add: slice internals backing array and slice header

16-slices/06-slice-expressions/2-example/main.go

@@ -17,18 +17,9 @@ func main() {
 	// think of this as search results of a search engine.
 	// it could have been fetched from a database
 	items := []string{
-		"pacman",
+		"pacman", "mario", "tetris", "doom",
-		"mario",
+		"galaga", "frogger", "asteroids", "simcity",
-		"tetris",
+		"metroid", "defender", "rayman", "tempest",
-		"doom",
-		"galaga",
-		"frogger",
-		"asteroids",
-		"simcity",
-		"metroid",
-		"defender",
-		"rayman",
-		"tempest",
 		"ultima",
 	}
 

16-slices/09-slice-internals-2-slice-header/1-theory/main.go

@@ -0,0 +1,46 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+
+	s "github.com/inancgumus/prettyslice"
+)
+
+func main() {
+	//
+	// each int element is 4 bytes (on 64-bit)
+	//
+	// let's say the ages point to 1000th.
+	// ages[1:] will point to 1004th
+	// ages[2:] will point to 1008th and so on.
+	//
+	// they all will be looking at the same
+	// backing array.
+	//
+
+	ages := []int{35, 15, 25}
+	red, green := ages[0:1], ages[1:3]
+
+	s.Show("ages", ages)
+	s.Show("red", red)
+	s.Show("green", green)
+
+	fmt.Println(red[0])
+	// fmt.Println(red[1]) // error
+	// fmt.Println(red[2]) // error
+
+	{
+		var ages []int
+		s.Show("nil slice", ages)
+
+		// or just:
+		s.Show("nil slice", []int(nil))
+	}
+}

16-slices/09-slice-internals-2-slice-header/2-example/main.go

@@ -19,7 +19,7 @@ type collection []string // #2
 
 // go is pass by copy
 // only the slice header is copied: 3 integer fields (24 bytes)
-// think of passing an array with millions of elements.
+// think of passing an array with millions of elements instead.
 
 func main() {
 	// SliceHeader lives here:

16-slices/exercises/16-internals-backing-array-fix/main.go

@@ -0,0 +1,60 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+	"math/rand"
+	"time"
+)
+
+// ---------------------------------------------------------
+// EXERCISE: Fix the backing array problem
+//
+//  You receive numbers from an API. After you're done working
+//  with it, the API needs to continue using those numbers.
+//
+//  But your program changes the numbers (changes the API's slice).
+//
+//  Fix the program so that your program doesn't modify
+//  the original numbers.
+//
+//
+// RESTRICTION
+//
+//  Fix your problem only in the designated area of the code below.
+//
+//
+// EXPECTED OUTPUT
+//
+//  Mine         : [-50 -100 -150]
+//  Original nums: [56 89 15]
+//
+//  Note: Original nums may vary (they're random)
+//        But your slice should look like the above (mine slice)
+//
+//        Yes, it should output only three numbers for the both slices!
+//
+// ---------------------------------------------------------
+
+func main() {
+	// API returns random numbers in an int slice
+	rand.Seed(time.Now().UnixNano())
+	nums := rand.Perm(100)
+
+	// ----------------------------------------
+	// RESTRICTIONS — ONLY ADD YOUR CODE HERE
+	//
+	mine := nums
+	//
+	// ----------------------------------------
+
+	mine[0], mine[1], mine[2] = -50, -100, -150
+	fmt.Println("Mine         :", mine)
+	fmt.Println("Original nums:", nums[:3])
+}

16-slices/exercises/16-internals-backing-array-fix/solution/main.go

@@ -0,0 +1,35 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+	"math/rand"
+	"time"
+)
+
+func main() {
+	rand.Seed(time.Now().UnixNano())
+	nums := rand.Perm(100)
+
+	// ----------------------------------------
+	// breaks the connection:
+	// mine and nums now have different backing arrays
+
+	// verbose solution:
+	// var mine []int
+	// mine = append(mine, nums[:3]...)
+
+	// better solution (almost the same thing):
+	mine := append([]int(nil), nums[:3]...)
+	// ----------------------------------------
+
+	mine[0], mine[1], mine[2] = -50, -100, -150
+	fmt.Println("Mine         :", mine)
+	fmt.Println("Original nums:", nums[:3])
+}

