Slice Header Quiz

What is a slice header?

The first element of a slice value
The first element of the backing array
A tiny data structure that describes all or some part of a backing array CORRECT
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?

Pointer, length, and capacity CORRECT
Length and capacity
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

array[5:]
array[:5] CORRECT
array[3:]
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

Pointer: 0, Length: 0, Capacity: 0 CORRECT
Pointer: 10, Length: 5, Capacity: 10
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[:]

1024 bytes
2024 bytes
3000 bytes
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)

[9 7 5 3 1] — All the values of the nums slice
A pointer to the backing array of the nums slice
A pointer, length and capacity as three different arguments
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.