Lesson test: Map, filter and reduce

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Map, filter and reduce

1.[3p]
What does `["1", "2", "3"].map(parseInt)` produce, and why?

[1, NaN, NaN], because map passes the index as a second argument and parseInt reads it as the number base

Correct

[1, 2, 3], the same as map(Number)

The answer is: [1, NaN, NaN], because map passes the index as a second argument and parseInt reads it as the number base

["1", "2", "3"], because parseInt needs a base to do anything

The answer is: [1, NaN, NaN], because map passes the index as a second argument and parseInt reads it as the number base

[NaN, NaN, NaN], because parseInt cannot read a string

The answer is: [1, NaN, NaN], because map passes the index as a second argument and parseInt reads it as the number base

2.[1p]
What does `[1, 2, 3, 4].reduce((total, v) => total + v, 0)` return?

CorrectNot quite: 10

3.[3p]
What happens when `reduce` is called on an empty array with no initial value?

It throws a TypeError

Correct

It returns undefined

The answer is: It throws a TypeError

It returns 0

The answer is: It throws a TypeError

It returns the empty array

The answer is: It throws a TypeError

4.[3p]
Match each method to the shape of what it returns.

map
filter
reduce
find
some

one value of any type
a boolean
an array of the same length
an array no longer than the original
the first matching element, or undefined

Show the answer

map: an array of the same length filter: an array no longer than the original reduce: one value of any type find: the first matching element, or undefined some: a boolean

5.[3p]
`[4, 7, 10, 3, 8].reduce((n, v) => v > 5 ? n + 1 : n)` is written without an initial value. What does it return?

CorrectNot quite: 7

6.[2p]
Filtering before mapping can change the answer, not merely the speed, because a predicate written for the original values does not mean the same thing applied to transformed ones.

True

Correct

False

The answer is: True

7.[3p]
In which of these cases is a plain loop the better choice?

Select all that apply

The walk should stop as soon as a condition is met

Correct

The minimum, the maximum and the sum are all wanted from one pass

Correct

Each element is compared with the one before it

Correct

Each element is transformed into exactly one new element

The answer is: The walk should stop as soon as a condition is met, The minimum, the maximum and the sum are all wanted from one pass, Each element is compared with the one before it

8.[2p]
Put these pipeline steps in the order that computes the total value of the paid orders.

.reduce((sum, a) => sum + a, 0)
.filter(o => o.paid)
.map(o => o.amount)

Show the answer

c, b, a

9.[2p]
What initial value should a `reduce` that multiplies numbers together be given? Write the number.

CorrectNot quite: 1

1.[3p] What does ["1", "2", "3"].map(parseInt) produce, and why?

2.[1p] What does [1, 2, 3, 4].reduce((total, v) => total + v, 0) return?

3.[3p] What happens when reduce is called on an empty array with no initial value?

4.[3p] Match each method to the shape of what it returns.

5.[3p] [4, 7, 10, 3, 8].reduce((n, v) => v > 5 ? n + 1 : n) is written without an initial value. What does it return?

6.[2p] Filtering before mapping can change the answer, not merely the speed, because a predicate written for the original values does not mean the same thing applied to transformed ones.

7.[3p] In which of these cases is a plain loop the better choice?

8.[2p] Put these pipeline steps in the order that computes the total value of the paid orders.

9.[2p] What initial value should a reduce that multiplies numbers together be given? Write the number.

1.[3p]
What does `["1", "2", "3"].map(parseInt)` produce, and why?

2.[1p]
What does `[1, 2, 3, 4].reduce((total, v) => total + v, 0)` return?

3.[3p]
What happens when `reduce` is called on an empty array with no initial value?

4.[3p]
Match each method to the shape of what it returns.

5.[3p]
`[4, 7, 10, 3, 8].reduce((n, v) => v > 5 ? n + 1 : n)` is written without an initial value. What does it return?

6.[2p]
Filtering before mapping can change the answer, not merely the speed, because a predicate written for the original values does not mean the same thing applied to transformed ones.

7.[3p]
In which of these cases is a plain loop the better choice?

8.[2p]
Put these pipeline steps in the order that computes the total value of the paid orders.

9.[2p]
What initial value should a `reduce` that multiplies numbers together be given? Write the number.