###NoMethodError

NoMethodError: undefined method `hello` for main:Object

Means that we tried to invoke the hello method, which has yet to be defined

###Wrong Number of Arguments - ArgumentError

ArgumentError
wrong number of arguments (2 for 1)
exercise.rb:1:in `hello'

exercise_spec.rb:5:in 'block (2 levels) in <top (required)>'

This tells us that the method hello is being invoked with 2 arguments but the method definition only takes 1

###NameError

NameError
undefined local variable or method `firs' for #<RSpec::Core::ExampleGroup::Nested_1:0x007f30f9b42360>
exercise.rb:2:in `hello'

exercise_spec.rb:5:in 'block (2 levels) in <top (required)>'

Telling us that there is most likely a typo in one of our variables or the variable is undefined

###TypeError

def add(a,b)
  a + " + " + b + " = " + (a + b)
end

Specs:

describe "add" do
  it "returns a string with 1 and 2 added" do
    expect( add(1,2) ).to eq("1 + 2 = 3")
  end
  it "returns a string with 5 and 7 added" do
    expect( add(5,7) ).to eq("5 + 7 = 12")
  end
end

Error given:

TypeError
String can't be coerced into Fixnum
exercise.rb:2:in `+'

exercise.rb:2:in `add'

exercise_spec.rb:5:in `block (2 levels) in <top (required)>'

Tells us that the string can't be converted into a number, try using string interpolation such as:

def add(a,b)
  "#{a} + #{b} = #{a + b}"
end

###syntax error

def hello
  "Hello World"

Error given

exercise_spec.rb:2:in `require': exercise.rb:2: syntax error, unexpected end-of-input, expecting keyword_end (SyntaxError) from exercise_spec.rb:2:in `<top (required)>'

syntax error's tells us we are missing or have an extra keyword, or character somewhere. In this case we are missing end in our hello method.