Understanding SS13 code

Other related guides: SS13 for experienced programmers and Guide to mapping
NOTE!! This guide uses terms very loosely. Experienced coders, please don't get at me over what an object is and what a method and so forth. This guide is intended solely for the new coder. The rest of you... don't read it as you'll only get mad at me.
==DreamMaker==
The program you will need for both coding and mapping is made out of six major components. The file tree, the object tree, the text editor, the icons editor, the map editor and the error report panel.
=== File tree ===
The file tree can be accessed by choosing the 'File' tab in the left-hand vertical panel. It shows the files which make up the code as they are sorted by folders. Double clicking any file will open it in it's respective editor. If you add a new file to the list (in windows explorer) hit the refresh button to refresh the list. The ticks next to files indicate if they're used in the compiling of the code. A common mistake is to forget to include a new file.
=== Objects tree ===
What an object is is explained further down in this guide, but for now we can say it contains a list of things, which you can place on the map. They are arranged in a hierarchy. This is also explained and described further down in the guide. The update button at the bottom will update the tree with new sprites and objects, if they have been added in any of the ticked .dm files.
=== Text editor ===
This editor is accessed whenever you open a file with the extension ".dm". It is intended for the writing and editing of the game code.
=== Icons editor ===
Intended for the editing of icon states and icons, it is accessed when you open a ".dmi" file. It displays a list of all the sprites (often called icon states) which are in the file. Rightclicking anywhere will yield the options to create a new movie or pixmap. A pixmap is a static image, while a movie is either an icon state which can point in several different directions (often called dirs) or is animated. The movie can have either 1 (south), 4 (NSEW) or 8 directions (full compass). Also note that a .dmi file may only have icons of a certain size. In the case of the singularity, which grows as it gains power, several .dmi files are needed.
=== Map editor ===
Usually opened at startup or when opening a ".dmm" map file, the map editor displays the map. By default it displays all 4 major groups: area, mob, object and turf. To choose the specifics of what to display, hit the layers tab in the top-most toolbar and select 'Only show selectable layers' and then choose which layers you want to be shown in the same (layers) tab. (Most commonly used to hide the areas overlay). Note that the map will not display if there are any errors in the .dm code. Correct any errors and then recompile it.
=== Error report panel ===
A text-based panel at the bottom of DreamMaker, the error report panel is intended to provide compiling error reports.
=== Compile and run ===
To compile updated code without running it, select the 'build' tab in the uppermost toolbar and shoose 'compile'. To compile and run hit the 'run' button in the same tab.
==Code components==
===Variables===
reference
Variables are intended to store data. Variables are created like this: (creates variable with no defined value)
var/i
To define a value in declaration do this:
 var/i = 5
or
 var/i = "Hello World"
Once a variable has been defined, you cannot define another one with the same name:
 var/i
 i = 2
 i = 6
 i = 12
==== List ====
reference
A list can be defined as either of the three, but special variables associated with lists (such as len for length) will only be available if you use the first declaration.
 var/list/a
 var/a[9]
 var/a = list()
==== Other Types ====
If you wish to store a coin in somewhere in the variables of an object and use the coin's defined procs or variables, you'll have to define the variable in which you store the coin as a coin. The second example also creates a variable called D and sets it to a new coin.
 var/obj/item/weapon/coin/C
 var/obj/item/weapon/coin/D = new/var/obj/item/weapon/coin(src)
==== Included variables ====
Variables which are built into byond itself and are not defined anywhere in code:
Atom vars
Client vars
Datum vars
Mob vars
===== Direction (dir) var =====
reference
North: 1
South: 2
East: 4
West: 8
Northeast: 5 (1 + 4)
Southeast: 6 (2 + 4)
Northwest: 9 (1 + 8)
Southwest: 10 (2 + 8)
They are numbered like this because dir uses the 'bitflag' methodology. You can read more about bitflags here. It is defined in binary, so...
0001 is north
0010 is south
0100 is east
1000 is west
Combining these numbers yields north-east (0101), north-west (1001), south-east (0110) and south-west (1010). Tho other combinations (east-west(1100), north-south (0011), north-east-west (1101) and such) are possible for special uses. Smoothwall code is an example.
===== Atom vars =====
These apply to all /obj, /turf, /area, /mob -type objects.
Contents: List of objects which are inserted into another object. (plasma tanks in radiation arrays, ore in the smelter, etc.)
Density: 0/1 - 0 means your mob can pass through (or over), 1 means you cannot
Desc: string - Description, displayed upon examine
Dir: 1-10 - the direction the object is facing
Icon: The .dmi file which contains the sprite for the icon
Icon_state: The name of the sprite in the file from Icon
Overlays: A list of images which are overlayed on the item.
Layer: If two objects are on the same tile and one has a higher layer number than the other, the one with the higher will be shown as above the one with the lower
Loc: The X,Y,Z positioning of an item
Luminosity: How much does it glow?
Name: The name, displayed when you hover your mouse over the item
Opacity: Can you see through it?
Pixel_x, pixel_y: Set if the item on the map is nudged a few pixels to either side. Used with APC's, Request Consoles, Fire alarms, etc.
Type: Type is the path of the object. A coin would have it's type variable set to: /obj/item/weapon/coin
=== Included instructions ===
Reference guides for included instructions (proc-s) can be found at the following links:
Area procs
Mob procs
Obj procs
Turf procs
=== Conditionals ===
Conditionals are statements which determine how the code will be executed, depending on a condition. The most common conditional is the IF statement
==== = vs. == ====
 var/a
 a = 14
 if (a == 14)
 world << "A has the value [a]"
 else
 world << "A is not 14"
As in the example above, the single = means you assign the value on the right to the variable on the left. In the example above, the variable a was assigned the value 14 (a = 14)
The double == is used to compare two values. It's most commonly used in the if statement. In the example above you can see we compared the variable a to 14 (a == 14). This determines how the if will react.
Comparison Operators
reference
The example shows a simple if statement. If statements work by first checking if the statement in the brackets, in the case above a == 14, is true or not. If it's true, it will proceed to execute the code, which is further indented from the if (in the case above: world << "A has the value [a]"). In the other case, if the statement is not true, it will jump to the else statement and execute the code, which is indented from that. It will only jump to the else statement if one is present. In the example, if a was not 14, it would execute world << "A is not 14"
Comparison operators include:
== Is Equal To
!= Is Not Equal To
< Less than
> More than
<= Less or equal
>= More or equal
==== Switch-case statement ====
reference
=== Loops ===
==== While ====
reference
==== For ====
reference
==== For (foreach) ====
reference
=== Procs ===
reference
==What is an object?==
SS13 is coded in Byond, which is an object oriented programming language. Please note that by object here i do not mean an item or machine in-game. For this intro alone i will speak of objects as I'll define them here.
An object can be any defined path in the game. Examples include:
 /obj/item/weapon/sword
 /turf/simulated/floor
 /area/
 /atom/
An object is made out of both variables and procs
Here's an example of an object:
 /turf/simulated/gold_spot
 var/spawned_gold = 0
 name = "Gold Area"
 desc = "This plot may have spawned gold!"
 
