From c1b7c64da2aa492576e9cbe67700b7281d88eb7d Mon Sep 17 00:00:00 2001
From: Robert Jeutter <robert.jeutter@gmx.de>
Date: Sat, 1 Jan 2022 19:47:24 +0100
Subject: [PATCH] go by example part 2

---
 5-go-by-example/11-range.go                | 38 +++++++++++++++
 5-go-by-example/12-function.go             | 24 ++++++++++
 5-go-by-example/13-muliple-return-value.go | 20 ++++++++
 5-go-by-example/14-variadic-function.go    | 26 ++++++++++
 5-go-by-example/15-closures.go             | 29 ++++++++++++
 5-go-by-example/16-recursion.go            | 29 ++++++++++++
 5-go-by-example/17-pointers.go             | 33 +++++++++++++
 5-go-by-example/18-structs.go              | 49 +++++++++++++++++++
 5-go-by-example/19-methods.go              | 32 +++++++++++++
 5-go-by-example/20-interfaces.go           | 55 ++++++++++++++++++++++
 10 files changed, 335 insertions(+)
 create mode 100644 5-go-by-example/11-range.go
 create mode 100644 5-go-by-example/12-function.go
 create mode 100644 5-go-by-example/13-muliple-return-value.go
 create mode 100644 5-go-by-example/14-variadic-function.go
 create mode 100644 5-go-by-example/15-closures.go
 create mode 100644 5-go-by-example/16-recursion.go
 create mode 100644 5-go-by-example/17-pointers.go
 create mode 100644 5-go-by-example/18-structs.go
 create mode 100644 5-go-by-example/19-methods.go
 create mode 100644 5-go-by-example/20-interfaces.go

diff --git a/5-go-by-example/11-range.go b/5-go-by-example/11-range.go
new file mode 100644
index 0000000..42d8783
--- /dev/null
+++ b/5-go-by-example/11-range.go
@@ -0,0 +1,38 @@
+package main
+
+import "fmt"
+
+// range iterates over elements in a variety of data structures
+func main() {
+
+	// use range to sum the numbers in a slice
+	nums := []int{2, 3, 4}
+	sum := 0
+	for _, num := range nums {
+		sum += num
+	}
+	fmt.Println("sum:", sum)
+
+	// range on arrays and slices provides both the index and value for each entry
+	for i, num := range nums {
+		if num == 3 {
+			fmt.Println("index:", i)
+		}
+	}
+
+	// range on map iterates over key/value pairs
+	kvs := map[string]string{"a": "apple", "b": "banana"}
+	for k, v := range kvs {
+		fmt.Printf("%s -> %s\n", k, v)
+	}
+
+	// range iterate over keys of a map
+	for k := range kvs {
+		fmt.Println("key:", k)
+	}
+
+	// range on strings iterates over Unicode code points
+	for i, c := range "go" {
+		fmt.Println(i, c)
+	}
+}
diff --git a/5-go-by-example/12-function.go b/5-go-by-example/12-function.go
new file mode 100644
index 0000000..f38a843
--- /dev/null
+++ b/5-go-by-example/12-function.go
@@ -0,0 +1,24 @@
+package main
+
+import "fmt"
+
+// function that takes two ints and returns their sum as an int
+// Go requires explicit returns
+func plus(a int, b int) int {
+	return a + b
+}
+
+// multiple consecutive parameters of the same type,
+// may omit the type name for the like-typed parameters up to the final parameter that declares the type
+func plusPlus(a, b, c int) int {
+	return a + b + c
+}
+
+func main() {
+	// Call a function
+	res := plus(1, 2)
+	fmt.Println("1+2 =", res)
+
+	res = plusPlus(1, 2, 3)
+	fmt.Println("1+2+3 =", res)
+}
diff --git a/5-go-by-example/13-muliple-return-value.go b/5-go-by-example/13-muliple-return-value.go
new file mode 100644
index 0000000..38912d8
--- /dev/null
+++ b/5-go-by-example/13-muliple-return-value.go
@@ -0,0 +1,20 @@
+package main
+
+import "fmt"
+
+// (int, int) in this function signature shows that the function returns 2 ints
+func vals() (int, int) {
+	return 3, 7
+}
+
+func main() {
+
+	// use the 2 different return values
+	a, b := vals()
+	fmt.Println(a)
+	fmt.Println(b)
+
+	// use the blank identifier _
+	_, c := vals()
+	fmt.Println(c)
+}
diff --git a/5-go-by-example/14-variadic-function.go b/5-go-by-example/14-variadic-function.go
new file mode 100644
index 0000000..7c17a47
--- /dev/null
+++ b/5-go-by-example/14-variadic-function.go
@@ -0,0 +1,26 @@
+package main
+
+import "fmt"
+
+// Variadic functions can be called with any number of trailing arguments
+
+// take an arbitrary number of ints as arguments
+func sum(nums ...int) {
+	fmt.Print(nums, " ")
+	total := 0
+	for _, num := range nums {
+		total += num
+	}
+	fmt.Println(total)
+}
+
+func main() {
+
+	// Variadic functions can be called in the usual way
+	sum(1, 2)
+	sum(1, 2, 3)
+
+	// apply multiple args to a variadic function
+	nums := []int{1, 2, 3, 4}
+	sum(nums...)
