So you have your movie ready to go, but something just isn't working. Debugging your actionscript can be a tense process but here are some tips to help you along.

Watching values

When working with complex chunks of actionscript it often becomes necessary to track values in order to diagnose problems in the code. There are three ways to do this:

• Trace Action. The Trace action allows you to send a value to an output window when you test the movie in Flash. Trace is excellent if you want to see a value at a number of specific points in your movie. Remember to set the string vs. expression option in the trace window. To get the value of a variable named frogs use an expression like "Number of Frogs = "&frogs.
• List Variables. This option can be found under the control menu when you test a movie. It lists every single variable contained in your file at the time you select it. This can be useful for mistakes in targetting variables such as typos.
• Text Fields. Probably the most useful way of tracking variables in Flash is using text fields. These need to be placed in the appropriate timeline but they have the advantage of continuously reflecting values as the movie runs. They can be placed in with the artwork or else off to the side.

Some problems arise because Flash is a streaming format. Obviously Flash can't execute any code which hasn't been downloaded yet. If you place chunks of code near the end of the movie and refer to it earlier on then you must use a preloader to ensure that it has been downloaded properly. This problem also comes up when you use load movie or load variables. You need to allow for the time it takes to perform the load action before you try to access any new variables or code. After you issue the Load Movie action, trap the user in a loop and test to see that the movie is fully loaded before you try to perform any actions on it. You can check this by testing if GetProperty("_levelX", _framesloaded) = GetProperty("_levelX", _totalframes).

Some common mistakes

• String vs. Expression. This one happens all the time. Forgetting to set a value as an expression in a Set Variable action can cause all sorts of havoc.
• Using numeric operators instead of string operators. Remember that Flash has a different set of operators for strings. Expressions such as "john"+"bob" will return a value of zero. This is because when Flash sees a numeric operator it attempts to convert both terms to a number. Make use of eq, &, gt, lt and the other string operators found in the expressions editor.
• Missing or incorrect instance names. Every instance of a movie clip must be given a unique name in order to target it. Forgetting to put these in or making an error in spelling will cause trouble.
• Knowing when to use eval. Sometimes you want Flash to assemble a variable name through some sort of expression and then return its value. For example the expression "name"&"1" will return "name1". But if you want to retrieve the value stored in a variable called name1 you must use the eval statement. The statement eval ("name"&"1") might return "john".
• Variable scoping and Tell Target. Remember that all variables are associated with a specific timeline. When using Tell Target it is easy to forget the variables declared outside of the Tell Target action are not local inside the Tell Target action.

Last word...

Don't be afraid to make a mess. Maintain multiple versions of your file and remember to back up your work so you can safely experiment as you hunt down and eliminate problems. Another good technique is to selectively cut down your work leaving only the part which is causing you the trouble.

Good luck!