Lab 2: Go Solution

Exercise 1: Basic Methods
Define the type dog as follows:

type dog struct {
  name string
  race string
  female bool
}
Write a method that will rename a dog which can be used as follows:

func main() {
    fido := dog {"Fido", "Poodle", false }
    fido.rename("Cocotte")
}
Exercise 2: Go Channel Basics
Test what will be printed by the following

package main

import (
    "fmt"
    "time"
    "strconv"
)

var i int

func makeCakeAndSend(cs chan string) {
    i = i + 1
    cakeName := "Strawberry Cake " + strconv.Itoa(i)
    fmt.Println("Making a cake and sending ...", cakeName)
    cs <- cakeName //send a strawberry cake
}

func receiveCakeAndPack(cs chan string) {
    s := <-cs //get whatever cake is on the channel
    fmt.Println("Packing received cake: ", s)
}

func main() {
    cs := make(chan string)
    for i := 0; i<3; i++ {
        go makeCakeAndSend(cs)
        go receiveCakeAndPack(cs)

        //sleep for a while so that the program doesn't exit
        //immediately and output is clear for illustration
        time.Sleep(1 * 1e9)
    }
} 
Exercise 3:
The following program is to be changed such that it uses arbitrary sized slices instead of fixed size arrays. In particular, the user entered size (variable sz) is to replace the hard-coded 32 in main and any function changed correspondingly.

package main

import "fmt"
import "math"
import "math/rand"

type Series struct {
        a, b float64
}

func (s Series) add(t, TP int) float64 {
        return s.a*math.Sin(2.0*math.Pi*float64(t)/float64(TP)) + s.b*math.Cos(2.0*math.Pi*float64(t)/float64(TP))
}

func fourier(c [32]Series, t, TP int) (res float64) {
        res = c[0].a
        for n := 1; n < 32; n++ {
                res += c[n].add(t, TP)
        }
        return
}

func main() {
        TP := 4
        sz := 1
        var res float64

        // Enter size of fourier series
        fmt.Print("Size of series (1 ... 512): ")
        // Depending on your environment you may have to remove \n in the scanf
        _, err := fmt.Scanf("%d \n", &sz)
        for err != nil || sz < 1 || sz > 512 {
                fmt.Println("Must be positive integer (1...512).")
                fmt.Println("Size of series (1 ... 512): ")
                _, err = fmt.Scanf("%d \n", &sz)
        }
        fmt.Printf("Size: %d\n", sz)

        var c [32]Series

        for t := 0; t < TP; t++ {
                for k := 0; k < 32; k++ {
                        c[k].a = rand.Float64()
                        c[k].b = rand.Float64()
                }
                res += fourier(c, t, TP)
                fmt.Printf("%f ", res)
                fmt.Println()
        }
}
Exercise 4 and Quiz: Semaphore
Please hand-in the answer to this question as a single go file on Virtual Campus during your lab session but at the latest by Friday 6:00pm! Remember, your submission will only count if you have signed the lab attendance sheet.
While go has laguage level support for CSP style concurrency, a resource guided by a semaphore can be easily implemented. The program below uses two go routines to access a variable, one routine counting up and one counting down. Each runs a loop and accesses the variable the same number of times. Naively, the variable should be the same as in the beginning. This is not the case. Use a global semaphore (the variable sema below) to signal the use of the variable Count by the functions increment and decrement.

package main

import (
        "fmt"
        "time"
)

var (
        Count      int = 0
        nFunctions int = 2
)

var sema = make(chan int)

func main() {
        ch := make(chan int)
        fmt.Printf("Count = %d\n", Count)
        go increment(ch, 1000)
        go decrement(ch, 1000)
        for i := 0; i < nFunctions*1000; i++ {
                fmt.Printf("Waiting %d\n", Count )
                <-ch
        }
        fmt.Printf("Count = %d\n", Count)
}

func increment(ch chan int, nSteps int) {
        for i := 0; i < nSteps; i++ {
                cnt := Count
                time.Sleep(3 * time.Millisecond)
                Count = cnt + 1
                ch <- 1
        }
        return
}

func decrement(ch chan int, nSteps int) {
        for i := 0; i < nSteps; i++ {
                cnt := Count
                time.Sleep(2 * time.Millisecond)
                Count = cnt - 1
                ch <- 1
        }
        return
}