Control statements and Functions

Control statement

if

if doesn't need parentheses in Go.

if x > 10 {
    //when x is greater than 10
    //program enters this block
    fmt.Println("x is greater than 10")
} else {
    //when x is smaller than 10
    //program enters this block
    fmt.Println("x is less than or equal to 10")
}

Go allows us to initialize and use variables in if like this:

// initialize x, then check if x greater than
if x := computedValue(); x > 10 {
    fmt.Println("x is greater than 10")
} else {
    fmt.Println("x is less than 10")
}

// the following code will not compile
fmt.Println(x)

For multiple conditions we use the else if block

if integer == 3 {
    fmt.Println("The integer is equal to 3")
} else if integer < 3 {
    fmt.Println("The integer is less than 3")
} else {
    fmt.Println("The integer is greater than 3")
}

goto

Go has a goto keyword, but be careful when you use it. goto reroutes the control flow to a previously defined label within the body of same code block.

func myFunc() {
    i := 0
Here:   // label ends with ":"
    fmt.Println(i)
    i++
    goto Here   // jump to label "Here"
}

The label name is case sensitive.

for

Go does not have while, do while. Just a for, which is the most powerful control logic. It can read data in loops and iterative operations, just like while. Like if, for doesn't need parenthesis.

for expression1; expression2; expression3 {
    //...
}
package main
import "fmt"

func main(){
    sum := 0;
    for index:=0; index < 10 ; index++ {
        sum += index
    }
    fmt.Println("sum is equal to ", sum)
}
// Print: sum is equal to 45

We can omit one or more expressions.

sum := 1
for ; sum < 1000;  {
    sum += sum
}

for {
    //this is an infinite loop
}

Using for like a while

sum := 1
for sum < 1000 {
    sum += sum
}

break and continue

break: jumps out of the loop. If you have nested loops, use break along with labels.

continue skips the current loop and starts the next one

for index := 10; index>0; index-- {
    if index == 5{
        break // or continue
    }
    fmt.Println(index)
}
// break prints 10、9、8、7、6
// continue prints 10、9、8、7、6、4、3、2、1

for can read data from slice and map when it is used together with range.

for k,v:=range map {
    fmt.Println("map's key:",k)
    fmt.Println("map's val:",v)
}

Because Go supports multi-value returns and gives compile errors when you don't use values that were defined, as discussed earlier, _ is used to discard certain return values.

for _, v := range map{
    fmt.Println("map's val:", v)
}

switch

Switch is an easier way to avoid long if-else statements.

switch sExpr {
case expr1:
    some instructions
case expr2:
    some other instructions
case expr3:
    some other instructions
default:
    other code
}

The type of sExpr, expr1, expr2, and expr3 must be the same.

Conditions don't have to be constants and it checks the cases from top to bottom until it matches conditions and executes only the matching case.

If there is no statement after the keyword switch, then it matches true.

The default case is when there is no match to the switch.

i := 10
switch i {
case 1:
    fmt.Println("i is equal to 1")
case 2, 3, 4:
    fmt.Println("i is equal to 2, 3 or 4")
case 10:
    fmt.Println("i is equal to 10")
default:
    fmt.Println("All I know is that i is an integer")
}

Cases can have more than one values separated by a comma. By default switch executes only the matching case, however we can make switch execute the matching case and all cases below it using the fallthrough statement.

integer := 6
switch integer {
case 4:
    fmt.Println("integer <= 4")
    fallthrough
case 5:
    fmt.Println("integer <= 5")
    fallthrough
case 6:
    fmt.Println("integer <= 6")
    fallthrough
case 7:
    fmt.Println("integer <= 7")
    fallthrough
case 8:
    fmt.Println("integer <= 8")
    fallthrough
default:
    fmt.Println("default case")
}

This program prints the following information.

integer <= 6 integer <= 7 integer <= 8 default case

Functions

func keyword is used to define a function.

func funcName(input1 type1, input2 type2) (output1 type1, output2 type2) {
    // function body
    // multi-value return
    return value1, value2
}
  • Functions may have zero, one or more than one arguments. The argument type comes after the argument name and arguments are separated by ,.
  • Functions can return multiple values.
  • There are two return values named output1 and output2, you can omit their names and use their type only.
  • If there is only one return value and you omitted the name, you don't need brackets for the return values.
  • If the function doesn't have return values, you can omit the return parameters altogether.
  • If the function has return values, you have to use the return statement somewhere in the body of the function.

