Question

I was just reading through ‘Learning Python’ by Mark Lutz and came across this code sample:

 >>> L = ['grail'] >>> L.append(L) >>> L ['grail', [...]]  

It was identified as a cyclic data structure.

So I was wondering, and here is my question:

What is a ‘cyclic data structure’ used for in real life programming?

There seems to be a little confusion, which i think stems from the very brief code sample… here’s a few more lines using the same object L

 >>> L[0] 'grail' >>> L[1][0] 'grail' >>> L[1][1][0] 'grail'  

Answer 1

Lots of things. Circular buffer, for example: you have some collection of data with a front and a back, but an arbitrary number of nodes, and the ‘next’ item from the last should take you back to the first.

Graph structures are often cyclic; acyclicity is a special case. Consider, for example, a graph containing all the cities and roads in a traveling salesman problem.

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Okay, here’s a particular example for you. I set up a collection of towns here in Colorado:

V=['Boulder', 'Denver', 'Colorado Springs', 'Pueblo', 'Limon'] 

I then set up pairs of cities where there is a road connecting them.

E=[['Boulder', 'Denver'],    ['Denver', 'Colorado Springs'],    ['Colorado Springs', 'Pueblo'],    ['Denver', 'Limon'],    ['Colorado Springs', 'Limon']] 

This has a bunch of cycles. For example, you can drive from Colorado Springs, to Limon, to Denver, and back to Colorado Springs.

If you create a data structure that contains all the cities in V and all the roads in E, that’s a graph data structure. This graph would have cycles.