Unit 2 Self-Check Questions

2.1.1

Examples like this one are useful to test your understanding of assignment to variables, but you wouldn't use a sequence of assignments like this in an actual program.

What value will this code display?

a ← 3
b ← a
a ← 4
DISPLAY(b)

3

Correct! The variable b is assigned the value of the variable a (which is 3 when

b ← a

is done).

4

The variable b is given the value of the variable a (which is 3 when

b ← a

is done); b doesn't remember that the 3 came from a so it doesn't change when a is changed.

a

The variable b has been set to the value of the variable a, not the letter "a" itself.

b

The script will display the value of the variable b, not the letter "b" itself.

2.1.1

What value will this script report?
set m to 9, set k to 5, set m to (m + 1), set k to (k - m), report k

-5

Correct!

3

The value of the variable m is always greater than the value of the variable k in this script, so subtracting m from k will give a negative number.

-4

The value of the variable m is 9 at the beginning, but it has been changed by the time it is subtracted from k.

5

The last set command sets the value of the variable k to the value k – m, not m – k.

2.1.2

We really shouldn't be encouraging students to mutate a block's inputs. There are really two issues here; one is with the x=x+1 style of programming altogether, and the second is that formal parameters specifically shouldn't generally be mutated; it's better to make a script variable that's initialized to the input value. (If we change this, we need to update the Guided Notes.) --MF, 6/26/21 with BH

mystery function (input):
if (input < 5) (repeat until (input = 5)
(
say (input) for (1) secs,
set (input) to (input + 1)
)
report (finished)

Which inputs to mystery function will report "finished"?

Any integer

Correct! No matter what the input number is (as long as it's an integer), the report block will eventually run. (This doesn't necessarily happen in every procedure, but here it will.)

Only integers greater than or equal to 5

Write out some examples of what happens when the input is less than 5. For example, if the input is 3, the sprite will say 3, the input will be changed to 4, the sprite will say 4, the input will be changed to 5, the repeat until loop will stop, and the function will report "finished."

Only integers less than 5

The report command isn't inside the if.

Only 5

The repeat until block checks the value of input every time through the loop, not just its initial value.

2.1.2

What will the sprite say if you run the same function with the input 1?

It does say 4 at the end, but it says other things before that.

How does repeat until work?

four speech bubbles saying '1', '2', '3', and then '4'

Correct! The repeat until block runs the code in its script slot until input = 5, then the computer skips down to the next command, report (finished), without the sprite ever saying 5.

five speech bubbles saying '1', '2', '3', '4', and then '5'

The repeat until block tests the value of input before running the code in its script slot.

The feedback is not finished because I realized that BH might object to changing the value of an input variable... --MF, 3/1/19

2.1.2

What will happen if you run the same function with the input 9?

The sprite will say 9, and the block will report "finished."

What does the if block do when input is 9?

The sprite will say 9, 8, 7, 6, and the block will report "finished."

What does the set instruction do to the value of input?

The sprite will say 9, 10, 11, 12, 13, … and keep counting, and the block will never report "finished."

When input is 9, will the code inside the if command run?

The sprite will say nothing, and the block will report "finished."

Correct! The code inside the if command will not run.

The feedback is not finished because I realized that BH might object to changing the value of an input variable... --MF, 3/1/19

2.1.5

AAP-2.N part b

Which of the following blocks will report a list of length 3?

Choose all that apply.

No, this makes a list of one item, even though it has more than a single word.

No, item 3 of the list is the word "kiwi" which isn't a list at all.

item 2 of {lemon, {pineapple, strawberry, kiwi}}

This is one of the two correct answers.

This is one of the two correct answers.

2.2.2

Imagine you make a variable capitals and use set to give this list of lists a name:
set (capitals) to {{Augusta, Maine}, {Boise, Idaho}, {Columbia, South Carolina}, {Des Moines, Iowa}}
Which of the following statements are true?

Choose all that apply.

reports a list with two items: "Augusta" and "Maine".

Correct.

reports the word "Augusta".

Correct.

reports the word "Iowa".

No. What is item 2 of "capitals"?

reports the number 8.

No. Each of the small lists is a single item in the list "capitals" .

all but first of (item (last) of (capitals))

reports a list with one item: "Iowa".

Correct.

reports the string "Boise".

The items of capitals are lists.

item (last) of (all but first of (capitals))

reports the string "Iowa".

The items of all but first of (capitals) are lists.

reports the number 2.

Correct.