 /turf/simulated/gold_spot/proc/spawn_gold()
 if(prob(50))
 new/obj/item/stack/sheet/gold(src)
 spawned_gold = 1
 
 /turf/simulated/gold_spot/New()
 ..()
 spawn_gold()
=== Hierarchy and Inheritance ===
Objects inherit all it's parent's variables and procs. This means that an object defined as /obj/item/weapon/storage/box has all the variables of /obj/item/weapon/storage, /obj/item/weapon, /obj/item, /obj and also /atom, but we'll get to that one later. Some procs are already defined for all objects, you can see these here. In the above example, New() was an instance of a proc which was inherited from the parent object. This is why New() does not have the proc/ prefix, because it is already defined in the parent object. You should however know that New() was redefined here, so it no longer acts the same way as in the parent objects. 
An example of an inherited variable are the name and desc (description) variables. Both were redefined tho. spawned_gold, however, is a new variable. One which the parent proc does not have.
If we define the following two objects...
 /turf/simulated/gold_spot/plot1
 /turf/simulated/gold_spot/plot2
...we just defined two child objects of /turf/simulated/gold_spot and they inherited all the procs and vars that /turf/simulated/gold_spot has including all those which /turf/simulated/gold_spot inherited from their parent (/turf/simulated).
=== ..() ===
 ..()
is something you will often see in the code. In other languages this is usually called super(), it calls the parent object's proc with the same name. Unlike some languages it is not automatically executed even if not added, nor is it required to be at the proc's start. These are usually added in New(), Del() and Attackby() procs to get to the original definition in the common parent. These common definitions usually ensure things are properly handled.
==Area, Mob, Obj and Turf organization==
 /datum
 /atom
 /area --All objects in here are areas, which are used in mapping to define the extent of an APC's power coverage, lighting, atmos, etc.
 /mob
 /dead --ghosts
 /living
 /carbon --humans and monkeys
 /silicon --AI and cyborgs
 /obj
 /effect --Landmarks, trigger objects, effects and decals
 /item
 /clothing --most of the clothing you can wear, belts and backpacks are in obj/item/weapon/storage
 /devices --electronic devices
 /stack --the things which you can stack, such as rods, floor tiles and materials
 /weapons --most items which you can pick up
 /machinery --machines which use power, process every tick and are generally unmovable
 /structure --objects which don't require processing every tick or power, examples include windows, grilles, bookcases etc.
 /turf
 /simulated
 /floor --floors with air
 /airless --floors without air (ALWAYS use these for areas exposed to space)
 /plating --plating
 /airless --airless plating
 /wall --walls
 /r-wall --reinforced walls
 /space --it's space
 /unsimulated --everything used on centcom and places where air movement is not simulated.
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