Introduction to Scripting Part 2

 

Recap

In the first tutorial, we covered the basics of scripting - how to write a simple script, use variables and conditional statements. You now know how to call all the built-in functions and write a fairly decent script. So, what's next?


Loops

You may find it useful to have a sequence of commands that are executed more than once. AGS supports this, using the while keyword. Its format is virtually identical to if:

  int counter = 1;
   while (counter < 10) {     
     counter ++;
   }

This means that the body of the while statement is repeatedly processed, as long as the condition is true. So, in this case, since counter starts as 1, the loop will be run 9 times, since on the tenth time counter will be 10 and therefore it will stop. 

NOTE: be careful with while loops - it is possible to crash your game by using a badly written one. Consider this:

  int counter = 1;
   while (counter > 0) {     
     counter ++;
   }

Here, counter starts off as 1, so the loop will run. Then, it will increase to 2. Since it is still greater than 0, the loop will run again. This particular loop will run forever, since counter can never become less than 1, and your game will grind to a halt.


Multiple conditions

On several occasions, you don't just want to decide what to do based on one variable - there may be a combination of things you want to consider.
For example, suppose that you want to display a message if the player has two specific inventory items. You could do this, and put two if statements inside each other:

  if (player.HasInventory(iFirstItem)) 
   {
     if (player.HasInventory(iSecondItem))    
     {   
       Display ("You have both the items!");     
     }
   }

However, this is quite unwieldy and if you have a lot of conditions it will look rather messy. So, AGS lets you do it this way:

  if ((player.HasInventory(iFirstItem)) &&     
       (player.HasInventory(iSecondItem)) )
   {     
     Display ("You have both the items!");     
   }

This makes use of the && operator. Note that you need an extra pair of outer parenthesis, to contain the whole expression. Each part of the expression goes inside parenthesis, as normal, and they are joined with the && symbol.

You can extend this to as many checks as you like. For example, this will display the message if the openedDoor variable is currently set to true, and the variable timer is between 5 and 10.

  if ((openedDoor == true) && (timer > 5) && (timer < 10))    
   { 
     Display ("It's all working out fine.");     
   }

'OR' expressions

Sometimes you will want to do some processing if one or another condition is true. For example, you might want to let the player open a door if he has either the door key or a chainsaw.

  if ((player.HasInventory(iDoorKey)) ||     
       (player.HasInventory(iChainsaw)) )
   {   
     player.ChangeRoom(10); 
   }

The operator here is the double-vertical-bar  ||  operator. It works similarly to the && operator, but it will process the commands inside it if either one expression, or the other, or both, are true. 


Doing one thing or another

Sometimes, you want the script to take one course of action if a variable is set, and another course of action if it isn't. Initially, you might think you would do this:

 if (timer == 5) {    
    // do something
  }
  if (timer != 5) {
    // do something else
  }

That would work -- however, there is a neater way, using the else keyword:

 if (timer == 5) {    
    // do something
  }
  else {
    // do something else
  }

This also allows you to modify the variable inside the first block of code, without affecting whether the second block gets run or not.

You can do as many tests as you like, using the else if keyword. So, a complete piece of code could look like this:

 if (timer == 5) {    
    // do something
  }
  else if (timer == 6) {   
    // do something different
  }
  else {
    // do this if it's not 5 or 6    
  }


Your Own Functions

You've probably noticed in the manual, it mentioning functions such as repeatedly_execute, and on_event, and how you can add them to your global script to do cool stuff. But you may be wondering, how exactly to go about it.

Remember in tutorial 1, we learnt about function parameters and how they could be int, string, etc. Well, you write your own functions like this:

 function dialog_request(int param)     
  {    
    // contents of function go here
  }

You start with the keyword function, then follow it by the function name, and then parenthesis listing the parameter types and names. For each parameter that you want, you need to write its type (int or string), followed by the name it will be known by inside the function. This name can be anything you like - it is similar to naming a variable.

There are some fixed functions, such as dialog_request and on_event, which are part of AGS and therefore you MUST use the correct number and type of parameters.  However, you may also add your own functions by naming them however you like, and having as many parameters as you need.

Functions are useful if you have a block of script code that you need to use in two different places - putting it in a function instead means you don't have to copy & paste, and that if you modify it, all other script that relies on it gets updated too.

To call your function from elsewhere in the script, just do it exactly like you call a built-in function - ie. just write its name, parameters then a semicolon.

I think a couple of examples are in order. First of all, let's show a fixed function, on_event:

 function on_event (EventType event, int data) 
  {    
    if (event == eEventGotScore) 
    {
      if (data == 5) 
      {
        aSpecialScoreSound.Play();
      }
      Display("You just got %d points!", data);   
    }
  }

With this script, whenever the player scores points, they will get a message telling them so. Also, if they happen to get 5 points at once, it will play audio clip aSpecialScoreSound.

As you can see, you use the function parameters just like any other script variables.

Our own function

Now, suppose we have a special animation of the player doing a dance, and we want to be able to play it from various points in the script. By far the easiest way to do this would be to put it in a function:

 function do_dance()  
  {       
    cEgo.LockView(10);
    cEgo.Animate(2, 5);    
    cEgo.UnlockView();
  }

This function runs view 10, loop 2, as the character's animation, waits until it finishes and then reverts to the default view. 

TIP: if you're wondering where to place your custom functions, just open up the global script (Game menu, Edit Global Script) and write them in there. The function must be outside all other functions.

Now, elsewhere in your script, when you want the player to dance, just do:

  do_dance();    

Returning a value

You may have noticed that some of the built-in functions, such as IsGamePaused, return a value to the script. You can do this from your own functions, using the return keyword. So:

 function add(int a, int b)      
  {    
    int result;
    result = a + b;
    return result;
  }

This function adds the two numbers together and returns the result (a useless function in practice since the + operator does the same thing, but it demonstrates the point).
Another part of your script could then do: 

  total = add(5, 10);    

for example.

Using functions from room scripts

You may notice that when you add your own function to your global script, you can call it fine from other places in the global script but attempting to use it in a room script gives a parse error. The manual explains how to solve this using the script header.


Conclusions

We've covered some of the more advanced topics of scripting. I'm sure there's a lot of stuff I've forgotten to mention, so feel free to comment on it on the forums. 
 

Go back to Introduction to Scripting Part 1

 

Return to tutorials index

Page created 7 June 2002; updated 19 August 2007. Copyright (c) 2002-2007 Chris Jones.

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