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(* This is a comment *)

(* In Mathematica instead of using these comments you can create a text cell
   and annotate your code with nicely typeset text and images *)

(* Typing an expression returns the result *)
2*2              (* 4 *)
5+8              (* 13 *)

(* Function Call *)
(* Note, function names (and everything else) are case sensitive *)
Sin[Pi/2]        (* 1 *)

(* Alternate Syntaxes for Function Call with one parameter *)
Sin@(Pi/2)       (* 1 *)
(Pi/2) // Sin    (* 1 *)

(* Every syntax in WL has some equivalent as a function call *)
Times[2, 2]      (* 4 *)
Plus[5, 8]       (* 13 *)

(* Using a variable for the first time defines it and makes it global *)
x = 5            (* 5 *)
x == 5           (* True, C-style assignment and equality testing *)
x                (* 5 *)
x = x + 5        (* 10 *)
x                (* 10 *)
Set[x, 20]       (* I wasn't kidding when I said EVERYTHING has a function equivalent *)
x                (* 20 *)

(* Because WL is based on a computer algebra system, *)
(* using undefined variables is fine, they just obstruct evaluation *)
cow + 5          (* 5 + cow, cow is undefined so can't evaluate further *)
cow + 5 + 10     (* 15 + cow, it'll evaluate what it can *)
%                (* 15 + cow, % fetches the last return *)
% - cow          (* 15, undefined variable cow cancelled out *)
moo = cow + 5    (* Beware, moo now holds an expression, not a number! *)

(* Defining a function *)
Double[x_] := x * 2    (* Note := to prevent immediate evaluation of the RHS
                          And _ after x to indicate no pattern matching constraints *)
Double[10]             (* 20 *)
Double[Sin[Pi/2]]      (* 2 *)
Double @ Sin @ (Pi/2)  (* 2, @-syntax avoids queues of close brackets *)
(Pi/2) // Sin // Double(* 2, //-syntax lists functions in execution order *)

(* For imperative-style programming use ; to separate statements *)
(* Discards any output from LHS and runs RHS *)
MyFirst[] := (Print@"Hello"; Print@"World")  (* Note outer parens are critical
                                                ;'s precedence is lower than := *)
MyFirst[]                                    (* Hello World *)

(* C-Style For Loop *)
PrintTo[x_] := For[y=0, y<x, y++, (Print[y])]  (* Start, test, incr, body *)
PrintTo[5]                                     (* 0 1 2 3 4 *)

(* While Loop *)
x = 0; While[x < 2, (Print@x; x++)]     (* While loop with test and body *)

(* If and conditionals *)
x = 8; If[x==8, Print@"Yes", Print@"No"]   (* Condition, true case, else case *)
Switch[x, 2, Print@"Two", 8, Print@"Yes"]  (* Value match style switch *)
Which[x==2, Print@"No", x==8, Print@"Yes"] (* Elif style switch *)

(* Variables other than parameters are global by default, even inside functions *)
y = 10             (* 10, global variable y *)
PrintTo[5]         (* 0 1 2 3 4 *)
y                  (* 5, global y clobbered by loop counter inside PrintTo *)
x = 20             (* 20, global variable x *)
PrintTo[5]         (* 0 1 2 3 4 *)
x                  (* 20, x in PrintTo is a parameter and automatically local *)

(* Local variables are declared using the Module metafunction *)
(* Version with local variable *)
BetterPrintTo[x_] := Module[{y}, (For[y=0, y<x, y++, (Print@y)])]
y = 20             (* Global variable y *)
BetterPrintTo[5]   (* 0 1 2 3 4 *)
y                  (* 20, that's better *)

(* Module actually lets us declare any scope we like *)
Module[{count}, count=0;        (* Declare scope of this variable count *)
  (IncCount[] := ++count);      (* These functions are inside that scope *)
  (DecCount[] := --count)]
count              (* count - global variable count is not defined *)
IncCount[]         (* 1, using the count variable inside the scope *)
IncCount[]         (* 2, incCount updates it *)
DecCount[]         (* 1, so does decCount *)
count              (* count - still no global variable by that name *)

(* Lists *)
myList = {1, 2, 3, 4}     (* {1, 2, 3, 4} *)
myList[[1]]               (* 1 - note list indexes start at 1, not 0 *)
Map[Double, myList]       (* {2, 4, 6, 8} - functional style list map function *)
Double /@ myList          (* {2, 4, 6, 8} - Abbreviated syntax for above *)
Scan[Print, myList]       (* 1 2 3 4 - imperative style loop over list *)
Fold[Plus, 0, myList]     (* 10 (0+1+2+3+4) *)
FoldList[Plus, 0, myList] (* {0, 1, 3, 6, 10} - fold storing intermediate results *)
Append[myList, 5]         (* {1, 2, 3, 4, 5} - note myList is not updated *)
Prepend[myList, 5]        (* {5, 1, 2, 3, 4} - add "myList = " if you want it to be *)
Join[myList, {3, 4}]      (* {1, 2, 3, 4, 3, 4} *)
myList[[2]] = 5          (* {1, 5, 3, 4} - this does update myList *)

(* Associations, aka Dictionaries/Hashes *)
myHash = <|"Green" -> 2, "Red" -> 1|>   (* Create an association *)
myHash[["Green"]]                       (* 2, use it *)
myHash[["Green"]] := 5                  (* 5, update it *)
myHash[["Puce"]] := 3.5                 (* 3.5, extend it *)
KeyDropFrom[myHash, "Green"]            (* Wipes out key Green *)
Keys[myHash]                            (* {Red, Puce} *)
Values[myHash]                          (* {1, 3.5} *)

(* And you can't do any demo of Wolfram without showing this off *)
Manipulate[y^2, {y, 0, 20}] (* Return a reactive user interface that displays y^2
                               and allows y to be adjusted between 0-20 with a slider.
                               Only works on graphical frontends *)