Programming in Lua - Homework 1

Feel free to run the following chunks in the interpreter, but it is better if you try to guess first what the output will be, from what you have learned in class.

1. What is the output of the following chunk, and why?

if nil then print ("true") else print ("false") end
if 0 then print ("true") else print ("false") end
if "" then print ("true") else print ("false") end
if false then print ("true") else print ("false") end

Answers:

false
true
true
false

Because only nil and false are false in Lua, everything else is true.

2. What is the output of the following chunk, and why?

a = 1
print(type(a))
a = print
print(type(a))
a("hello world!")

Answer:

number
function
hello world!

First a is a number, then we assign the value of the print function to it, which is a function. Then we call a, which actually calls the print function.

3. What is the output of the following chunk, and why?

function f()
  i = 10
end

function g()
  local i = 5
end

print(i)
f()
print(i)
g()
print(i)

Answer:

nil
10
10

In the first print the variable i does not exist, so its value is nil, then f assigns to 10 to the global variable i, and this is what the second print prints. Function g assigns 5 to a local variable i that is shadowing the global i, so the global i is unaffected, and the last print still prints 10.

4. Write a chunk that counts odd numbers backwards from 99 to 1 (99, 97, 95, …, 5, 3, 1), printing each number and them printing their sum. Do it in three ways: with a while loop, with a repeat loop, and using the numeric for.

Answers:

local i = 99
local s = 0
while i > 0 do
  print(i)
  s = s + i
  print(s)
  i = i - 2
end

local i = 99
local s = 0
repeat
  print(i)
  s = s + i
  print(s)
  i = i - 2
until i == 0

local s = 0
for i = 99, 1, -2 do
  print(i)
  s = s + i
  print(s)
end

5. Write a function quadratic(a, b, c) that returns the two solutions of the quadratic equation with coeficients a, b, and c. Use the quadratic formula. You can take the square root of a number in Lua with the math.sqrt function. Do not worry about the case where the input to the square root is negative.

function quadratic(a, b, c)
  local sqr_delta = math.sqrt(b * b - 4 * a * c)
  local two_a = 2 * a
  return (-b + sqr_delta)/two_a, (-b - sqr_delta)/two_a
end