[ < ] [ > ] [ << ] [ Up ] [ >> ] [Top] [Contents] [Index] [ ? ]

### 3.4.1 Efficient loops

This might seem rather trivial, in how many ways can you mess up a loop anyway? Well... actually, quite a few. Let's start with the most common mistake. Assume you have a large array, let's call it 'big_arr', and let's assume you want to loop over all elements in that array, what do you do? "Simple!", you exclaim, "A 'for' loop, of course!". Sure... but how do you write it? Well, the most common implementation usually looks something like this:

 ``` int i; for (i = 0; i < sizeof(big_arr); i++) { // Here we do something with the array } ```

Ok... now why is this bad? Well, if you review the chapter on how the `for` statement works, you'll find that the three parts inside the round brackets actually gets executed. The first one once at the start, the second (or middle) part every time the loop is run and the third part also every time the current loop is finished.

This obviously means that the `sizeof()` function gets executed every time the loop is run. rather a waste of time given the fact that the array hardly is intended to change size. If it was, that would have been another matter, but as it isn't... No. Write like this instead:

 ``` int i, sz; for (i = 0, sz = sizeof(big_arr); i < sz ; i++) { // Here we do something with the array } ```

See? The variables 'i' and 'sz' gets assigned their respective values at the start of the loop, and only then. The counter 'i' gets set to 0 and the size variable 'sz' gets set to the size of the array. During the entire loop after that, 'i' is compared with 'sz' instead of repeatedly recompute the size of the array.

Believe it or not, this is a very common mistake, all people do it. While the savings in doing as I suggest might not seem that great, well... multiply this small gain in one loop by all the loops in the mud and all the number of times that those loops are run and you'll end up with quite a big number. The added cost in doing this is one local variable, which is a small enough price to pay.

Keep this general problem in mind since a lot of cases don't use arrays perhaps, but mappings or other general containers of items you want to loop through. The solution apart from specifics in identifying the size of that container is always the same.

 [ < ] [ > ] [ << ] [ Up ] [ >> ] [Top] [Contents] [Index] [ ? ]

This document was generated by Ronny Wikh on July, 8 2003 using texi2html