answers student question

25_QUESTIONS-FROM-STUDENTS/04_goroutines_closing-chan/01_broken-code/main.go

@@ -0,0 +1,48 @@
+package main
+
+import "fmt"
+
+// THIS CODE DOES NOT RUN
+// see next folder for fixed code with notes
+
+var done chan bool
+
+func main() {
+	done = make(chan bool)
+	c := fanIn(incrementor("1"), incrementor("2"))
+	for n := range c {
+		fmt.Println(n)
+	}
+}
+
+func incrementor(s string) <-chan string {
+	c := make(chan string)
+	go func() {
+		for i := 0; i < 20; i++ {
+			c <- fmt.Sprintf("Process: "+s+" printing:", i)
+		}
+		done <- true
+	}()
+	return c
+}
+
+// FAN IN
+func fanIn(input1, input2 <-chan string) <-chan string {
+	c := make(chan string)
+	go func() {
+		for {
+			c <- <-input1
+		}
+	}()
+	go func() {
+		for {
+			c <- <-input2
+		}
+	}()
+	go func() {
+		<-done
+		<-done
+		close(c)
+	}()
+	return c
+}

25_QUESTIONS-FROM-STUDENTS/04_goroutines_closing-chan/02_fixed-code/main.go

@@ -0,0 +1,114 @@
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	// read NOTE ONE below
+	c1 := incrementor("1")
+	c2 := incrementor("2")
+	// NOTE TWO
+	// the flow of code continues here in main
+	// incrementor is called
+	// launches goroutines
+	// returns channels which will eventually be closed
+	// by the code that is running in the goroutines
+	// launched by incrementor
+	// program flow in main continues on down vertically here
+
+	// NOTE THREE
+	// you're passing into "fanIn" two channels
+	// fanIn HAS TO pull values off of those channels
+	// otherwise: DEADLOCK
+	// fanIn will need to launch goroutines to pull those values
+	c := fanIn(c1, c2)
+
+	// NOTE SEVEN
+	// we are held up here
+	// pulling values off of c
+	// until c is closed
+	// and all values have been pulled from c
+	// at which point ...
+	for n := range c {
+		fmt.Println(n)
+	}
+
+	// NOTE EIGHT
+	// our program is done
+	// flow of code exits out of main
+	// program ends
+}
+
+func incrementor(s string) <-chan string {
+	c := make(chan string)
+	go func() {
+		for i := 0; i < 20; i++ {
+			// NOTE NINE
+			// use Sprint here, or Sprintln, not Sprintf
+			// this code is formatted for Sprintln
+			// Sprintf would need this
+			// c <- fmt.Sprintf("Process: %v, printing %v", s, i)
+			c <- fmt.Sprint("Process: "+s+" printing:", i)
+		}
+		// NOTE ONE
+		// incrementor
+		// every time it's called
+		// create a channel, put values on the channel
+		// ******** important ********
+		// have some other goroutine somewhere
+		// pulling values off the channel
+		// ***************************
+		// close the channel
+		// return that closed channel
+		// these "incrementor" goroutines are off and running
+		close(c)
+	}()
+	return c
+}
+
+// FAN IN
+func fanIn(input1, input2 <-chan string) <-chan string {
+	c := make(chan string)
+	done := make(chan bool)
+
+	// NOTE FOUR
+	// these goroutines will pull values off the "incrementor" channels
+	// I'm defining the func to have a channel as a parameter and
+	// I'm passing the channels in as an argument
+	// as this is good practice to avoid different channels
+	// accessing the same data and creating a race condition
+	// not needed here, but good practice
+	// and good to know about
+
+	go func(x <-chan string) {
+		for n := range x {
+			c <-n
+		}
+		done <- true
+	}(input1)
+
+	go func(x <-chan string) {
+		for n := range x {
+			c <-n
+		}
+		done <- true
+	}(input2)
+
+	// NOTE FIVE
+	// this will signal when we're done writing values to c
+	go func() {
+		<-done
+		<-done
+		close(c)
+	}()
+
+	// NOTE SIX
+	// all of the above code
+	// just flows straight through
+	// goroutines are launched
+	// and even though they're not done processing
+	// program flow comes to here and this func returns
+	// the channel c
+	return c
+}