The function f(x) is returning the approximate length of rivers when given a river name as input. For example, it returned "420" for the Mississippi River, which is close to the length of 2,320 miles, and "450" for the Volga River, corresponding to its length of 2,193 miles. The function outputs the length of other rivers like the Nile River, Amazon River, and so on based on the given inputs.

