[tcortina@linux3 ~]$ cd private
[tcortina@linux3 ~]$ mkdir 15110
[tcortina@linux3 ~/private]$ ls
15110
[tcortina@linux3 ~/private]$ cd 15110
[tcortina@linux3 ~/private]$ mkdir lab2
[tcortina@linux3 ~/private]$ mkdir practice
[tcortina@linux3 15110]$ ls
lab2  practice
[tcortina@linux3 15110]$ cd practice
[tcortina@linxu3 practice]$ pwd
/afs/cs.cmu.edu/user/tcortina/private/15110/practice
[tcortina@linux3 practice]$ irb
irb(main):001:0> 15 + 110
=> 125
irb(main):002:0> 15 - 110
=> -95
irb(main):003:0> 15 * 110
=> 1650
irb(main):004:0> 15 / 110
=> 0
irb(main):005:0> 15.0 / 110.0
=> 0.136363636363636
irb(main):006:0> 15.0 / 110
=> 0.136363636363636
irb(main):007:0> 99 / 100
=> 0
irb(main):008:0> 1.0 / 3.0
=> 0.333333333333333
irb(main):009:0> 2.0 / 3.0
=> 0.666666666666667
irb(main):010:0> 7.0 / 9.0
=> 0.777777777777778
irb(main):011:0> 110 % 15
=> 5
irb(main):012:0> 42 % 18
=> 6
irb(main):013:0> 42 % 6
=> 0
irb(main):014:0> 17 % 2
=> 1
irb(main):015:0> 18 % 2
=> 0
irb(main):016:0> 19 % 2
=> 1
irb(main):017:0> 20 % 2
=> 0
irb(main):018:0> 15 % 110
=> 15
irb(main):019:0> 5 ** 3
=> 125
irb(main):020:0> 8 ** 2.4
=> 147.03338943962
irb(main):021:0> 9 ** 0.5
=> 3.0
irb(main):022:0> 9 ** (1/2)
=> 1
irb(main):023:0> 27 ** (1.0/3.0)
=> 3.0
irb(main):024:0> 5 * 6 + 7
=> 37
irb(main):025:0> 5 * (6 + 7)
=> 65
irb(main):026:0> 3 * 4 / 5
=> 2
irb(main):027:0> 3 * (4 / 5)
=> 0
irb(main):028:0> 2 ** 3 ** 4
=> 2417851639229258349412352
irb(main):029:0> (2 ** 3) ** 4
=> 4096
irb(main):030:0> x = 6
=> 6
irb(main):031:0> y = 14.5
=> 14.5
irb(main):032:0> z = 42 % 18
=> 6
irb(main):033:0> x
=> 6
irb(main):034:0> y
=> 14.5
irb(main):035:0> x + z
=> 12
irb(main):036:0> gravity = 9.8
=> 9.8
irb(main):037:0> interest_rate = 0.015
=> 0.015
irb(main):038:0> 7up = 2.39
SyntaxError: compile error
(irb):38: syntax error
7up = 2.39
   ^
        from (irb):38
        from :0
irb(main):039:0> courseFall2011 = 15110
=> 15110
irb(main):040:0> gravity
=> 9.8
irb(main):041:0> gravitY
NameError: undefined local variable or method `gravitY' for 

main:Object
        from (irb):41
        from :0
irb(main):042:0> base = 15
=> 15
irb(main):043:0> height = 110
=> 110
irb(main):044:0> triangle_area = 0.5 * base * height
=> 825.0
irb(main):045:0> base = 20
=> 20
irb(main):046:0> triangle_area = 0.5 * base * height
=> 1100.0
irb(main):047:0> base = 25
=> 25
irb(main):048:0> triangle_area
=> 1100.0
irb(main):049:0> triangle_area = 0.5 * base * height
=> 1375.0
irb(main):050:0> course = 15110
=> 15110
irb(main):051:0> course + 2
=> 15112
irb(main):052:0> course = "15110"
=> "15110"
irb(main):053:0> course + 2
TypeError: can't convert Fixnum into String
        from (irb):53:in `+'
        from (irb):53
        from :0
irb(main):054:0> s1 = " "
=> " "
irb(main):055:0> s2 = ""
=> ""
irb(main):056:0> male = true
=> true
irb(main):057:0> married = false
=> false
irb(main):058:0> f = 32
=> 32
irb(main):059:0> c = 5/9 * (f-32)
=> 0
irb(main):060:0> f = 80
=> 80
irb(main):061:0> c = 5/9 * (f-32)
=> 0
irb(main):062:0> c = (5.0/9.0) * (f-32)
=> 26.6666666666667
irb(main):063:0> side = 109
=> 109
irb(main):064:0> square = side * side
=> 11881
irb(main):065:0> triangle = 0.5 * side / 2 * side / 2
=> 1485.125
irb(main):066:0> area = square - triangle
=> 10395.875

At this point, I use gedit to create a 
Ruby file that contained one function
that computes the area of a countertop
that is in the shape of a square:

def compute_area(side)
    area = side * side
    return area
end

By saving the file as "countertop.rb",
keywords are highlighted in red. Constants in pink.

In this function definition, there is one
parameter, side. When we call this function,
we need to supply one value for the side.
It returns the answer back to wherever we
call the function (in this case, irb).

In irb again:

irb(main):067:0> load "countertop.rb"
=> true
irb(main):068:0> compute_area(15)
=> 225
irb(main):069:0> compute_area(110)
=> 12100
irb(main):070:0> x = 42
=> 42
irb(main):071:0> compute_area(x)
=> 1764
irb(main):072:0> compute_area(15) + compute_area(110)
=> 12325

Editing countertop.rb, we replace
the return statement with a print statement.

def compute_area(side)
    area = side * side
    print area, "\n"
end

The function now prints its answer 
along with a newline ("\n")
and returns nothing to wherever we call
the function.  So when we call the function
now, the function prints the answer but
irb displays "nil" since it got nothing
back from the function. This is fine unless
we want to use the answer in another computation
as shown below.

irb(main):073:0> load "countertop.rb"
=> true
irb(main):074:0> compute_area(15)
225
=> nil
irb(main):075:0> compute_area(110)
12100
=> nil
irb(main):076:0> compute_area(15) + compute_area(110)
225
12100
NoMethodError: undefined method `+' for 

nil:NilClass
        from (irb):6
        from :0

The reason the previous expression fails is that 
each call prints out its own answer, but neither answer
is returned back to irb (since there is no return statement)
so irb can't add the computed results together.

Using the Math module:

irb(main):077:0> Math.sqrt(16)
=> 4.0
irb(main):078:0> Math.log10(100.0)
=> 2.0
irb(main):079:0> Math::PI
=> 3.14159265358979
irb(main):080:0> Math.sin(Math::PI / 2)
=> 1.0
irb(main):081:0> include Math
=> Object
irb(main):082:0> sqrt(16)
=> 4.0
irb(main):083:0> PI
=> 3.14159265358979
irb(main):084:0> quit