Is Python a

Loop Coding TechniquesThe for loop subsumes most counter-style loops. It’s generally simpler to code andquicker to run than a while, so it’s the first tool you should reach for whenever youneed to step through a sequence. But there are also situations where you will need toiterate in more specialized ways. For example, what if you need to visit every secondor third item in a list, or change the list along the way? How about traversing morethan one sequence in parallel, in the same for loop?You can always code such unique iterations with a while loop and manual indexing,but Python provides two built-ins that allow you to specialize the iteration in a for:• The built-in range function returns a list of successively higher integers, whichcan be used as indexes in a for. *• The built-in zip function returns a list of parallel-item tuples, which can be usedto traverse multiple sequences in a for.Because for loops typically run quicker than while-based counter loops, it’s to youradvantage to use tools that allow you to use for when possible. Let’s look at each ofthese built-ins in turn.Counter Loops: while and rangeThe range function is really a general tool that can be used in a variety of contexts.Although it’s used most often to generate indexes in a for, you can use it anywhereyou need a list of integers:>>> range(5), range(2, 5), range(0, 10, 2)([0, 1, 2, 3, 4], [2, 3, 4], [0, 2, 4, 6, 8])With one argument, range generates a list of integers from zero up to but not includingthe argument’s value. If you pass in two arguments, the first is taken as the lowerbound. An optional third argument can give a step; if used, Python adds the step toeach successive integer in the result (steps default to 1). Ranges can also be nonpositiveand nonascending, if you want them to be:>>> range(-5, 5)[-5, -4, -3, -2, -1, 0, 1, 2, 3, 4]>>> range(5, -5, -1)[5, 4, 3, 2, 1, 0, -1, -2, -3, -4]* Python today also provides a built-in called xrange that generates indexes one at a time instead of storing allof them in a list at once like range does. There’s no speed advantage to xrange, but it’s useful as a space optimizationif you have to generate a huge number of values. At this writing, however, it seems likely that xrangemay disappear in Python 3.0 altogether, and that range may become a generator object that supports the iterationprotocol to produce one item at a time, instead of all at once in a list; check the 3.0 release notes forfuture developments on this front.Loop Coding Techniques | 265