1. More Lua

2. Expressions Arithmetic operators */-+ “real” numbers only ^ exponentiation a%b == a - floor(a/b)*b x%1 gives fractional part, x-x%1 integer part angle = (angle + delta) % (2*math.pi) keeps angle in [0,2π)

3. Relational operators = == ~= return true or false if values are different types, they are not equal compares as references tables, fucntions, userdata -- must be ref to same object to be equal ordering (, =) defined only for strings and numbers other types only compare for equality 2<15 ~= “2” < “15”. 2 < “15” gives error

4. Logical operators - and, or not false and nil are false, anything else is true short circuit evaluation and returns first argument if false, else second argument. or returns first argument if true, else second argument. ex) type(v)==”table” and v.tag=value ex) x=(x or v) --assigns v to x if x is not intialized ex) a and b or c -- same as a?b:c in C/C++ max = (x>y) and x or y ‘and’ has higher precedence than ‘or’ not always returns true or false not 0 == true not nil == true

5. String Concatenation.. print(‘Hello ‘.. ‘World’) Precedence (if in doubt, use parens!) ^ not # - (unary) * / % + -.. = ~= == and or

6. Table Constructors expressions that create and initialize tables ex) species={‘Acamarian’, ‘Andorian’, ‘Denebian’, ‘Antaran’, ‘Klingon’,‘Human’} first value has index 1, not 0 ex) ship={ammo=100, health=100} same as: ship={}; ship.ammo=100; ship.health=100 constructor just initializes and creates a new table (all tables are same type) ex) linked list stack! create one with lines from a file list = nil for line in io.lines() do list = {next=list, value=line} traverse it local p = list while p do print ( p.value ) p = p.next end

7. can mix keyed values and lists in constructor ex) polyline = {color=‘blue’, thickness=2, npoints = 4, {x=0, y = 0}, {x=-10, y=0}, {x=-10, y=1}, {x=0, y=1} } print(polyline[2].x) --> -10 can’t use constructors for keys that are negative numbers or non-identifier strings. for those,use this syntax: ex) expletives={[“$^%”] = ‘golly’, [“-1”] = ‘oops’, [“#%*”] = ‘shucks’} {x=0,y=0} same as {[“x”]=0, [“y”]=0} can use ; instead of, as separator can have one after the last entry (easier metaprogramming)

8. Statements Assignment: changing the value of a variable or table field multiple assignment a,b = 10, 2*x x,y = y,x -- swaps x and y since rhs evaluated before any assignments if rhs list shorter than lhs list, rest get nil if lhs list shorter than right, extra values discarded functions can return multiple values

9. Local variables and Blocks lunch = 10 --global variable local snack = ‘tribble’ -- local variable scope of local = block (body of control structure) or chunk x = 10 local i = 1 while i <= x do local x = i*2 print(x) i = i + 1 end if i > 10 then local x x = 20 print (x+2) -- would print 22 else print (x) -- 10 (global) end print (x) -- 10 (global)

10. can create blocks with do and end do local x = r*cos(theta) local y = r*sin(theta) dropBomb(posx+x, posy+y) end common idiom: init a local with a global of the same name (faster, can change it without side effects) local foo = foo

11. Control Structures If then else if age<21 then booze=0 end if weight > 200 then exercise() hungry = 0 end elseif useful ( no switch stmt!), only needs one ‘end’ if collar == ‘up’ then spaz = spaz + 2 elseif collar == ‘buttondown’ then spaz = spaz + 1 elseif collar == ‘missing’ then spaz = spaz - 1 elseif collar == ‘lipstick’ then spaz = spaz - 2 end

12. while -- loops until condition is false a={0,1,2,3,4,5,'hahah', 6, 7} local i = 1 while a[i] do print (a[i]) i = i + 1 end

13. repeat-until -- post-test loop (1 or more iterations) scope of locals includes the condition -- print first non-empty line repeat line = io.read() until line ~= '' print (line) x = 99 local sqr = x/2 repeat sqr = ( sqr + x/sqr) / 2 local error = math.abs ( sqr^2 - x ) until error < x/ 10000 -- ‘error’ in scope here print ('x',x) print ('sqr',sqr)

14. for numeric for for var=initval, testval, step do end step optional (default=1) all vals evaluated ONCE at start of loop control variable is a local, only visible in body never change the value of the control var! use break to exit loop early

15. Generic for executes body once for each value of an iterator function a={'dog','cat',5, {1,2,3}, shoppinglist='socks', setdate=function(dt) date=dt end} for i,v in ipairs(a) do print (i, v) end print ('keys=') for k in pairs(a) do print (k) end

16. break and return break to exit the innermost loop its within return returns a value from a function (or just exits it if no value returned) break and return must be the last statement of a block for i,v in ipairs(a) do print (i, v) if v == 5 then break end printKeys = function (t) print ('keys until 5=') for k in pairs(t) do print (k) if ( t[k] == 5 ) then return end printKeys(a)

17. Functions ex) print ( ‘a’, a ) args in parentheses exception: if a single argument, and its a literal string or a table constructor, parens not needed ex) print “Hello,World” ex) f {x=0,y=10} special syntax for OOP o:foo(x) same as o.foo(o,x)

18. function definition parameters are local variables a={'dog','cat',5, {1,2,3}, shoppinglist='socks', setdate=function(dt) date=dt end} function summation (t) local sum = 0 for k in pairs(t) do print (k) if type(k)=='number' then sum = sum + k end return sum end sum = summation(a) print ('sum = ', sum)

19. if fewer args than params, extra params get nil if more args than params, extra args discarded ex) default argument idiom function incrCount (n) n = n or 1 count = count + n end can return multiple values, list them after return

20. variable number of arguments function summation (...) local sum = 0 for i,v in ipairs{...} do print (v) if type(v)=='number' then sum = sum + v end return sum end sum = summation(1,2,3,4) print ('sum = ', sum) function argsim(... ) local a, b, c =... print (a,b,c) return a,b end function foo1 (... ) print ('calling foo with args',... ) foo (... ) end

21. named arguments Lua uses positions to match args with params to get named args with default values, pass a table to the function, and write the function to extract the named args from the table function foo (options) _foo ( options.x or 0, options.y or 0, options.width or 1, options.background or “white”) end

22. More About Functions Lua functions are first-class values - can be stored in variables, tables, passed as args, etc. like any other values have “lexical scoping” can access variables of their enclosing functions are anonymous the syntax function foo(x) return 2*x end is syntactic sugar for foo = function(x) return 2*x end

23. can pass an anonymous function to another function as a parameter ex) table.sort(atable, function (a,b) return (a.name > b.name) end) a “higher-order function” is a function taking a function as an parameter ex) function derivative ( f, delta ) delta = delta or 1e-4 return function (x) return(f(x+delta) - f(x))/delta end c = derivative(math.sin) print(cos(10), c(10))

24. Closures when a function b() is written inside a function a(), the inner function (b) has access to the variables of the enclosing function (a) even after the enclosing function exits! the function retains a reference to the variable, so if it was a local (to a()) it will not be deleted as long as b() exists. the variable so referenced is called “non-local” or a “Lua upvalue”

25. closure example function newCounter() local i = 0 return function () i = i+1 return i end c1 = newCounter() print(c1()) c2 = newCounter() print(c2()) print(c1()) print(c2())

26. closures are useful for callback functions function digitButton(digit) return Button{ label=tostring(digit) action = function() add_to_display(digit) end }

27. can redefine existing function with closures do localoldSin = math.sin localk = math.pi/180 math.sin = function (x) return oldSin(x*k) end end