91 lines
2.7 KiB
Markdown
91 lines
2.7 KiB
Markdown
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# Slice Header Quiz
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## What is a slice header?
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1. The first element of a slice value
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2. The first element of the backing array
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3. A tiny data structure that describes all or some part of a backing array *CORRECT*
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4. A data structure that contains the elements of a slice
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> **3:** Yes! It's just a tiny data structure with three numeric fields.
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>
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> **4:** A slice doesn't contain any elements on its own.
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## What are the fields of a slice value?
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1. Pointer, length, and capacity *CORRECT*
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2. Length and capacity
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3. Only a pointer
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## Which slice value does the following slice header describe?
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SLICE HEADER:
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+ Pointer : 100th
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+ Length : 5
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+ Capacity: 10
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Assume that the backing array is this one:
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```go
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var array [10]string
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```
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1. array[5:]
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2. array[:5] *CORRECT*
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3. array[3:]
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4. array[100:]
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> **1**: This slice's capacity is 5, it can only see the elements beginning with the 6th element.
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>
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> **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.
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>
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> **3**: This slice's capacity is 7, it can only see the elements beginning with the 4th element.
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>
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> **4**: This is an error. The backing array doesn't have 100 elements.
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>
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## Which one is the slice header of the following slice?
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```go
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var tasks []string
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```
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1. Pointer: 0, Length: 0, Capacity: 0 *CORRECT*
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2. Pointer: 10, Length: 5, Capacity: 10
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3. Pointer: 0, Length: 1, Capacity: 1
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> **1:** A nil slice doesn't have backing array, so all the fields are equal to zero.
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## What is the total memory usage of this code?
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```go
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var array [1000]int64
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array2 := array
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slice := array2[:]
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```
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1. 1024 bytes
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2. 2024 bytes
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3. 3000 bytes
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4. 16024 bytes *CORRECT*
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> **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.
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## What value does this code pass to the sort.Ints function?
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```go
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nums := []int{9, 7, 5, 3, 1}
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sort.Ints(nums)
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```
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1. [9 7 5 3 1] — All the values of the nums slice
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2. A pointer to the backing array of the nums slice
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3. A pointer, length and capacity as three different arguments
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4. The slice header that is stored in the nums variable *CORRECT*
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> **1:** No, a slice value doesn't contain any elements. So it cannot pass the elements.
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>
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> **2:** Sorry but not only that.
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>
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> **3:** Nope. Remember, they are packed in a tiny data structure called the ....?
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>
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> **4:** Yep! A slice value is a slice header (pointer, length and capacity). A slice variable stores the slice header.
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>
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