learngo/16-slices/questions/4-backing-array.md

Backing Array Quiz

Where does a slice store its elements?

1. In the slice value
2. In a global backing array that is shared by all the slices
3. In a backing array that is specific to a slice
4. In a backing array that the slice references CORRECT

1: A slice value doesn't store any elements. It's just a simple data structure.

2: There is not a global backing array.

3: A backing array can be shared among slices. It may not be specific to a slice.

4: Yep! A slice stores its elements in a backing that the slice references (or points to).

When you slice a slice, what value does it return?

// example:
s := []string{"I'm", "a", "slice"}
s[2:] // <-- slicing

1. It returns a new slice value with a new backing array
2. It returns the existing slice value with a new backing array
3. It returns a new slice value with the same backing array CORRECT

3: Yes! Slicing returns a new slice that references to some segment of the same backing array.

Why are slicing and indexing a slice efficient?

1. Slices are fast
2. Backing arrays are contiguous in memory CORRECT
3. Go uses clever algorithms

2: Yes. A slice's backing array is contiguous in memory. So, accessing an element of a slice is very fast. Go can look at a specific memory location to find an element's value very fast.

Which one is the backing array of "slice2"?

arr := [...]int{1, 2, 3}
slice1 := arr[2:3]
slice2 := slice1[:1]

1. arr CORRECT
2. slice1
3. slice2
4. A hidden backing array

1: Yes! When a slice is created by slicing an array, that array becomes the backing array of that slice.

4: Nope. That only happens when a slice doesn't being created from an array.

Which one is the backing array of "slice"?

arr := [...]int{1, 2, 3}
slice := []int{1, 2, 3}

1. arr
2. slice1
3. slice2
4. A hidden backing array CORRECT

1: Nope, the slice hasn't created by slicing an array.

4: Yes! A slice literal always creates a new hidden array.

Which answer is correct for the following slices?

slice1 := []int{1, 2, 3}
slice2 := []int{1, 2, 3}

1. Their backing array is the same.
2. Their backing arrays are different. CORRECT
3. They don't have any backing arrays.

2: That's right. A slice literal always creates a new backing array.

Which answer is correct for the following slices?

slice1 := []int{1, 2, 3}
slice2 := []int{1, 2, 3}
slice3 := slice1[:]
slice4 := slice2[:]

1. slice1 and slice2 have the same backing arrays.
2. slice1 and slice3 have the same backing arrays. CORRECT
3. slice1 and slice4 have the same backing arrays.
4. slice3 and slice4 have the same backing arrays.

2: Yep! A slice that is being created by slicing shares the same backing with the sliced slice. Here, slice3 is being created from slice1. That is also true for slice2 and slice4.

What does the backing array of the nums slice look like?

nums := []int{9, 7, 5, 3, 1}
nums = nums[:1]
	
fmt.Println(nums) // prints: [9]

1. [9 7 5 3 1] CORRECT
2. [7 5 3 1]
3. [9]
4. []

What does this code print?

arr   := [...]int{9, 7, 5, 3, 1}
nums  := arr[2:]
nums2 := nums[1:]

arr[2]++
nums[1]  -= arr[4] - 4
nums2[1] += 5

fmt.Println(nums)

1. [5 3 1]
2. [6 6 6] CORRECT
3. [9 7 5]

2: Yes! Because the backing array of nums and nums2 is the same: arr. See the explanation here: https://play.golang.org/p/xTy0W0S_8PN