A simple program to calculate maximum value.

package main
import "fmt"

// returns the greater value between a and b
func max(a, b int) int {
    if a > b {
        return a
    }
    return b
}

func main() {
    x := 3
    y := 4
    z := 5

    max_xy := max(x, y) // call function max(x, y)
    max_xz := max(x, z) // call function max(x, z)

    fmt.Printf("max(%d, %d) = %d\n", x, y, max_xy)
    fmt.Printf("max(%d, %d) = %d\n", x, z, max_xz)
    fmt.Printf("max(%d, %d) = %d\n", y, z, max(y,z)) // call function here
}

In a function call, if two or more arguments have the same data type, then we can put the data type only after the last argument.

func max(a,b int, c,d string): this means we have four arguments, a,b: integers and c,d: string.

Multi-value return

package main
import "fmt"

// return results of A + B and A * B
func SumAndProduct(A, B int) (int, int) {
    return A+B, A*B
}

func main() {
    x := 3
    y := 4

    xPLUSy, xTIMESy := SumAndProduct(x, y)

    fmt.Printf("%d + %d = %d\n", x, y, xPLUSy)
    fmt.Printf("%d * %d = %d\n", x, y, xTIMESy)
}

SumAndProduct will return two values without names. Go allows us to have named return arguments. By using named arguments, the respective variables are returned automatically, we would just need to use return.

If functions are going to be used outside of current program, it is better to explicitly write return statements for the sake of readability.

func SumAndProduct(A, B int) (add int, multiplied int) {
    add = A+B
    multiplied = A*B
    return
}
// Since return arguments are named, the function automatically
// returns them

Variadic arguments to functions

In many cases, we do not know how many arguements can be passed, in such cases, we use variadic arguments.

func myfunc(arg ...int) {}

arg …int tells Go that this is a function that has variable arguments. Notice that these arguments are type int. In the body of function, the arg becomes a slice of int.

for _, n := range arg { fmt.Printf("And the number is: %d\n", n) }

Pass by value and pointers

Argument are passed by value to the functions, the argument change inside the function doesn't affect the arguments used to call the function.

package main
import "fmt"

// simple function to add 1 to a
func add1(a int) int {
    a = a+1 // we change value of a
    return a // return new value of a
}

func main() {
    x := 3

    fmt.Println("x = ", x)  // should print "x = 3"

    x1 := add1(x)  // call add1(x)

    fmt.Println("x+1 = ", x1) // should print "x+1 = 4"
    fmt.Println("x = ", x)    // should print "x = 3"
}

The original value of x doesn't change, because we passed x as a value, so the function add1 created a copy of x. Despite having the same names, the both variables are totally independant of each other.

In cases where we want to be able to modify the argument's value, we use pass by reference using pointers.

In reality, a variable is nothing but a pointer to a location in memory. Each variable has a unique memory address. So, if we want to change the value of a variable, we must change its memory address. Therefore the function add1 has to know the memory address of x in order to change its value. Here we pass &x to the function, and change the argument's type to the pointer type *int. Be aware that we pass a copy of the pointer, not copy of value.

package main
import "fmt"

// simple function to add 1 to a
func add1(a *int) int {
    *a = *a+1 // we changed value of a
    return *a // return new value of a
}

func main() {
    x := 3

    fmt.Println("x = ", x)  // should print "x = 3"

    x1 := add1(&x)  // call add1(&x) pass memory address of x

    fmt.Println("x+1 = ", x1) // should print "x+1 = 4"
    fmt.Println("x = ", x)    // should print "x = 4"
}

Advantages of pointers:

