Go Programming Language

Wed, 21 Jun 2023 00:00:00 +0000

Go is a compiled language developed by Robert Griesemer, Rob Pike, and Ken Thompson at Google. It is primarily used in cloud-native and web development projects.

Some of the popular projects created with Go

Docker: Container Engine
Kubernetes: Container Orchestrator
Hugo: Static Site Generator (Also used to generate this blog)
Helm: Package manager for Kubernetes

Packages

A Package abstraction is created in Go to group functions that are related to each other or associated with a specific task. At the start of each source file (*.go) a package name is assigned to it using the package keyword, for example, package main.

To import a package we use the import keyword with the name of the package enclosed in "" like import "fmt". The constants, functions, and variables imported from a package are referred to with their first letter capitalized like math.Pi and math.Pow().

package main

import "fmt"

func main () {
	return 0
}

Packages could be bundled together as modules. The go.mod file declares the path to a Go module and all packages within it. A Go module is initialized using the go mod init command followed by the name of the module

go mod init exampleModule

This will generate a go.mod file in the directory specifying the name of the module (exampleModule) and its dependencies.

Standard Input and Output

The fmt package provides functions for standard input and output, these help return output from the program and take inputs from the user respectively. The fmt.Println() function outputs its arguments to the terminal and fmt.Scanln() takes input from the terminal and assigns them to variables. Variations of Println and Scanln provided by fmt:

fmt.Print(): Standard output without any newline at the end.
fmt.Scan(): Values are read from standard input and the newline is interpreted as space.
fmt.Printf(): Standard output in the specified format.
fmt.Scanf(): Reads the value from the terminal in the specified format.

package main

import "fmt"

func main () {
        var name string
        fmt.Print("Your name:")
        fmt.Scan(&name)

        fmt.Println("Hello", name)
}

// Output
// Your name: Joe
// Hello Joe

Building and Executing

To compile and build a Go package as a binary we use the go build command.

go build hello.go

This will create a binary named hello (or hello.exe in Windows) in the current directory that could be executed directly by the host OS.

We can execute a Go package directly from the source file using the go run command.

go run hello.go

Constants

const keyword is used to define constants, values that don’t have to be changed throughout the program. Go supports numeric, character, string, and boolean constants.

package main

import "fmt"

func main () {
        const pi = 3.14
        fmt.Println("Approximate value of Pi is:", pi)
}

// Output
// Approximate value of Pi is: 3.14

Variables and Datatypes

To initialize a variable in Go we have to use the var keyword followed by the name of the variable and its datatype like var intExample int.

Go has the following datatypes

bool for boolean values. Either true or false.
string for storing single or multiple characters.
rune for storing the Unicode code point of a character.
int for integers. Go also supports int8, int16, int32, and int64 where the digits after int signifies the number of bits required to represent the value.
uint for unsigned integers (integers without + or -). uint8/byte, uint16, uint32, uint64 are also available in Go.
Variables of the uintptr type are used to store integer representations of memory addresses. They are useful for performing pointer arithmetic provided by unsafe.Pointer.
float32 and float64 for values that require decimal precision.
complex64 and complex128 are used to represent 64 and 128-bit complex numbers like $5\ +\ i6$ (where $i$ represents $\sqrt{(-1)}$)

package main

import "fmt"

func main () {
        var intExample int
        intExample = 3
        fmt.Println("Integer Example:", intExample)

        var floatExample float64
        floatExample = 2.3
        fmt.Println("Float Example:", floatExample)

        var stringExample string
        stringExample = "Hello, World"
        fmt.Println("String Example:", stringExample)

        var boolExample bool
        boolExample = true
        fmt.Println("Boolean Example:", boolExample)

        var complexExample complex64
        complexExample = complex(5, 6)
        fmt.Println("Complex Number Example:", complexExample)
}

// Output
// Integer Example: 3
// Float Example: 2.3
// String Example: Hello, World
// Boolean Example: true
// Complex Number Example: (5+6i)

To declare and assign values to a variable without explicitly specifying its datatype we use := operand, the variable’s datatype will be inferred during compile time by the value on the right side of the operand.

package main

import "fmt"

func main () {
        intExample := 3
        fmt.Println("Integer Example:", intExample)

        floatExample := 2.3
        fmt.Println("Float Example:", floatExample)

        stringExample := "Hello, World"
        fmt.Println("String Example:", stringExample)

        boolExample := true
        fmt.Println("Boolean Example:", boolExample)

        complexExample := complex(5, 6)
        fmt.Println("Complex Number Example:", complexExample)
}

