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Unit 1: A tour of Go

Please finish all materials on https://go.dev/tour/ or https://go.dev/tour/list except "Generics" and "Concurrency"

Note: if you stuck with module try to read https://go.dev/blog/using-go-modules.

FAQ

Defer, why and how?

Defer is used to delay some function call when return from function is happened. In most cases defer is used to perform cleanup and closing resources.

For an instance when we are working with files we need to close it. For example we have function that opens file and call other funciton(s) performs processing of opened file which can return errors.

Every time we are facing with return function file must be closed by calling file.Close():

func routine(filename string) error
file, err := os.Open(filename)
if err != nil {
	return err
}

err = Foo(file)
if err != nil {
	file.Close()
	return err
}

file.Close()
return nil

Such approach is not convinient and causes places for potential errors. instead of that we can use defer statement:

func routine(filename string) error
file, err := os.Open(filename)
if err != nil {
	return err
}

defer file.Close() //starting from this point no matter where routine returns file.Close() will be called.

err = Foo(file)
if err != nil {
	return err
}
return nil

Note that defer is a statement that delays function call and arguments to this call. The actual call and argument resolving will happen "during" return statement

Slices and arrays. Mechanincs and difference

Both arrays and slices represent solid piece of memory. Arrays is static structure points to fixed amount of memory, while slice is dynamic and can point to flexible amount of memory.

Arrays and slices have 2 major properties:

  • length len() - number of elements defined in array or slice
  • capacity cap() - total length of memory allocated for slice or array

Array properties are known at compilation time, while slice properties are known only at runtime.

Array:

nums := [5]int{1, 2, 3} // nums == [1 2 3 0 0], len()=5, cap()=5
nums[0]=0 				// nums == [0 2 3 0 0], len()=5, cap()=5
nums[3]=4 				// nums == [0 2 3 4 0], len()=5, cap()=5

Note: [...]int{1, 2, 3} will define array of len()=3 cap()=3. Three dots direct compiler to count number of elements in definition and set cap and len to this value.

Slice:

As we saw for array its cap and len are always the same. Slice is slightly trickier.

nums := make([]int, 0,2)	// nums[], mem[0 0], cap=2, len=0
nums[0]=1					// panic, len=0, there is no first element
nums = append(nums, 1)  	// nums[1], mem[1 0], cap=2, len=1
append(nums[1:],2)			// nums[1], mem[1 2], cap=2, len=1
nums = nums[:cap(nums)]		// nums[1 2], mem[1 2], cap=2, len=2
nums := []int{1, 2, 3} 	// nums == [1 2 3 0 0], len()=3, cap()=3
nums[0]=0 				// nums == [0 2 3 0 0], len()=3, cap()=3
nums[3]=4 				// nums == [0 2 3 4 0], len()=3, cap()=3
nums2:=make([]int,0,1) 	// nums2 == [], 		len()=0, cap()=1
nums2 = append(nums2, 9)// nums2 == [9], 		len()=1, cap()=1
nums2 = append(nums2, 8)// nums2 == [9 8], 		len()=2, cap()=2

As one might see we can append new elements to the end of the slice and capacity of slice will be increased if needed. For arrays we can't append elements. Slices and arrays are different types as float and int for example:

append(nums, 5)			// compile error: nums is not slice
nums=[]int{0}			// compile error: nums is array and []int{0} is slice

We can easily make slice from array and slice:

nums := [5]int{1, 2, 3}	// [1 2 3 0 0], len()=5, cap()=5
slice := nums[:] 		// [1 2 3 0 0], len()=5, cap()=5

Note that when we make slice from another slice or array, created slice points to the same memory as original slice or array:

nums := [3]int{1, 2}
slice := nums[0:1]
// nums: [1 2 0] len()=3, cap()=3, slice [1] len()=1, cap()=3

slice[0]=10 // We changed element in slice but element 0 is also changed in nums:
// nums: [10 2 0] len()=3, cap()=3, slice [10] len()=1, cap()=3
// the reason is that element 0 of slice points to the same memory as element 0 of nums

// We can even append element to slice and it will appear in nums:
slice = append(slice, 11, 12)
// nums: [10 11 12] len()=3, cap()=3, slice [10,11,12] len()=3, cap()=3

What if one will continue to add elements to slice built on top of array?

slice = append(slice, 13)
// nums: [10 11 12] len()=3, cap()=3, slice [10,11,12,13] len()=4, cap()=4
slice[0]=9000
// nums: [10 11 12] len()=3, cap()=3, slice [9000,11,12,13] len()=4, cap()=4

As one can see, when we exceed capacity of slice array, append will alocate new slice and copy all values from old slice and append values to the end of new slice. Values are remain but slice doesn't point to the same memory as array anymore. Changing element in slice will not affect orifinal array.

The same situation will happen for slice of slice. Just keep in mind slice internals with cap, len and underlying memory.

