3) == 5).to_s)
    puts('nth_prime(4) == 7: ' + (nth_prime(4) == 7).to_s)
    puts('nth_prime(5) == 11: ' + (nth_prime(5) == 11).to_s)
puts("===============================================")