// Output
// Integer Example: 3
// Float Example: 2.3
// String Example: Hello, World
// Boolean Example: true
// Complex Number Example: (5+6i)

If a variable is initialized without any value then a zero value will be assigned to it by default. Zero values for different datatypes are 0 (for int), 0.0 (for float64), "" (for string), and false (for bool).

package main

import "fmt"

func main () {
    var example1 int
    fmt.Println(example1)
    
    var example2 float64
    fmt.Println(example2)
    
    var example3 string
    fmt.Println(example3)
    
    var example4 bool
    fmt.Println(example4)
}

// Output
// 0
// 0
// 
// false

Type Conversion

Datatypes could also be used as functions for type conversion of variables for example float64(intExample).

package main

import "fmt"

func main () {
        intExample := 4
        fmt.Printf("Float conversion of intExample: %0.2f", float64(intExample))
}

// Output
// Float conversion of intExample: 4.00

Functions

Functions allow us to aggregate frequently used instructions as a single statement. To define a function in Go we use the func keyword followed by the name of the function, its arguments (inputs), and the datatype of the return values like func exampleFunc (inputValue int) (outputValue int) { }

package main

import "fmt"

func square (x int) (int) {
        return x*x
}

func sumAndDifference (x int, y int) (int, int){
        return x+y, x-y
}

func main () {
        value1 := 5
        fmt.Println("Square of", value1, "is:", square(value1))

        value2 := 8
        sum, difference := sumAndDifference(value1, value2)
        fmt.Println("Sum of", value1, "and", value2, "is:", sum)
        fmt.Println("Difference of", value2, "from", value1, "is:", difference)
}

// Output
// Square of 5 is: 25
// Sum of 5 and 8 is: 13
// Difference of 8 from 5 is: -3

Anonymous Functions

Functions are first-class citizens in Go i.e. they could be defined inside variables, passed as arguments, and returned as values from other functions.

Functions with no name (called Anonymous functions) could be created to perform a set of instructions once in a program.

package main

import "fmt"

func main () {
	// Anonymous Function
	func (x int) {
			fmt.Println("Hello, this is not a random number:", x)
	}(1)

	// Function as Variable
	funcExample := func (x int){
			fmt.Println("Example Function")
			fmt.Println("Input:", x)
	}

	funcExample(3)
}

// Output
// Hello, this is not a random number: 1
// Example Function
// Input: 3

Conditions

Go supports if, else if, and else for defining conditional statements.&& (AND) and || (OR) operators could be used to create complex conditional expressions.

package main

import "fmt"

func evaluateGrades (marks int) string {
    if marks < 101{
    	if marks < 40{
    		return "D"
    	} else if marks >= 40{
    		return "C"
    	} else if marks >= 50{
    		return "B"
    	} else if (marks >= 60){
    		return "A"
    	} 
    }
    return "INVALID"
}

func main () {
	marks := 4
	fmt.Println("Grade for marks:", marks, "is:", evaluateGrades(marks))
	
	marks = 40
	fmt.Println("Grade for marks:", marks, "is:", evaluateGrades(marks))

	marks = 51
	fmt.Println("Grade for marks:", marks, "is:", evaluateGrades(marks))

	marks = 101
	fmt.Println("Grade for marks:", marks, "is:", evaluateGrades(marks))
}

// Output
// Grade for marks: 4 is: D
// Grade for marks: 40 is: C
// Grade for marks: 51 is: C
// Grade for marks: 101 is: INVALID

Switch Cases

Sometimes a switch could be relatively clean compared to multiple if and else if statements.

Unlike if-else the first case which satisfies the condition will be executed and the following cases will be ignored. If none of the case conditions are satisfied then the default statement is executed.

package main

import "fmt"

func evaluate (value1, value2, operation int) (int) {
    switch{
        case operation==1:
            return value1+value2
        case operation==2:
            return value1-value2
        case operation==3:
            return value1*value2
        case operation==4:
            return value1/value2
        default:
            return (-1)
    }
}

func main () {
    var value1, value2, operation int
    
	fmt.Println("Calculator Program:")
	fmt.Print("Enter value1:")
	fmt.Scanln(&value1)
	fmt.Print("Enter value2:")
	fmt.Scanln(&value2)
	
	fmt.Println("\nSelect Operation to Perform")
	fmt.Println("1) ADD")
	fmt.Println("2) SUBTRACT")
	fmt.Println("3) MULTIPLY")
	fmt.Println("4) DIVIDE")
	fmt.Print("Enter the operation number:")
	fmt.Scanln(&operation)
	
	fmt.Println("Result of Operation is:", evaluate(value1, value2, operation))
}

// Output
// Calculator Program:
// Enter value1:56
// Enter value2:54
// Select Operation to Perform
// 1) ADD
// 2) SUBTRACT
// 3) MULTIPLY
// 4) DIVIDE
// Enter the operation number:3
// Result of Operation is: 3024

Additional Datatypes

Arrays

We create arrays to store a sequence of values with the same datatype. Go arrays have explicitly defined sizes associated with their datatype.

