learngo/16-slices/questions/5-slice-header.md

Slice Header Quiz

What is a slice header?

1. The first element of a slice value
2. The first element of the backing array
3. A tiny data structure that describes all or some part of a backing array CORRECT
4. A data structure that contains the elements of a slice

3: Yes! It's just a tiny data structure with three numeric fields.

4: A slice doesn't contain any elements on its own.

What are the fields of a slice value?

1. Pointer, length, and capacity CORRECT
2. Length and capacity
3. Only a pointer

Which slice value does the following slice header describe?

SLICE HEADER:

• Pointer : 100th
• Length : 5
• Capacity: 10

Assume that the backing array is this one:

var array [10]string

1. array[5:]
2. array[:5] CORRECT
3. array[3:]
4. array[100:]

1: This slice's capacity is 5, it can only see the elements beginning with the 6th element.

2: That's right. array[:5] returns a slice with the first 5 elements of the array (len is 5), but there are 5 more elements in the backing array of that slice, so in total its capacity is 10.

3: This slice's capacity is 7, it can only see the elements beginning with the 4th element.

4: This is an error. The backing array doesn't have 100 elements.

Which one is the slice header of the following slice?

var tasks []string

1. Pointer: 0, Length: 0, Capacity: 0 CORRECT
2. Pointer: 10, Length: 5, Capacity: 10
3. Pointer: 0, Length: 1, Capacity: 1

1: A nil slice doesn't have backing array, so all the fields are equal to zero.

What is the total memory usage of this code?

var array [1000]int64

array2 := array
slice := array2[:]

1. 1024 bytes
2. 2024 bytes
3. 3000 bytes
4. 16024 bytes CORRECT

4: array is 1000 x int64 (8 bytes) = 8000 bytes. Assigning an array copies all its elements, so array2 adds additional 8000 bytes. A slice doesn't store anything on its own. Here, it's being created from array2, so it doesn't allocate a backing array as well. A slice header's size is 24 bytes. So in total: This program allocates 16024 bytes.

What value does this code pass to the sort.Ints function?

nums := []int{9, 7, 5, 3, 1}
sort.Ints(nums)

1. [9 7 5 3 1] — All the values of the nums slice
2. A pointer to the backing array of the nums slice
3. A pointer, length and capacity as three different arguments
4. The slice header that is stored in the nums variable CORRECT

1: No, a slice value doesn't contain any elements. So it cannot pass the elements.

2: Sorry but not only that.

3: Nope. Remember, they are packed in a tiny data structure called the ....?

4: Yep! A slice value is a slice header (pointer, length and capacity). A slice variable stores the slice header.