map and filter

The map and filter functions are a key part of a much larger group of functions for dealing with sequences. Of that group, map is certainly the one you will use the most, and filter is a close second. Their role is to modify sequences.

They accept one or more sequences as input, and return a sequence: sequence in, sequence out:

Figure 4.1: A schematic diagram of map and filter working together

In the preceding diagram, we can see map and filter working together, where filter eliminates items from the original list while map changes them.

The first question to ask when solving a problem involving collections is: "Do I want to obtain a list of values, or a single value?" If the answer is a list, then map, filter, or similar functions are what you need. If you need some other kind of value, the solution is probably a reduction of some kind, which we will discuss in the next chapter. But even then, as you break the problem down...