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Plain English explanation of Big O

I've seen some questions here and on SO talking about the most efficient way to find or sort this or that. The questions usually talk about the efficiency of a certain algorithms in terms of O(...). As a wannabe-programmer, I would like to start learning how to program algorithmically.

So, what is O(...)? How do I calculate it, where can I learn about this?

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closed as not a real question by Mark Trapp Jan 29 '12 at 7:53

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Have you looked at the Wikipedia article on time complexity? –  PersonalNexus Jan 29 '12 at 6:42
    
@PersonalNexus, Not yet, but I will. –  mowwwalker Jan 29 '12 at 6:44
    
Ideally, you would do some basic research (such as looking at the relevant Wikipedia article) and only then ask about specific things that aren't clear here. A basic question such as "what is big O notation" isn't really a good fit for the Stack Exchange Q&A format. –  PersonalNexus Jan 29 '12 at 6:47
    
@PersonalNexus, I didn't know what it was called, so I didn't know how to search for it. –  mowwwalker Jan 29 '12 at 6:48
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1 Answer

up vote 5 down vote accepted

The O(...) refers to Big-O notation, which is a simple way of describing how many operations an algorithm takes to do something.

In Big-O notation, the cost of an algorithm is represented by its most costly operation at large numbers. If an algorithm took n^3 + n^2 + n steps, it would be represented O(N^3). An algorithm that counted each item in a list would operate in O(N) time, called linear time.

For a list of the names http://en.wikipedia.org/wiki/Big_O_notation#Orders_of_common_functions

If you would like to learn more, a free course from Stanford is being offered on Algorithms in February.

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+ 1 for the link to free courses from Stanford. –  Christian P Jan 29 '12 at 8:33
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