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As we learn in computer science classes, global optimizations (such as algorithm and data structure choices) determine in large part the overall performance of our programs. For larger values of "n," or the number of input elements, the complexity of running time can dominate any local optimization concerns. This complexity is expressed in O-notation, where complexity or "order" is expressed as a function of n. Table 10.1 shows some examples.
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Notation |
Name |
Example |
|
O(1) |
constant |
array index, simple statements |
|
O(logn) |
logarithmic |
binary search |
|
O(n) |
linear |
string comparison, sequential search |
|
O(nlogn) |
nlogn |
quicksort and heapsort |
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O(n2) |
quadratic |
simple selection and insertion sorting methods (two loops) |
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O(n3) |
cubic |
matrix multiplication of nxn matrices |
|
O(2n) |
exponential |
set partitioning (traveling salesman) |
Array access or simple statements are constant-time operations, or O(1).
Well-crafted quicksorts run in nlogn time or O(nlogn).
Two nested for loops take on the order of nxn or O(n2)
time. For low values of n, choose simple data structures and algorithms.
As your data grows, use lower-order algorithms and data structures that
will scale for larger inputs.
Use built-in functions whenever possible (like the Math object),
because these are generally faster than custom replacements. For critical
inner loops, measure your changes because performance can vary among different
browsers.
Refactoring is the art of reworking your code to a more simplified or efficient form in a disciplined way. Refactoring is an iterative process:
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Refactoring clarifies, refines, and in many cases speeds up your code. Here's a simple example that replaces an assignment with an initialization. So instead of this:
function foo() {
var i;
// ....
i = 5;
}Do this:
function foo() {
var i = 5;
// ....
}
For More Information - Refactoring is a discipline unto itself. In fact, entire books have been written on the subject. See Martin Fowler's book, Refactoring: Improving the Design of Existing Code (Addison-Wesley, 1999). See also his catalog of refactorings at http://www.refactoring.com/.
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12. Kernighan and Pike, The Practice of Programming, 41. Back
13. Andrew Hunt and David Thomas, The Pragmatic Programmer: From Journeyman to Master (Boston, MA: Addison-Wesley, 1999), 179. Back
From Speed Up Your Site: Web Site Optimization, Chapter 10.
Copyright 2003. Reproduced by permission of New Riders.
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Created: January 8, 2003
Revised: January 8, 2003
URL: http://webreference.com/programming/optimize/speedup/chap10/1/2.html