Individual elements in an array could be accessed by their index (starting from 0). For example, to access the 3rd element in the array we can simply call array[2].

An array is defined as array := [5]int{2, 4, 8, 16, 32}. len() function returns the length of the array.

package main

import "fmt"

func main() {
    var arrayExample [5]int
    fmt.Println("Zero Value Array:", arrayExample)
    
    arrayExample[1]=2
    fmt.Println("Changed arrayExample:", arrayExample)
    
    arrayExample2 := [5]int{2, 3, 1, 3, 21}
    fmt.Println("Another arrayExample:", arrayExample2)
}

// Output
// Zero Value Array: [0 0 0 0 0]
// Changed arrayExample: [0 2 0 0 0]
// Another arrayExample: [2 3 1 3 21]

Slices

A segment of an array could be extracted by creating its slice. To extract a segment starting from index 2 till index 5 in an array called arrayExample we can simply write arrayExample[2:5]

package main

import "fmt"

func main() {
    arrayExample2 := [7]int{2, 3, 1, 3, 21, 234, 23}
    
    lowIndex := 2
    highIndex := 5
    fmt.Println("Slice of arrayExample starting from index",
                lowIndex, "to", highIndex, "is:",
                arrayExample2[lowIndex:highIndex])

    fmt.Println("Slice of arrayExample starting from index",
                lowIndex, "till (including)", highIndex, "is:",
                arrayExample2[lowIndex:highIndex+1])
    
    fmt.Println("Slice Length:", len(arrayExample2[lowIndex:highIndex+1]))
    fmt.Println("Capacity of the original array:",
                cap(arrayExample2[lowIndex:highIndex+1]))
}

// Output
// Slice of arrayExample starting from index 2 to 5 is: [1 3 21]
// Slice of arrayExample starting from index 2 till (including) 5 is: [1 3 21 234]
// Slice Length: 4
// Capacity of the original array: 5

Any changes performed on the slice will also affect the original array.

The cap() function is used to return the capacity of the original array.

The length of the slice is not part of its datatype, so slices are also used as dynamically-sized arrays. A slice is created just like an array but without any size in its declaration like var sliceExample []int

package main

import "fmt"

func main() {
    var capitals []string
    capitals = append(capitals, "delhi")
    capitals = append(capitals, "london")
    fmt.Println("Slice of Capitals:", capitals)
    fmt.Println("Length of Capitals Slice:", len(capitals))
    fmt.Println("Capacity of Capitals Slice:", cap(capitals))
}

// Output
// Slice of Capitals: [delhi london]
// Length of Capitals Slice: 2
// Capacity of Capitals Slice: 2

Maps

To store key-value pairs in Go we have Maps. We can define a map that stores string keys mapped to bool values with the help of make() function like mapExample := make(map[string]bool).

package main

import "fmt"

func main() {
    mapExample := make(map[string]bool)
    mapExample["yes"] = true
    mapExample["no"] = false
    fmt.Println("Example of Map:", mapExample)
}

// Output
// Example of Map: map[no:false yes:true]

Structs

Go is a procedural language but to create data structures with multiple fields we have struct. By creating a struct we are adding a new datatype to our program, that’s why the type keyword is used in its declaration.

While creating an instance of the struct we can pass the value to its fields like maps where the key is the name of the field.

package main

import "fmt"

type dog struct {
    name string
    age int
}

func main() {
    dog1 := dog{name: "DoggyMcDogFace", age: 1}
    fmt.Println("Dog1:", dog1)

    dog2 := dog{name: "AnotherDog", age: 3}
    fmt.Println("Dog2:", dog2)
}

// Output
// Dog1: {DoggyMcDogFace 1}
// Dog2: {AnotherDog 3}

Methods

Methods refer to the functions created to be executed on a specific datatype (like a struct). Their declaration is similar to functions with the addition of struct argument after func keyword like func (d dog) showFields() () { }

package main

import "fmt"

type dog struct {
    name string
    age int
}

func (d dog) showFields() (){
    fmt.Println("Name of Dog:", d.name)
    fmt.Println("Age of Dog:", d.age)
}

func main() {
    dog1 := dog{name: "DoggyMcDogFace", age: 1}
    dog1.showFields()

    dog2 := dog{name: "AnotherDog", age: 3}
    dog2.showFields()
}

// Output
// Name of Dog: DoggyMcDogFace
// Age of Dog: 1
// Name of Dog: AnotherDog
// Age of Dog: 3

Interfaces

Interfaces are used to group common methods.