Read more about slices in go blog article

Quiz

Q1. How will you add the number 3 to the right side?

numbers := []int{1, 1, 2}

  1. numbers.append(3)
  2. numbers.insert(3, 3)
  3. append(numbers, 3)
  4. numbers = append(numbers, 3)

Q2. From where is the variable fooVar accessible if it is declared outside of any functions in a file in package fooPackage located inside module fooModule

  1. anywhere inside fooPackage, not the rest of fooModule
  2. by any application that imports fooModule
  3. from anywhere in fooModule
  4. by other packages in fooModule as long as they import fooPackage

Q3. What should the idiomatic name be for an interface with a single method and the signature Serve() error

  1. Servable
  2. Server
  3. ServeInterface
  4. IServe

Q4. Which is not valid loop construct?

  1. for i,r:=0,rand.Int(); i < r%10; i++ { ... }
  2. for { ... }
  3. { ... } for false
  4. for _,c := range "foobar" { ... }

Excercises

E0 is for illustration how to work and submit Excercises. Let's make

E0. Exercise: Loops and Functions

More: https://go.dev/tour/flowcontrol/8 let's create implementation in unit1/exercises/e0/main.go, wherer unit1 is number of current unit, e0 is number of current exercise.

If task has multiple steps to do, we assume that the last steps is final. Input values must be the same as in Exercie definition if other is not mentioned.

At first we've been asked to implement Sqrt function with partial implementation of Newton method and send output of computation steps on each iteration of method:

package main

import (
	"fmt"
)

func Sqrt(x float64) float64 {
	z := 1.0
	for i:=0; i<10; i++ {
		delta := (z*z - x) / (2*z)
		z -= delta
		fmt.Println(z)
	}
	return z
}

func main() {
	fmt.Println(Sqrt(2))
}

Next we've asked to change loop condition to stop once the value has stopped changing (or only changes by a very small amount). We can achieve that by adding epsilon constant that will be compared with delta. Note that their absolute values should be compared. One approach is to use Abs() function form math package, or we can notice that x argument must be positive, let's make our own condition based on partial comparison of positive-only numbers:

package main

import (
	"fmt"
)

func Sqrt(x float64) float64 {
	epsilon := 0.0000000000001
	z := 1.0
	for {
		delta := (z*z - x) / (2 * z)
        // or: if math.Abs(delta) < epsilon {
		if delta < epsilon && delta > -epsilon {
			break
		}
		z -= delta
		fmt.Println(z)
	}
	return z
}

func main() {
	fmt.Println(Sqrt(2))
}

If you haven't managed to solve all steps publish code for the step you succeeded and make a comment in code. If you exercise requires to make multiple files or even packages, don't hesitate to create them in unit1/exercises/eX/ folder

As soon as you implemented the code and place it into unit1/exercises/e0/main.go, make PR to this repo.

E1. Fibonacci closure

More: https://go.dev/tour/moretypes/26

Note: Edit only fibonacci function definition:

func fibonacci() func() int {}

Keep main function untouch. Don't add additional Prints to output. It is checked in tests.

Share your implementation unit1/exercises/e1/main.go in github PR.

E2. Stringers

More: https://go.dev/tour/methods/18

Implement func (ip IPAddr) String() string method for IPAddr structure. Note that receiver variable ip is not pointer.

Keep main function untouch. Don't add additional Prints to output. It is checked in tests.

Share your implementation unit1/exercises/e2/main.go in github PR.

Note: about interfaces, behaviour and who is calling String()

E3. Errors

More: https://go.dev/tour/methods/20

Note: errors are just values and must be treated as values. In this scenario error value will be ErrNegativeSqrt struct that satisfy error type. Nil is valid error, pointer to struct with Error() string method is valid error too.

Keep main function untouch. Don't add additional Prints to output. It is checked in tests.

Share your implementation unit1/exercises/e3/main.go in github PR.

E4. rot13Reader

More: https://go.dev/tour/methods/23

Add more info about reader and the entire workflow of reaeder.

'A' is a byte. It is treated as 8 bit number, so it can be compared and added/substracted.

not that A-Z and a-z letters have their own ranges and should be mapped to letter from their group: letter from A-Z must be mapped to letter from A-Z. Same for a-z.

https://www.rapidtables.com/code/text/ascii-table.html

Keep main function untouch. Don't add additional Prints to output. It is checked in tests.

Share your implementation unit1/exercises/e4/main.go in github PR.

E5. Images

More: https://go.dev/tour/methods/25

Note: read the docs for golang.org/x/tour/pic package.

Keep pic.ShowImage(m) in main function untouch. Don't add additional Prints to output. It is checked in tests.

Share your implementation unit1/exercises/e5/main.go in github PR.

Note about what to do

Summary of what you learnt

Don't worry if you haven't deep understanding why interfaces are needed. You got almost all Go syntax. Key point after this unit is to be able to read the code and tear down it to known primiteves.