Loop Coding Techniques

The for loop subsumes most counter-style loops. It’s generally simpler to code and quicker to run than a while, so it’s the first tool you should reach for whenever you need to step through a sequence. But there are also situations where you will need to iterate in more specialized ways. For example, what if you need to visit every second or third item in a list, or change the list along the way? How about traversing more than 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:

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  • The built-in range function produces a series of successively higher integers, which can be used as indexes in a for.
  • The built-in zip function returns a series of parallel-item tuples, which can be used to traverse multiple sequences in a for.

Because for loops typically run quicker than while-based counter loops, it’s to your advantage to use tools like these that allow you to use for when possible. Let’s look at each of these built-ins in turn.