Interfaces enable polymorphism in Go, which means a method’s behavior is determined by the datatype it is applied upon.

package main

import "fmt"

type ubuntu struct {
    releaseName string
    releaseVersion float64
}

func (u ubuntu) showUpdateCommand() {
    fmt.Println("Update command for", u.releaseName, "is")
    fmt.Println("\tapt-get update && apt-get upgrade")
}

type fedora struct {
    releaseName string
    releaseVersion float64
}

func (f fedora) showUpdateCommand() {
    fmt.Println("Update command for", f.releaseName, "is")
    fmt.Println("\tdnf update")
}

type distro interface {
    showUpdateCommand()    
}

func main() {
    var exampleDistro, exampleDistro2 distro
    exampleDistro = ubuntu{releaseName: "Lunar Lobster", releaseVersion: 23.04}
    fmt.Println("Example of a distro:", exampleDistro)
    
    exampleDistro.showUpdateCommand()
    
    exampleDistro2 = fedora{releaseName: "Fedora 39", releaseVersion: 39.00}
    fmt.Println("Example of another distro:", exampleDistro2)
    
    exampleDistro2.showUpdateCommand()
}

// Output
// Example of a distro: {Lunar Lobster 23.04}
// Update command for Lunar Lobster is
// 	 apt-get update && apt-get upgrade
// Example of another distro: {Fedora 39 39}
// Update command for Fedora 39 is
//	  dnf update

For Loops

Loops allow us to execute a statement multiple times until some condition is valid.

A for loop requires three statements:

initializing statement: This statement will be executed once before the loop has started.
conditional expression: This expression will be evaluated after each execution of the loop. The loop will stop if this expression evaluates to false.
incrementing statement: This statement is executed after each iteration of the loop

package main

import "fmt"

func main() {
    // Initializing Statement: i:=0 (i is initialized with value 0)
    // Conditional Expression: i<10 (until i is less than 10)
    // Incrementing Statement: i++  (increment i by 1)
    for i:=0; i<10; i++ {
        fmt.Println("Value of i:", i)
    }
}

// Output
// Value of i: 0
// Value of i: 1
// Value of i: 2
// Value of i: 3
// Value of i: 4
// Value of i: 5
// Value of i: 6
// Value of i: 7
// Value of i: 8
// Value of i: 9

For Loop over an Array

len(array) will return the length of an array, by using its length we can calculate the last index of an array i.e. len(array)-1. This will help us write a loop that accesses each element of an array using its index.

package main

import "fmt"

func main() {
    arrayExample := [6]int{123, 21, 3, 54, 65, 45}
    for i:=0; i<len(arrayExample); i++ {
        fmt.Println("Element at index ", i, "in arrayExample is", arrayExample[i])
    }
}

// Output
// Element at index  0 in arrayExample is 123
// Element at index  1 in arrayExample is 21
// Element at index  2 in arrayExample is 3
// Element at index  3 in arrayExample is 54
// Element at index  4 in arrayExample is 65
// Element at index  5 in arrayExample is 45

Range

To iterate over data structures such as slices, maps, and strings we can take the help of range. Each iteration of a loop implemented with range will return two values depending on the data structure passed to it:

for string, it will return the byte position of the character and character
for array/slices, it will return the index and the element
for maps, it will return the key and value

package main

import "fmt"

func main() {
    var sliceExample []float64
    sliceExample = append(sliceExample, 4.3)
    sliceExample = append(sliceExample, 5.2)
    sliceExample = append(sliceExample, 64.3)
    sliceExample = append(sliceExample, 34.8)
    fmt.Println("Example of a Slice:", sliceExample)
    
    for index, value := range sliceExample {
        fmt.Println("Value at index", index, "in sliceExample is", value)
    }
    
    stringExample := "Hello, World"
    fmt.Println("Example of a String:", stringExample)
    for pos, char := range stringExample {
        fmt.Printf("%#U character is at position %d\n", char, pos)
    }
    
    mapExample := make(map[int]string)
    mapExample[1] = "one"
    mapExample[2] = "two"
    mapExample[4] = "four"
    fmt.Println("Example of a Map:", mapExample)
    for key, value := range mapExample {
        fmt.Println("Value for key", key, "in mapExample is", value)
    }
}

