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GameMaker Use
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GameMaker Use

Some general GameMaker Advice

Drag & Drop vs Code

GameMaker is at first glance built very much around the drag & drop features, for "programming without programming", which is indeed a fantastic way for people with no skill at all in programming to get something done.

In general, my view is that you no tol is perfect for everything, and there is a lot to gain by choosing the right tool for each occasion. For a simple quick game, Drag & Drop is perfectly adequate. If someone tells you "you should use code, Drag & Drop is for children", ignore him. He just wants to show off and let everyone know he can code. Would a soldier use a flame thrower to light a candle at his dinner table? No. Use the right tool for the rght occasion.

However, if you start with drag & drop, and after a while expand your initially small game into something that grows into something more complex, you will soon run into the limitations of drag & drop. In particular, reuse is difficult. For example, if you created 15 baddies, all as different objects, with different sprites, just to realize you made the same mistake in all of them. You need to change each and every of them.

With proper use of coding, you would have the same piece of code doing this for all your 15 objects, and change only in one place.

So, if you know you'll expand your project, extensive use of drag & drop may not be the best of ideas. On the other hand, don't overestimate your own expansion plans. By the time you know enough to realistically expand your little game, you may have come to realize it was not that good anyway.


GM and programming

It might appear that you can do a lot with GM without knowing anything about
computer programming. In a sense, that is not true at all, since what you do
with the Drag & Drop symbols is a kind of programming, just with an unusual
syntax.

However, it is true that with very limited understanding of even the most
basic programming, you can get far. But not everywhere. Sooner or later,
you'll end up in problems that requires some programming skills. Be prepared
for that, and accept the fact that if you cannot program, you will have some
work in front of you to get through those problems

It'll sometimes strike you that people who know an awful lot about GM,
actually sometimes knows very little when it comes to basic programming. And
hence run into problems in unexpected situations.

Mixing run-time and design-time concepts

I often note that people on the forums mix up what is design time concepts with what is run time concepts. More specifically, many try to create variable names from strings in runtime. Although GM offers a function "execute_script" that can take a string, and execute the code that it corresponds to, which actually makes it possible to create variable names in runtime, I strongly discourage any such ideas.

Why?, you might ask. There are many reasons, some of which are very practical and hands on:

Moreover, there are very strong conceptual reasons for not doing this:
- Variable names are not part of the semantics of a program. It is used to uniquely identify a variable, but the name as such has no meaning. If you change it to something else, also unique, it does not change the semantics of the program. This fundamental fact is no longer true if you use variable names in runtime.
- You need an interpreter and all the symbol information to go with your executable
- Any type of compiler optimization will be almost impossible
- The amount of faults that cannot be detected at compile time increases
- The only possible way for this to be needed is in a situation where the user of the program knows what variable names the programmer used when designing the program. Except for debugging purposes, this is in principle never the case. If you think you ended up in such a need, think again. You will be better off with solving it another way

 

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Common errors

This is a list of some of the most trivial and common errors likely to cause
newcomers headaches.

Random numbers

In GML you can generate a random number using
random(X)
This will give a random floating point number from (and including) zero, to (but excluding) X. To generat a random integer between 0 and X, you do
floor(random(X+1))
where floor is used to round to the nearest lower integer.
A common error, and one that you often see people suggesting that you do, is using round() instead of floor(). Round rounds to the nearest integer, regardless whether it is smaller or bigger. Using
floor(random(5))

to generate a random integer from 0 to 5 works, in the sense that it produces numbers in the expected range. But the probability of getting them will not be even! You will get

0 if 0-0.5

1 if 0.5-1.5

2 if 1.5-2.5

3 if 2.5-3.5

4 if 3.5-4.5

5 if 4.5-5

I.e. the chance of getting 0 or 5 is only half the chance of getting 1, 2, 3 or 4. So this way of implementing a 6-sided die

dieroll = floor(random(X+1)) + 1

will not be correct (unless your die is very weird).

More generally, by the way, a random integer between X and Y (inclusively) can be generated with

floor(random(Y-X+1))+X


Conflicting names

You can use the same name for several type of things in GM without GM
complaining, until suddenly things just don't work. It can be a variable and
a script having the same name. Or an object and a sprite. It will result in
errors that may be difficult to understand

UNinitialized Variables

Any variable you use must have been given a valuer before you try to access it. Often, this is simply a matter of setting some default (or initial) value in the create event of an object, but sometimes there are other ways that are more suitable (like setting it from the object creating it, just after creation)

You can use a feature in GM to set all variables to 0, so that you need not bother about initializeing them to avoid the "uninitialized variable" error. Although this feature is a perfectly valid GM feature, I would strongly suggest that you instead make sure you do initialize variables explicitly.

