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] // a.[](0), object a, method []=, arg 0 // a.[](1), object a,method [], arg 1  x + y // x.+(y) Compare to other languages overloaded operators

 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 || []  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) How does this work? Remember short-circuit evaluation? Caveat: lvalue must be nil or false

 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

1-6 operators unary (1 operand) binary (2 operands) ternary (3 operands) infix - most prefix - Scheme Typical associativity rules Left to right, except assignment and **, which are right to left Sometimes unary operators associate right to left 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? typical precedence: parentheses unary operators ** (if the language supports it) *, /, % +, -

 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