App Academy Prep

Question 1

Add, subtract, multiply, and divide numbers

Answer 1

3 + 3 == 6
3 * 4 == 12
4 - 3 == 1
8 / 2 == 4

Question 2

create variable, assign it 3

Answer 2

my_favorite_number = 3

Question 3

# recall my_favorite_number, use it in an expression, save it
# to a variable

Answer 3

my_favorite_square = my_favorite_number * my_favorite_number

Question 4

# block of code that will take in a number, and return its
# square

Answer 4

def square(num)
  # squares num; the last expression in a method gets returned
  # implicitly
  num * num
end

square(3) == 9

Question 5

# search for `target_item` in `items`; return the array index
# where it first occurs

Answer 5

def find_item(items, target_item)
  i = 0
  while i < items.count
    current_item = items[i]
    if current_item == target_item
      # found item; return its index; stop here and exit the
      # method
      return i
    end
    i += 1
  end

Question 6

return nil to mean item not found

Answer 6

nil

end

Question 7

Use if, else, elsif to describe negative, small, and big numbers

Answer 7

def how_big(num)
  if num < 0
    "negative"
  elsif num < 10
    "pretty small"
  elsif num < 100
    "not too big"
  elsif num < 1000
    "plenty big"
  else
    "way too big"
  end
end

Question 8

What is an array?

Answer 8

Arrays are lists of items.

Question 9

How do you create an empty array

Answer 9

empty_array = []

Question 10

Create an array with some_prime numbers

Answer 10

some_primes = [2, 3, 5, 7]

Question 11

some_primes = [2, 3, 5, 7]

fetch 2 and 3 from this array

Answer 11

some_primes[0] == 2 # first prime

some_primes[1] == 3 # second prime

Question 12

Set the first element of the array to 2

Answer 12

some_primes[0] = 2 # set the first element of the array to 2

Question 13

Add an element to the end of an array

Answer 13

some_primes &laquo_space;11 # add the next prime to the array

Question 14

get the number of items in an array

Answer 14

[2, 3, 5, 7].count == 4

Question 15

What is a while loop?

Answer 15

A while loop repeatedly executes a block as long as its “condition” is true

Question 16

How do you loop through items in an array?

Answer 16

To loop through the items in an array, use an “index variable” (often named i) and go one-by-one through the positions in the array.
i = 0
while i < items.count
current_item = items[i]

# Do something with current_item here…

i += 1
end

Question 17

Q: Write a method, pow, that takes two (non-negative, integer) numbers, base and exponent and returns base raised to the exponent power. (No fair using Ruby’s base ** exponent notation!).

Answer 17

def pow(base, exponent)
  result = 1

  i = 1
  while i <= exponent
    result = result * base
    i += 1
  end

result
end

Question 18

Q: Write a method, sum which takes an array of numbers and returns the sum of the numbers.

Answer 18

def sum(nums)
  total = 0

i = 0
while i < nums.count
total += nums[i]

i += 1   end

# return total
total
end

Question 19

Q: Write a method, is_prime?, that takes a number num and returns true if it is prime and false otherwise.

You may wish to use the modulo operation: 5 % 2 returns the remainder when dividing 5 by 2: 1. If num is divisible by i, then num % i == 0. (You would not be expected to already know about modulo for the challenge)

Answer 19

Works for values greater than 1

def is_prime?(num)
  i = 2
  while i

Question 20

Q: Using your is_prime? method, write a new method, primes that takes a (non-negative, integer) number max and returns an array of all prime numbers less than max.

Answer 20

def primes(max)
  primes_arr = []

  i = 2
  while i < max
    if is_prime?(i)
      # i is prime; add it to the array
      primes_arr << i
    end

i += 1   end

# return primes_arr
primes_arr
end