CS 1400 Final Review

What is executed first in this code?

[Lecture 19]

• The return statement

• sum(vals)

• The multiply

Answer explanation

The multiply cannot execute until its inputs are known, and the return won't happen until its operation is completed.

Which operation is performed first?

[Lecture 19]

• add

• divide

• subtract

• print

Answer explanation

The subtraction can't finish until both of its values are known, so it goes to func1.

The addition can't finish until both of its values are known, so it goes to func2.

The division can happen because y was passed as a parameter.

Which of the following are true of recursion or recursive functions?

[Lecture 19]

• Recursion involves a base case and a recursive step.

• The base case of a recursive function is the first iteration of the recursion.

• Recursive functions invoke themselves to solve a smaller part of the same problem.

• Recursive solutions involve iterating by adding one to a counting variable.

Answer explanation

Right Answers:

Recursive solutions always identify a base case and a recursive step/case.

Recursion involves a function calling itself repeatedly to whittle down the problem to the most trivial case.

Wrong Answers:

The base case is the simplest version of a problem (e.g. for the factorial problem it is when N is 1).

Recursive solutions progress by repeatedly calling the function with smaller versions of the problem, not by increasing by one.

Write two functions:

1) iter_factorial takes an integer as the parameter, returns the factorial of that parameter. The factorial must be solved iteratively (e.g. without recursion).

2) recurs_factorial takes an integer as the parameter, returns the factorial of that parameter. The factorial must be solved recursively.

[Lecture 19/Lab 12]

Answer explanation

What is the base case in this code?

[Lecture 20]

• number // 2

• 1 + mystery

• return 1

• number <= 1

Answer explanation

The base case is the simplest version of a problem.

The only answer option that involves checking the parameter (to decide if it's time to stop recursing) is number <= 1.

What causes recursion to progress towards the base case?

[Lecture 20]

• number // 2

• 1 + mystery

• return 1

• number <= 1

Answer explanation

Recursion progresses through the recursive step. This step has 2 parts: calling the function again, and making the problem smaller. In the case of the factorial example, this looks like factorial(n - 1).

For this problem, technically the whole line return 1 + mystery(number // 2) is the recursive step, but this question is specifically asking what is progressing this code to the base case. In other words, it's asking what part of the code is making the problem smaller. The only code doing that is number // 2.

What is the value of mystery(1)?

[Lecture 20]

• 0.5

• 0

• 1

• Infinite recursion (Stack Overflow)

Answer explanation

1 is included in the base case (if number <= 1), so 1 is returned.

What is the value of mystery(40)?

[Lecture 20]

• 1

• 5

• 6

• 7

Answer explanation

Frame 1: mystery(40) - not base case, so recurse -> 1 + mystery(20)

Frame 2: mystery(20) - not base case, so recurse -> 1 + mystery(10)

Frame 3: mystery(10) - not base case, so recurse -> 1 + mystery(5)

Frame 4: mystery(5) - not base case, so recurse -> 1 + mystery(2)

Frame 5: mystery(2) - not base case, so recurse -> 1 + mystery(1)

Frame 6: mystery(1) - base case

At this point, we can just start backing up through the stack frames:

Frame 6 returns 1

Frame 5 returns 1 + [Frame 6] = 1 + 1 = 2

Frame 4 returns 1 + [Frame 5] = 1 + 2 = 3

Frame 3 returns 1 + [Frame 4] = 1 + 3 = 4

Frame 2 returns 1 + [Frame 3] = 1 + 4 = 5

Frame 1 returns 1 + [Frame 2] = 1 + 5 = 6

What does the function os.path.isfile(file_name) do?

[Lecture 20]

• Returns the full absolute path of file_name

• Returns true if file_name is a directory/folder

• Returns true if file_name is a file

• Returns the relative path of file_name

Answer explanation

This is a review of A10, where you had to use this function to determine if what was being looked at was a file, or the base case.

Which of the following are examples of input for interactive programs?

[Lecture 21]

• Pressing a key

• Moving the mouse

• Clicking the mouse

• Touching the screen