+}
diff --git a/5-go-by-example/15-closures.go b/5-go-by-example/15-closures.go
new file mode 100644
index 0000000..a97ee4d
--- /dev/null
+++ b/5-go-by-example/15-closures.go
@@ -0,0 +1,29 @@
+package main
+
+import "fmt"
+
+// Go supports anonymous functions, which can form closures. Anonymous functions are useful when you want to define a function inline without having to name it.
+
+// intSeq returns another function, which is defined anonymously in the body of intSeq
+// the returned function closes over the variable i to form a closure
+func intSeq() func() int {
+	i := 0
+	return func() int {
+		i++
+		return i
+	}
+}
+
+func main() {
+	// call intSeq, assigning the result (a function) to nextInt
+	nextInt := intSeq()
+
+	// see the effect of the closure
+	fmt.Println(nextInt())
+	fmt.Println(nextInt())
+	fmt.Println(nextInt())
+
+	// confirm that the state is unique to that particular function
+	newInts := intSeq()
+	fmt.Println(newInts())
+}
diff --git a/5-go-by-example/16-recursion.go b/5-go-by-example/16-recursion.go
new file mode 100644
index 0000000..7a3914e
--- /dev/null
+++ b/5-go-by-example/16-recursion.go
@@ -0,0 +1,29 @@
+package main
+
+import "fmt"
+
+// fact function calls itself until it reaches the base case of fact(0)
+func fact(n int) int {
+	if n == 0 {
+		return 1
+	}
+	return n * fact(n-1)
+}
+
+func main() {
+	fmt.Println(fact(7))
+
+	// Closures can also be recursive, but this requires the closure to be declared with a typed var explicitly before it’s defined
+	var fib func(n int) int
+
+	// Go knows which function to call with fib as it is defined previously in main
+	fib = func(n int) int {
+		if n < 2 {
+			return n
+		}
+		return fib(n-1) + fib(n-2)
+
+	}
+
+	fmt.Println(fib(7))
+}
diff --git a/5-go-by-example/17-pointers.go b/5-go-by-example/17-pointers.go
new file mode 100644
index 0000000..0a2c440
--- /dev/null
+++ b/5-go-by-example/17-pointers.go
@@ -0,0 +1,33 @@
+package main
+
+import "fmt"
+
+// Go supports pointers, allowing you to pass references to values and records within your program
+
+// zeroval has an int parameter, so arguments will be passed to it by value
+// zeroval will get a copy of ival distinct from the one in the calling function
+func zeroval(ival int) {
+	ival = 0
+}
+
+// zeroptr has an *int parameter, meaning that it takes an int pointer
+// the *iptr code in the function body then dereferences the pointer from its memory address to the current value at that address
+// assigning a value to a dereferenced pointer changes the value at the referenced address
+func zeroptr(iptr *int) {
+	*iptr = 0
+}
+
+func main() {
+	i := 1
+	fmt.Println("initial:", i)
+
+	zeroval(i)
+	fmt.Println("zeroval:", i)
+
+	// &i syntax gives the memory address
+	zeroptr(&i)
+	fmt.Println("zeroptr:", i)
+
+	// Pointers can be printed
+	fmt.Println("pointer:", &i)
+}
diff --git a/5-go-by-example/18-structs.go b/5-go-by-example/18-structs.go
new file mode 100644
index 0000000..691ebce
--- /dev/null
+++ b/5-go-by-example/18-structs.go
@@ -0,0 +1,49 @@
+package main
+
+import "fmt"
+
+// Go’s structs are typed collections of fields. They’re useful for grouping data together to form records.
