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Extra Credit - Observables

Please download the homework here.

Overview​

In this assignment, you will work with observables.

Learning Objectives​

  • Practice writing and reasoning about observables and observers

Student Expectations​

Students will be graded on their ability to:

  • Correctly implement the functions specified below
  • Follow the coding, bad practice, and testing guidelines
  • Design full-coverage unit tests for the functions they implemented
    • Note that you can delete the uncredited functions should you choose not to implement them to avoid writing tests for them.

Programming Tasks​

For all functions below, use the provided Observable class.

  1. Write a function classifyObservables that takes in an array obsArr of Observables where each Observable updates with a type string, number, or boolean. Return an object with three observables, one for each named type. Each of the three observables updates anytime an Observable of that type in the input array updates.

  2. Write a function obsStrCond that takes a nonempty array funcArr of functions with type string => string, a function f with type string => boolean, and an Observable o with type Observable<string>. It returns an Observable<string> that updates when o updates, in the following way. If f returns true for the string obtained by applying the composition of the functions in funcArr (with the function at index 0 being applied first) to the update value of o, then the returned observable should update with that string. If f returns false, update the returned observable with the unchanged update value of o.

  3. Write a function statefulObserver that takes an Observable<number> o as input and returns a new Observable<number> which only updates if the current update value from o is divisible by the previous update value from o.

Optional Programming Tasks​

The following questions were taken directly from past exams and do not count for credit. They are provided for additional practice only. You can see the autograder tests for these on gradescope, but they are uncredited. Solutions are on Canvas under Exams and Solutions.

  1. [FALL 2022 MIDTERM 2] Write a function mergeMax that takes two Observables o1 and o2 with type Observable<number>, subscribes functions to each, and returns a new Observable which updates its subscribers whenever it receives a value which is no smaller than any value received so far from o1 or o2. Subscribers should be sent objects of the type { obs: number, v: number }, where v is the value received, and obs is either 1 or 2, indicating the source of the value.

  2. [SPRING 2022 FINAL EXAM] Write a function merge(o1: Observable<string>, o2: Observable<string>): Observable<string> returning a new Observable<string> which, whenever either o1 or o2 are updated, will be updated with the same value.

  3. [SPRING 2022 MIDTERM 2] Consider the Observable class from lecture, receiving updates with positive values. Implement a method greaterAvg returning a new Observable<number> that is updated with every number that is at least 50% larger than the average of the previous two numbers. Hint: use this.

  4. [FALL 2021 FINAL EXAM] Consider the Observable class provided in observable.ts, receiving updates with numeric values. In observables.ts implement a method signChange() that returns a new Observable<number> which will be updated with every nonzero value v that is not preceded by a nonzero value of the same sign. Hint: use this.

    • Write a function usingSignChange that takes an array of numbers numArr and an observer function f. You should subscribe f to the observer returned by the signChange method and then update the observer returned by SignChangeObservable with every number in numArr starting at index 0. The function should return nothing.