STRATEGY DEVELOPMENT
I comment out ten lines. If that doesn’t work,
I might comment
out whole functions or sections of code. When the compiler finally runs properly, you know that the last section of commented out code comprises the bug.
Next, you work backwards.
Start with making the offending commented-out section smaller and smaller until the error comes back. Now, you have finally zeroed in on the actual source of the problem.
My favored manner of bug fixing is to comment most of the relevant decision information onto the screen, which is done using the Comment() function. I then run the visual backtester, watching how the data behaves in relation to the visual information.
On screen comments are essentially jury-rigged break points. Controlling how and when they appear allows the coder to step through the code to uncover the issue. The only difference is that comments do not forcefully prevent the code from running. The text which appears is very small. Aside from that, I really like that fact that it’s so robust. The comment
function always works without a hitch, making it the best friend of a coder that’s troubleshooting code.
Taking screenshots takes this to the next level. Whenever consumers ask questions about why an expert advisor behaved a certain way, the easiest answers come with screenshots. Commenting the imitation break points usually provide bullet proof
FX
Our default template includes a true/false variable called WriteScreenshots. Traders control whether they want to bother with this bug fixing feature or not. Te only downside to it is that every recorded action eats up about 50kb of hard drive space.
Log files offer the last bug fixing option. The major drawback is that they are so ugly and burdensome to read. I almost always prefer screenshots for this reason.
Nonetheless, log
files do have their place. The main way to use them is as error catchers. Whenever a
course of action
goes awry due to a problem with either MetaTrader itself or with a broker transaction, the log file is the easiest place to catch it.
answers - the coder and client can actually see what the Ea thought at the time it made a decision. MQL4 offers the WindowScreenShot() function to do this.
Te Expert advisors that we program always take screenshots during pivotal actions, such as entering a trade or modifying an exit condition. Te goal is to provide a visual account of every decision with an eye to answering future inquiries about the action.
Bug fixing MQL files is a skill that takes awhile to learn. The methods at the programmer’s disposal are very different from those available to higher level
languages. Once
you get accustomed to working with the much more basic tools in the MetaEditor and MetaTrader, the bug fixing process goes a lot quicker.
Shaun Overton FX TRADER MAGAZINE April - June 2012 85
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