first

course/10-advanced_functions/exercises/5-closures/code.go

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+package main
+
+import "fmt"
+
+func adder() func(int) int {
+	// ?
+}
+
+// don't touch below this line
+
+type emailBill struct {
+	costInPennies int
+}
+
+func test(bills []emailBill) {
+	defer fmt.Println("====================================")
+	countAdder, costAdder := adder(), adder()
+	for _, bill := range bills {
+		fmt.Printf("You've sent %d emails and it has cost you %d cents\n", countAdder(1), costAdder(bill.costInPennies))
+	}
+}
+
+func main() {
+	test([]emailBill{
+		{45},
+		{32},
+		{43},
+		{12},
+		{34},
+		{54},
+	})
+
+	test([]emailBill{
+		{12},
+		{12},
+		{976},
+		{12},
+		{543},
+	})
+
+	test([]emailBill{
+		{743},
+		{13},
+		{8},
+	})
+}

course/10-advanced_functions/exercises/5-closures/complete.go

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+package main
+
+import "fmt"
+
+func adder() func(int) int {
+	sum := 0
+	return func(x int) int {
+		sum += x
+		return sum
+	}
+}
+
+// don't touch below this line
+
+type emailBill struct {
+	costInPennies int
+}
+
+func test(bills []emailBill) {
+	defer fmt.Println("====================================")
+	countAdder, costAdder := adder(), adder()
+	for _, bill := range bills {
+		fmt.Printf("You've sent %d emails and it has cost you %d cents\n", countAdder(1), costAdder(bill.costInPennies))
+	}
+}
+
+func main() {
+	test([]emailBill{
+		{45},
+		{32},
+		{43},
+		{12},
+		{34},
+		{54},
+	})
+
+	test([]emailBill{
+		{12},
+		{12},
+		{976},
+		{12},
+		{543},
+	})
+
+	test([]emailBill{
+		{743},
+		{13},
+		{8},
+	})
+}

course/10-advanced_functions/exercises/5-closures/expected.txt

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+You've sent 1 emails and it has cost you 45 cents
+You've sent 2 emails and it has cost you 77 cents
+You've sent 3 emails and it has cost you 120 cents
+You've sent 4 emails and it has cost you 132 cents
+You've sent 5 emails and it has cost you 166 cents
+You've sent 6 emails and it has cost you 220 cents
+====================================
+You've sent 1 emails and it has cost you 12 cents
+You've sent 2 emails and it has cost you 24 cents
+You've sent 3 emails and it has cost you 1000 cents
+You've sent 4 emails and it has cost you 1012 cents
+You've sent 5 emails and it has cost you 1555 cents
+====================================
+You've sent 1 emails and it has cost you 743 cents
+You've sent 2 emails and it has cost you 756 cents
+You've sent 3 emails and it has cost you 764 cents
+====================================

course/10-advanced_functions/exercises/5-closures/readme.md

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+# Closures
+
+A closure is a function that references variables from outside its own function body. The function may access and *assign* to the referenced variables.
+
+In this example, the `concatter()` function returns a function that has reference to an *enclosed* `doc` value. Each successive call to `harryPotterAggregator` mutates that same `doc` variable.
+
+```go
+func concatter() func(string) string {
+	doc := ""
+	return func(word string) string {
+		doc += word + " "
+		return doc
+	}
+}
+
+func main() {
+	harryPotterAggregator := concatter()
+	harryPotterAggregator("Mr.")
+	harryPotterAggregator("and")
+	harryPotterAggregator("Mrs.")
+	harryPotterAggregator("Dursley")
+	harryPotterAggregator("of")
+	harryPotterAggregator("number")
+	harryPotterAggregator("four,")
+	harryPotterAggregator("Privet")
+
+	fmt.Println(harryPotterAggregator("Drive"))
+	// Mr. and Mrs. Dursley of number four, Privet Drive
+}
+```
+
+## Assignment
+
+Keeping track of how many emails we send is mission-critical at Mailio. Complete the `adder()` function.
+
+It should return a function that adds its input (an `int`) to an enclosed `sum` value, then return the new sum. In other words, it keeps a running total of the `sum` variable within a closure.
+