16-slices/exercises/17-internals-backing-array-sort/main.go

@@ -0,0 +1,52 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+)
+
+// ---------------------------------------------------------
+// EXERCISE: Sort the backing array
+//
+//  1. Sort only the middle 3 items.
+//
+//  2. All the slices should see your change.
+//
+//
+// RESTRICTION
+//
+//  Do not sort manually. Sort by slicing then by using the sort package.
+//
+//
+// EXPECTED OUTPUT
+//
+//  Original: [pacman mario tetris doom galaga frogger asteroids simcity metroid defender rayman tempest ultima]
+//
+// Sorted  : [pacman mario tetris doom galaga asteroids frogger simcity metroid defender rayman tempest ultima]
+//
+//
+// HINT:
+//
+//   Middle items are         : [frogger asteroids simcity]
+//
+//   After sorting they become: [asteroids frogger simcity]
+//
+// ---------------------------------------------------------
+
+func main() {
+	items := []string{
+		"pacman", "mario", "tetris", "doom", "galaga", "frogger",
+		"asteroids", "simcity", "metroid", "defender", "rayman",
+		"tempest", "ultima",
+	}
+
+	fmt.Println("Original:", items)
+	fmt.Println()
+	fmt.Println("Sorted  :", items)
+}

16-slices/exercises/17-internals-backing-array-sort/solution/main.go

@@ -0,0 +1,33 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+	"sort"
+)
+
+func main() {
+	items := []string{
+		"pacman", "mario", "tetris", "doom", "galaga", "frogger",
+		"asteroids", "simcity", "metroid", "defender", "rayman",
+		"tempest", "ultima",
+	}
+
+	fmt.Println("Original:", items)
+
+	mid := len(items) / 2
+	smid := items[mid-1 : mid+2]
+
+	// sorting the smid will affect the items
+	// as well. their backing array is the same.
+	sort.Strings(smid)
+
+	fmt.Println()
+	fmt.Println("Sorted  :", items)
+}

16-slices/exercises/18-internals-slice-header/main.go

@@ -0,0 +1,115 @@
+package main
+
+import (
+	"fmt"
+	"runtime"
+	"runtime/debug"
+)
+
+// ---------------------------------------------------------
+// EXERCISE: Observe the memory allocations
+//
+//  In this exercise, your goal is to observe the memory allocation
+//  differences between arrays and slices.
+//
+//  You will create, assign arrays and slices then you will print
+//  the memory usage of your program on each step.
+//
+//  Please follow the instructions inside the code.
+//
+//
+// EXPECTED OUTPUT
+//
+//  Note that, your memory usage numbers may vary. These are on my
+//  own system. However, the size of the arrays and slices should be
+//  the same on your own system as well (if you're on 64-bit machine).
+//
+//
+//  <<< initial memory usage >>>
+//          > Memory Usage: 104 KB
+//  <<< after declaring an array >>>
+//          > Memory Usage: 78235 KB
+//  <<< after copying the array >>>
+//          > Memory Usage: 156365 KB
+//  <<< inside passArray >>>
+//          > Memory Usage: 234495 KB
+//  <<< after slicings >>>
+//          > Memory Usage: 234497 KB
+//  <<< inside passSlice >>>
+//          > Memory Usage: 234497 KB
+//
+//  Array's size : 80000000 bytes.
+//  Array2's size: 80000000 bytes.
+//  Slice1's size: 24 bytes.
+//  Slice2's size: 24 bytes.
+//  Slice3's size: 24 bytes.
+//
+//
+// HINTS
+//
+//  I've declared a few function to help you.
+//
+//    report function prints the memory usage.
+//    Just call it with a message that matches to the expected output.
+//
+//    passArray function accepts a [size]int array, so you can pass it
+//    your array. It automatically prints the memory usage.
+//
+//    passSlice function accepts an int slice, so you can pass it
+//    your one of your slices. It automatically prints the memory usage.
+//
+// ---------------------------------------------------------
+
+const size = 1e7
+
+func main() {
+	// stops the gc: prevents cleaning up the memory
+	debug.SetGCPercent(-1)
+
+	// run the program to see what this prints
+	report("initial memory usage")
+
+	// 1. allocate an array with 10 million int elements
+	//    this array's size is equal to ~80MB
+	//    hint: use the `size` constant
+	//
+	// 2. print the memory usage
+
+	// 3. copy the array to a new array (just assign)
+	// 4. print the memory usage
+
+	// 5. pass the array to passArray function
+
+	// 6. convert the array to a slice (by slicing)
+	// 7. slice only the first 1000 elements of the array
+	// 8. slice only the elements of the array between 1000 and 10000
+	// 9. print the memory usage
+
+	// 10. pass the one of the slices to passSlice function
+
+	// 11. print the sizes of the arrays and slices
+	//     hint: use the unsafe.Sizeof function
+}
+
+// observe that passing an array affects the memory usage dramatically
+//
+// passes [size]int array — about 80MB!
+func passArray(items [size]int) {
+	items[0] = 100
+	report("inside passArray")
+}
+
+// observe that passing a slice doesn't affect the memory usage
+//
+// only passes 24-bytes of slice header
+func passSlice(items []int) {
+	items[0] = 100
+	report("inside passSlice")
+}
+
+func report(msg string) {
+	var m runtime.MemStats
+	runtime.ReadMemStats(&m)
+	fmt.Printf("<<< %s >>>\n", msg)
+	fmt.Printf("\t> Memory Usage: %v KB\n", m.Alloc/1024)
+}