+
+// this person struct type has name and age fields
+type person struct {
+	name string
+	age  int
+}
+
+// newPerson constructs a new person struct with the given name
+// safely return a pointer to local variable as a local variable will survive the scope of the function
+func newPerson(name string) *person {
+	p := person{name: name}
+	p.age = 42
+	return &p
+}
+
+func main() {
+	// create a new struct
+	fmt.Println(person{"Bob", 20})
+
+	// name the fields when initializing a struct
+	fmt.Println(person{name: "Alice", age: 30})
+
+	// omitted fields will be zero-valued
+	fmt.Println(person{name: "Fred"})
+
+	// & prefix yields a pointer to the struct
+	fmt.Println(&person{name: "Ann", age: 40})
+
+	// encapsulate new struct creation in constructor functions (ideomatic)
+	fmt.Println(newPerson("Jon"))
+
+	// access struct fields with a dot
+	s := person{name: "Sean", age: 50}
+	fmt.Println(s.name)
+
+	// use dots with struct pointers
+	// the pointers are automatically dereferenced
+	sp := &s
+	fmt.Println(sp.age)
+
+	// structs are mutable
+	sp.age = 51
+	fmt.Println(sp.age)
+}
diff --git a/5-go-by-example/19-methods.go b/5-go-by-example/19-methods.go
new file mode 100644
index 0000000..6137fc5
--- /dev/null
+++ b/5-go-by-example/19-methods.go
@@ -0,0 +1,32 @@
+// methods defined on struct types
+
+package main
+
+import "fmt"
+
+type rect struct {
+	width, height int
+}
+
+// area method has a receiver type of *rect
+func (r *rect) area() int {
+	return r.width * r.height
+}
+
+// Methods can be defined for either pointer or value receiver types
+func (r rect) perim() int {
+	return 2*r.width + 2*r.height
+}
+
+func main() {
+	r := rect{width: 10, height: 5}
+
+	// call 2 methods defined for the struct
+	fmt.Println("area: ", r.area())
+	fmt.Println("perim:", r.perim())
+
+	// Go automatically handles conversion between values and pointers for method calls
+	rp := &r
+	fmt.Println("area: ", rp.area())
+	fmt.Println("perim:", rp.perim())
+}
diff --git a/5-go-by-example/20-interfaces.go b/5-go-by-example/20-interfaces.go
new file mode 100644
index 0000000..110e1bb
--- /dev/null
+++ b/5-go-by-example/20-interfaces.go
@@ -0,0 +1,55 @@
+// Interfaces are named collections of method signatures
+
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+// basic interface for geometric shapes
+type geometry interface {
+	area() float64
+	perim() float64
+}
+
+// implement interface on rect and circle types
+type rect struct {
+	width, height float64
+}
+type circle struct {
+	radius float64
+}
+
+// implement geometry on rects
+func (r rect) area() float64 {
+	return r.width * r.height
+}
+func (r rect) perim() float64 {
+	return 2*r.width + 2*r.height
+}
+
+// implementation for circles
+func (c circle) area() float64 {
+	return math.Pi * c.radius * c.radius
+}
+func (c circle) perim() float64 {
+	return 2 * math.Pi * c.radius
+}
+
+// call methods that are in the named interface
+func measure(g geometry) {
+	fmt.Println(g)
+	fmt.Println(g.area())
+	fmt.Println(g.perim())
+}
+
+func main() {
+	r := rect{width: 3, height: 4}
+	c := circle{radius: 5}
+
+	// circle and rect struct types both implement the geometry interface
+	// use instances of these structs as arguments to measure
+	measure(r)
+	measure(c)
+}