To express what some have tried to get at, in a different way...
When the order makes no difference to the behavior of the program, the difference is clearly a matter of what's most readable to humans. Here's a linguistic reason why putting the "topic" on the left makes sense:
In linguistics, the topic of a sentence is what is being talked about, and the comment is what is being said about the topic. In this example, we can assume that
position is the topic, and "less than array length" is the comment. In English, and in many other languages, the topic is usually expressed before the comment. "The tendency to place topicalized constituents sentence-initially (topic fronting) is widespread." http://en.wikipedia.org/wiki/Topic_%28linguistics%29#Realization_of_topic.E2.80.93comment
So a good rule of thumb is to think of your line of code as a sentence (or clause, in this case), decide what the sentence is about, and put that first if you can. Often, what the sentence is "about" will be a variable rather than a constant. But sometimes the comment will also involve a variable, so you can't just go by that.