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and other languages…

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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

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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

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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

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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

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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) *, /, % +, -

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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.

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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

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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

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Copyright © 1998 by Addison -Wesley Longman, Inc. 1 Chapter 6 Arithmetic Expressions - Their evaluation was one of the motivations for the development.

Copyright © 1998 by Addison -Wesley Longman, Inc. 1 Chapter 6 Arithmetic Expressions - Their evaluation was one of the motivations for the development.

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