and other languages…

puts “yes”
• // global, method of Kernel

Math.sqrt 2
• // object Math, method sqrt

message.length
• // object message, method length, no args

a.each {|x| puts x}
• // object a, method each, associated block

a[0] = a[1]
overloaded operators
• // a.[](0), object a, method []=, arg 0
• // a.[](1), object a,method [], arg 1

x+y
• // x.+(y)
Compare to other languages
 lvalue
= rvalue
 lvalue must be address, rvalue must be
value
x = 1
 x += 1 # but not x++.
• Abbreviated assignment pseudooperators
 x,y,z
= 1,2,3 # parallel assignment
 x=y=0 # chained assignment, right assoc
 MAX = 5; MAX = 6 # constant not really
constant… but Ruby gives warning


Goal: return array if it exists, or a new one
results = results || []
How does this work?
Remember short-circuit evaluation?
Caveat: lvalue must be nil or false

results ||= []

Example of a Ruby idiom
• English idioms: pulling my leg, spill the beans
• Programming: means of
expressing a recurring
construct or simple task. Knowing the idioms
associated with a programming language and how to use
them is an important part of gaining fluency in that
language (wikipedia)
 x,y
= y,x # swap
 x = y; y = x # sequential, both now same
value
 x = 1,2,3 # x = [1,2,3]
 x,y = [1,2] # same as x=1; y=2
 a,b,c = 1,2 # a=1; b=2; c=nil
 x, *y = 1,2,3 # x=1; y=[2,3]
• * is called splat, can only have one per
assignment
 *x, y
= 1,2,3 # x=[1,2]; y=3
infix - most
prefix - Scheme
unary (1 operand)
binary (2 operands)
operators
Typical associativity rules
• Left to right, except assignment
and **, which are right to left
• Sometimes unary operators
associate right to left
ternary (3 operands)
typical precedence:
• parentheses
• unary operators
• ** (if the language supports it)
• *, /, %
• +, -
• APL is different; all operators have equal precedence and all operators
associate right to left, precedence and associativity rules can be
overridden with parentheses
would this be more or less confusing?
1-6
 local
variables and method names both
start with lower case letter
 So how does Ruby know?
 If prior assignment, it’s a variable.
Otherwise, method invocation.
 Conditional Expressions
• C-based languages (e.g., C, C++)
• Also in Perl, JavaScript and Ruby
• An example:
average = (count == 0)? 0 : sum / count
• Evaluates as if written like
if (count == 0)
average = 0
else
average = sum /count
 Language
•
•
•
•
•
•
•
•
•
•
Concepts
overloaded operators
associativity
infix vs prefix
arity
precedence
lvalue/rvalue
parallel assignment
constants
short-circuit evaluation
conditional expressions
 Ruby
• Abbreviated
assignment
pseudooperators
• ||= idiom
• parallel assignment
options
• splat