The auto-initialize feature can easily hide bugs for you in a very nasty way. You'd rather get a "not initialized"-error, and fix it (which is easy), than having some variable that you forgot to set the value of (by misspelling it when you tried to set it, for example) being zero, and causing strange behaviour. That can be very hard to find



Drawing and draw events

A sprite is displayed by default, at its x and y coordinate. But once you
start putting something into its draw event, you must also explicitly draw
the sprite in the draw event. You also must make sure that it is using the
right sprite, and that it is positioned where you want it (normally at 0,0,
with the relative box ticked)

Another common mistake is to draw outside the draw event. That is not
possible, but will not generate an error either. Things just won't be drawn.
ALWAYS draw in the draw event (including displaying of text)

Some additional info on this topic

Vertical Speed

In GM vertical speed has downwards direction. vspeed = 16 means that an
object is going downwards on screen.

This is in line with the coordinate system of the GM screen, which has it's
origin (0, 0) at the top left corner, and positive values to the right and
down

Object and Instance Identities

An object is the definition of how instances of a certain object type shall
look. (it is normally called "class" in the litterature of object
orientation). An object in itself does not "exist", but so does only hte
instances of that object. The object is like the recipe, while the instances
are the cakes.

An instance has an id (one of the built in local variables in all objects).
This id (a variable called "id") is returned from the instance_create()
function. It can also be obtained from within the object by just using the
"id" variable. Each instance of an object has a unique id. that id is really
just a number, and if you put it in a variable (say "myobject"), you can use
it in statements like:
myobject.x = 50;
to set the x coordinate of that object to 50, from another object.

By keeping track of objects id:s when created, you can do a lot. Link
objects togetehr, create lists of objects, etc.

Variables and their scope

Every variable has a scope, in which it is known and useable. The scope is
is either global, meaning that the variable is known everywhere, and every
little piece of your program can access the same variable. Or it is local to
an instance. In this case, the variable is really part of the instance. If
more instances are created, each of them will have its own variable with
that particular name.

Local variables are really having names that includes the instance identity.
"playerid.weapon" is the variable that is called "weapon" inside the
instance with id playerid, which can also be referred to with its full name
from other objects. "playerid.weapon" is the unique name of that variable,
but within the object with identity playerid, it is possible to address that
variable with "weapon" only. It is assumed that variables which are used
without specifying the id before the "." are variables local to the current
object.

Loops

Loops are extremely important in programming. Still, in GM you hardly see
them if you use D&D. Hoe come? Well, first of all, you are limited by not
being able to make proper loops with D&D, it is probably one of the reasons
you'll start using scripting sooner or later. Secondly, the core of GM is
its event loop, which is the loop that is repeated over and over again, and
on each iteration generates events for all your objects. Your whole program,
no matter how trivial, is really just a huge loop.

If statements - the brackets

The piece of code that shall be executed only if a condition is fulfilled
shall be put after the if-construct, and within brackets ( "{...}") (or in
D&D between the start block and end block symbols). If not, only the one
single line appearing immidately after the if-construct will be conditional,
the lines after it will always be executed.

if (x == 1){
y=2;
z=3;
}
will assign both y and z if x is 1, but none of them if x is not one, while

if (x == 1)
y=2;
z=3;

will always assign z, but will assign y only if x is 1.

If-statements - comparisons

Remember that if you have if x< 0, and if x > 0, then you will not have a
case for x = 0. Always check that a set of conditions actually covers all
cases you want covered.

Destroyed objects

Once an object is destroyed, it cannot do anything more. Hence, if an object
is destroyed by itself, there is no point putting any code after the
instance_destroy(), since it will never execute. Just think about it. Who would consider writing a goodbye letter after commiting suicide?

Alarms not firing

If you use an alarm to make things happen periodically, do not forget to
restart the alarm in the alarm event. I.e. when the alarm goes off, make
sure you restart it again, otherwise nothing more will happen after the
first time the alarm goes off.

FPS

It may be tempting to increase the FPS (frames per seconds, set with
room_speed) to a high value, to make animatinos look smoother. Beware,
however, since once the set room_speed exceeds the value the computer your
game is running on can handle, the game will run at uncontrolled speed. If
you set room_speed = 100, and you run some stuff in the background on a slow
PC, the outcome may well be that the actual speed varies between 30 and 50,
and you have no control over it. It will not look good

Sprite Origin

What is the sprite origin? It is the point of a sprite which will be the one ending up on the position you place the object thathas the srite. I.e. the x, y you specify will be exactly where the sprite origin is. If x, y is 0, 0 (top left corner), and the sprites origin is in the bottom right corner of the sprite, the entire sprite will be outside the screen. If the origin is in the top left corner of the sprite, the sprite will appear in the top left corner of the screen when put at 0, 0.

Compare this to lying down on the flor in a room. If someone tells you to lie on a spot painted on the floor in the middle of the room, then what  part of your body do you place on the spot? Whichever one you chose, that one is the origin of your body (if you view it as a sprite)









- Design time and run time are different, and mixing them has a lot of side effects, including

Generally, wanting to use variable names in run time is like wanting to be able to tell from a painting how the colors were arranged on the palette of the painter. It simply is not relevant. It does in no way affect how the painting looks, and there is no sensible way to include the information in the painting.


- It is error prone
- Resulting errors are very hard to find
- It results in inefficient code
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