Come on TS, you can't be easier?
This syntax is actually very common amongst C-like languages. The general syntax is
for(expression; expression; expression) { } and whatever expression you put in the 3 spots, it will be syntactically valid. For example,
for("this"; "language"; "sucks") {} is perfectly valid and won't even result in an infiniteloop. (Though I'm not sure if this works in other languages, which may have stricter restrictions on that sort of thing)
The thing to note is that a for loop is really just a condensed while loop, where, for example,
for(%variable = 5; %variable < 100; %variable += 7) { echo(%variable); } is expanded into
%variable = 5;
while(%variable < 100)
{
echo(%variable);
%variable += 7;
}