2.2.3

Which of the scripts below will produce the following sprite behavior?
sprite saying Abraham then Lincoln
(When you run the script, though, it actually says Abraham Lincoln only once.)

Choose all that apply.

for each (item) of {Abraham, Lincoln} [say (item) for (1) secs]

Correct.

No, when a sprite says a list, we see a visual representation of the list.

say (item (1) of {Abraham Lincoln, George Washington, Barack Obama}) for (2) secs

No, this says "Abraham Lincoln" all at once, not in two separate speech balloons.

say (item (1) of {Abraham, George, Barack} for (1) secs, then say (item (1) of {Lincoln, Washington, Obama} for (1) secs

Correct.

2.2.3

AAP-2.O part b

inputList ← [3, -1, 2, 10, -5]

FOR EACH item IN inputList
{
  IF(item > 0  AND  item * item > 4)
  {
    DISPLAY(item)
  }
}

What will be displayed as a result of running the code segment given?

9, 1, 4, 100, 25

What does the

IF

instruction test? What does the

DISPLAY

instruction in this code display?

9, 100

What does the

DISPLAY

instruction in this code display?

3, 2, 10

Is 2 * 2 > 4 ?

3, 10

Correct!

2.2.3

PROCEDURE Mystery(numberList, targetNumber)
{
  counter ← 0
  FOR EACH number IN numberList
  {
    IF(number > targetNumber)
    {
      counter ← counter + 1
    }
  }
  RETURN(counter)
}
myMathGrades ← [100, 80, 90, 80, 60, 100, 50, 100]
Mystery(myMathGrades, 60)

What will be returned if the code above is run?

[100, 80, 90, 80, 100, 100]

What type of value does the procedure return?

7

Is 60 > 60 ?

6

Correct!

60

What is the procedure doing?

2.3.2

There's no built in ≤ block in Snap!. Suppose we wanted to build one. Which two of the following Boolean expressions are equivalent to the expression num less than or equal to 23 predicate

?

Can num be both less than 23 and equal to 23 at the same time?

Correct! This picture shows an expression that will report true if num is less than or equal to 23.

Correct! If num is not greater than 23, then it is either less than 23 or equal to 23.

Imagine that the value of num is 22.5. Will this expression and the expression given in the question report the same response?

2.3.2

Which of the following expressions will report true?

I only

II only

I and II

Neither I nor II

2.4.1

AAP-2.L.2

Algorithms that look the same may have different results. The following incomplete code fragment was designed to test if

number

is odd:

IF (MISSING CONDITION)
{
  DISPLAY "It is odd."
}

Which of the following can be used in place of the MISSING CONDITION?

number MOD 1 = 0

MOD

returns the remainder when the first input is divided by the second. Whether a number is odd or even depends on divisibility by 2.

number MOD 1 = 1

MOD

returns the remainder when the first input is divided by the second. Whether a number is odd or even depends on divisibility by 2.

number MOD 2 = 0

number MOD 2

returns the remainder when

number

is divided by 2. If that remainder is zero, is the number odd or even?

number MOD 2 = 1

Correct!

number MOD 2

returns the remainder when

number

is divided by 2. If

number MOD 2

is 1, then the number is odd.

2.4.1

What's the value of 11 mod (2 + 3)

?

1

Correct!

0.2

Remainders are always whole numbers.

2

Mod gives you the remainder, not the quotient.

2.2

It's true that division and mod have some similarities, but mod gives you just the remainder.

2.4.3

When will "Error. Invalid entry." be displayed?

PROCEDURE greaterThanOrEqualTo(a, b)
    IF(a > b)
    {
        RETURN(true)
    }
    ELSE
    {
        RETURN(a = b)
    }
    DISPLAY("Error. Invalid entry.")
}

When

a > b

is true.

Think about what

RETURN

does. Like report in Snap!,

RETURN

ends the procedure. Once it is run, no other code in that procedure is run, and the flow of control returns to the place where the procedure was called.

When

a > b

is false.

Think about what

RETURN

does. Like report in Snap!,

RETURN

ends the procedure. Once it is run, no other code in that procedure is run, and the flow of control returns to the place where the procedure was called.

Always

It looks like that because that's the last line of procedure code, but think about what

RETURN

does. Like report in Snap!,

RETURN

ends the procedure. Once it is run, no other code in that procedure is run, and the flow of control returns to the place where the procedure was called.

Never

Correct!

Unit 2: Abstraction

Lab 1: Games

Lab 2: Making Lists

Lab 3: Making Decisions

Lab 4: Making Computers Do Math