Slides
Numbers
The following operations work on numbers:
 +  addition
   subtraction
 *  multiplication
 /  division
 %  modulus
 **  exponentiation
LAB: Playing With Numbers
Answer the following questions using irb:
 How many seconds are in an hour?
 How many minutes are in a week?
 How many seconds old are you?
 How many years old is someone who is 1 billion seconds old?
Order of operations
Q: What is 1 plus 2 times 3?
Order of operations
Q: What is 1 plus 2 times 3?
A: It depends!
(1 + 2) * 3 == 9
1 + (2 * 3) == 7
Parentheses Are Free
When in doubt, use parentheses!
Strings vs. Numbers
Hmmm....
1 + 2
"1" + "2"
"1 + 2"
Strings plus Numbers
Hmmm again...
"1" + 2
Uhoh!
TypeError: can't convert Fixnum into String
The problem is that Strings and Numbers are different TYPES, aka different CLASSES.
Don't panic! The solution is easy.
Type Conversion
Numbers know a message that converts them into strings. to_s
means "to string".
"1" + 2.to_s
Likewise, strings know a message that converts them into numbers.
1 + "2".to_i
to_i
means "to integer".
Try this in irb!
Advanced Number Theory (optional)
WTFixnum?
The error said can't convert Fixnum into String
.
Q: What is a Fixnum?
A: It's one type of number.
Math is hard
There are many types of numbers!
Each is useful in different situations.
Without getting into too much detail, the two main number types in Ruby are:

Fixnum
 for integers like 12 or 1023 
Float
 for decimals like 3.14
(Other number types include Complex, Rational, and Bignum.)
Number to Number
You can convert from one type of number to another by sending a message:

to_i
turns a Float into a Fixnum 
to_f
turns a Fixnum into a Float
Try this:
3.to_f
3.14.to_i
String to Number
to_f
and to_i
also work on Strings:
"3.14".to_f
"3.14".to_i
and to_s
works on numbers:
3.14.to_s
Arithmetic
Try this in irb:
1 + 2
3  4
5 * 6
7 / 8
Whoa! What just happened?
Integer Arithmetic
7 and 8 are Integers
so the result is an Integer
7/8 is somewhere between 0 and 1
but there is no integer between 0 and 1
so the computer has to round down to 0
Floating Point Arithmetic
7.0/8.0
7.0 and 8.0 are Floats
so the result is a Float
and 0.875
can fit in a float
Okay, that's enough math!
Outline
 Numbers
 LAB: Playing With Numbers
 Order of operations
 Order of operations
 Parentheses Are Free
 Strings vs. Numbers
 Strings plus Numbers
 Type Conversion
 Advanced Number Theory (optional)
 WTFixnum?
 Math is hard
 Number to Number
 String to Number
 Arithmetic
 Integer Arithmetic
 Floating Point Arithmetic
 Okay, that's enough math!