16-slices/exercises/18-internals-slice-header/solution/main.go

@@ -0,0 +1,62 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+	"runtime"
+	"runtime/debug"
+	"unsafe"
+)
+
+const size = 1e7
+
+func main() {
+	// stops the gc: prevents cleaning up the memory
+	debug.SetGCPercent(-1)
+
+	report("initial memory usage")
+
+	var array [size]int
+	report("after declaring an array")
+
+	array2 := array
+	report("after copying the array")
+
+	passArray(array)
+
+	slice1 := array[:]
+	slice2 := array[1e3:]
+	slice3 := array[1e3:1e4]
+	report("after slicings")
+
+	passSlice(slice3)
+
+	fmt.Println()
+	fmt.Printf("Array's size : %d bytes.\n", unsafe.Sizeof(array))
+	fmt.Printf("Array2's size: %d bytes.\n", unsafe.Sizeof(array2))
+	fmt.Printf("Slice1's size: %d bytes.\n", unsafe.Sizeof(slice1))
+	fmt.Printf("Slice2's size: %d bytes.\n", unsafe.Sizeof(slice2))
+	fmt.Printf("Slice3's size: %d bytes.\n", unsafe.Sizeof(slice3))
+}
+
+func passArray(items [size]int) {
+	items[0] = 100
+	report("inside passArray")
+}
+
+func passSlice(items []int) {
+	report("inside passSlice")
+}
+
+func report(msg string) {
+	var m runtime.MemStats
+	runtime.ReadMemStats(&m)
+	fmt.Printf("<<< %s >>>\n", msg)
+	fmt.Printf("\t> Memory Usage: %v KB\n", m.Alloc/1024)
+}

16-slices/exercises/README.md

@@ -42,3 +42,13 @@ These are warm-up exercises that will reinforce your knowledge of slices.
 2. **[Slicing by arguments](https://github.com/inancgumus/learngo/tree/master/16-slices/exercises/14-slicing-by-args)**
 
 3. **[Slicing the Housing Prices](https://github.com/inancgumus/learngo/tree/master/16-slices/exercises/15-slicing-housing-prices)**
+
+---
+
+## Exercises Level IV - Internals
+
+1. **[Fix the backing array problems](https://github.com/inancgumus/learngo/tree/master/16-slices/exercises/16-internals-backing-array-fix)**
+
+2. **[Sort the backing array](https://github.com/inancgumus/learngo/tree/master/16-slices/exercises/17-internals-backing-array-sort)**
+
+3. **[Observe the memory allocations](https://github.com/inancgumus/learngo/tree/master/16-slices/exercises/18-internals-slice-header)**

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

@@ -0,0 +1,131 @@
+# 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?
+```go
+// 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 is 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"?
+```go
+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"?
+```go
+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?
+```go
+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?
+```go
+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?
+```go
+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?
+```go
+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

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

@@ -0,0 +1,90 @@
+# 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:
+```go
+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?
+```go
+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?
+```go
+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?
+```go
+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.
+> 
+

16-slices/questions/README.md

@@ -5,3 +5,5 @@
 * [Appending](2-appending.md)
 
 * [Slicing](3-slicing.md)
+
+* [Backing Array](4-backing-array.md)

x-tba/slices/10-slice-internals-2-slice-header/1-theory/main.go

@@ -1,21 +0,0 @@
-// For more tutorials: https://blog.learngoprogramming.com
-//
-// Copyright © 2018 Inanc Gumus
-// Learn Go Programming Course
-// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
-//
-
-package main
-
-import s "github.com/inancgumus/prettyslice"
-
-func main() {
-	ages := []int{35, 15, 25}
-	first, last := ages[0:1], ages[1:3]
-
-	s.Show("ages", ages)
-	s.Show("first", first)
-	s.Show("last", last)
-
-	s.Show("nil slice", []int(nil))
-}