// Output
// Example of a Slice: [4.3 5.2 64.3 34.8]
// Value at index 0 in sliceExample is 4.3
// Value at index 1 in sliceExample is 5.2
// Value at index 2 in sliceExample is 64.3
// Value at index 3 in sliceExample is 34.8
// Example of a String: Hello, World
// U+0048 'H' character is at position 0
// U+0065 'e' character is at position 1
// U+006C 'l' character is at position 2
// U+006C 'l' character is at position 3
// U+006F 'o' character is at position 4
// U+002C ',' character is at position 5
// U+0020 ' ' character is at position 6
// U+0057 'W' character is at position 7
// U+006F 'o' character is at position 8
// U+0072 'r' character is at position 9
// U+006C 'l' character is at position 10
// U+0064 'd' character is at position 11
// Example of a Map: map[1:one 2:two 4:four]
// Value for key 1 in mapExample is one
// Value for key 2 in mapExample is two
// Value for key 4 in mapExample is four

Substitute for `while` Loop

Go doesn’t have while loops but we can use for loops for the same purpose by executing it with just the conditional expression.

package main

import "fmt"

func main() {
    fmt.Println("Enter an odd number to end the loop")
    
    flag := false
    for flag==false {
            var input int
            fmt.Printf("Enter a value:")
            fmt.Scanln(&input)
            
            if input%2!=0{
                flag=true
            }
    }
    
    fmt.Println("Loop ended")
}

// Output
// Enter an odd number to end the loop
// Enter a value:2
// Enter a value:42
// Enter a value:52
// Enter a value:68
// Enter a value:69
// Loop ended

Pointers

Pointers for a datatype will store the address of a value rather than the value itself. So an integer pointer (*int) will store the address of an integer value.

To fetch the address of a value we use & operand. For example, to fetch the address of the value stored in the intExample variable we call &intExample. Similarly to fetch the value stored at an address we use * operand like *(&intExample).

package main

import "fmt"

func main() {
    intExample := 2
    fmt.Println("Value of intExample:", intExample)
    fmt.Println("Memory address of intExample:", &intExample)
    
    var pointerExample *int
    pointerExample = &intExample
    fmt.Println("Value of pointerExample:", pointerExample)
    fmt.Println("Value stored at pointerExample address:", *pointerExample)
}

// Output
// Value of intExample: 2
// Memory address of intExample: 0xc000022050
// Value of pointerExample: 0xc000022050
// Value stored at pointerExample address: 2

When methods are defined on the pointer of the struct rather than the struct themselves then they can manipulate the value of fields in the original instance rather than its copy.

package main

import "fmt"

type car struct {
    color string
}

func (c car) changeColor(color string) {
    c.color = color
}

func (c *car) changeColorPointer(color string) {
    c.color = color
}

func main() {
    newCar := car{color: "red"}
    fmt.Println("newCar: ", newCar)
    
    // This will not work as the change is not performed 
    // on the original memory address but on its copy
    newCar.changeColor("blue")
    fmt.Println("newCar after changing color: ", newCar)
    
    var newCarPtr *car
    newCarPtr = &newCar
    newCarPtr.changeColorPointer("blue")
    fmt.Println("newCar after changing color using pointer: ", newCar)
}

// Output
// newCar:  {red}
// newCar after changing color:  {red}
// newCar after changing color using pointer:  {blue}

Errors

In Go, errors are like other return types. So a function can return two values one being the output of the function and the other value could be an error interface. We can declare this in the function’s signature like func sampleFunc(value int) (int, error) { }.

A custom error is created using errors.New() function is available in the errors package.

package main

import (
    "fmt"
    "errors"
)

func divide(value1, value2 int) (int, error) {
    if value2==0 {
        return -1, errors.New("Divide by Zero Error. Change value2")
    } 
    return value1/value2, nil
}

func main() {
    var value1, value2 int
    fmt.Printf("Enter value1:")
    fmt.Scanln(&value1)
    fmt.Printf("Enter value2:")
    fmt.Scanln(&value2)
    
    quotient, err := divide(value1, value2)
    fmt.Println("Quotient:", quotient)
    if err!= nil {
        fmt.Println("Error:", err)
    }
}

// Output
// Enter value1:1
// Enter value2:0
// ERROR!
// Quotient: -1
// Error: Divide by Zero Error. Change value2

To catch an unexpected error and exit the program we call the panic() function with a string or the error as its argument. If we don’t want the program to abort on panic() we can follow it up with a recover() function.

Resources

Go by Example
Learn Go in 12 Minutes

Procedural on avni.sh