  • Allows us to use more functions to operate on one variable.
  • Low cost by passing memory addresses (8 bytes), copy is not an efficient way, both in terms of time and space, to pass variables.
  • string, slice and map are reference types, so they use pointers when passing to functions by default. (Attention: If you need to change the length of slice, you have to pass pointers explicitly)

defer

Defer postpones the execution of a function till the calling function has finished executing. You can have many defer statements in one function; they will execute in reverse order when the program reaches its end. In the case where the program opens some resource files, these files would have to be closed before the function can return with errors. Let's see some examples.

func ReadWrite() bool {
    file.Open("file")
    // Do some work
    if failureX {
        file.Close()
        return false
    }

    if failureY {
        file.Close()
        return false
    }

    file.Close()
    return true
}

We saw some code being repeated several times. defer solves this problem very well. It doesn't only help you to write clean code but also makes your code more readable.

func ReadWrite() bool {
    file.Open("file")
    defer file.Close()
    if failureX {
        return false
    }
    if failureY {
        return false
    }
    return true
}

If there are more than one defers, they will execute by reverse order. The following example will print 4 3 2 1 0.

for i := 0; i < 5; i++ {
    defer fmt.Printf("%d ", i)
}

Functions as values and types

Functions are also variables in Go, we can use type to define them. Functions that have the same signature can be seen as the same type.

type typeName func(input1 inputType1 , input2 inputType2 [, ...]) (result1 resultType1 [, ...])

This makes Go a functional language as functions are a first class citizen.

package main
import "fmt"

type testInt func(int) bool // define a function type of variable

func isOdd(integer int) bool {
    if integer%2 == 0 {
        return false
    }
    return true
}

func isEven(integer int) bool {
    if integer%2 == 0 {
        return true
    }
    return false
}

// pass the function `f` as an argument to another function

func filter(slice []int, f testInt) []int {
    var result []int
    for _, value := range slice {
        if f(value) {
            result = append(result, value)
        }
    }
    return result
}

func main(){
    slice := []int {1, 2, 3, 4, 5, 7}
    fmt.Println("slice = ", slice)
    odd := filter(slice, isOdd)    // use function as values
    fmt.Println("Odd elements of slice are: ", odd)
    even := filter(slice, isEven)
    fmt.Println("Even elements of slice are: ", even)
}

It's very useful when we use interfaces. As you can see testInt is a variable that has a function as type and the returned values and arguments of filter are the same as those of testInt. Therefore, we can have complex logic in our programs, while maintaining flexibility in our code.

Panic and Recover

Go doesn't have try-catch structure like Java does. Instead of throwing exceptions, Go uses panic and recover to deal with errors. However, you shouldn't use panic very much, although it's powerful.

Panic is a built-in function to break the normal flow of programs and get into panic status. When a function F calls panic, F will not continue executing but its defer functions will continue to execute. Then F goes back to the break point which caused the panic status. The program will not terminate until all of these functions return with panic to the first level of that goroutine. panic can be produced by calling panic in the program, and some errors also cause panic like array access out of bounds errors.

Recover is a built-in function to recover goroutines from panic status. Calling recover in defer functions is useful because normal functions will not be executed when the program is in the panic status. It catches panic values if the program is in the panic status, and it gets nil if the program is not in panic status.

The following example shows how to use panic.

var user = os.Getenv("USER")

func init() {
    if user == "" {
        panic("no value for $USER")
    }
}

The following example shows how to check panic.

func throwsPanic(f func()) (b bool) {
    defer func() {
        if x := recover(); x != nil {
            b = true
        }
    }()
    f() // if f causes panic, it will recover
    return
}

main and init functions

Go has two retentions which are called main and init, where init can be used in all packages and main can only be used in the main package. These two functions are not able to have arguments or return values. Even though we can write many init functions in one package, I strongly recommend writing only one init function for each package.

Go programs will call init() and main() automatically, so you don't need to call them by yourself. For every package, the init function is optional, but package main has one and only one main function.

Programs initialize and begin execution from the main package. If the main package imports other packages, they will be imported in the compile time. If one package is imported many times, it will be only compiled once. After importing packages, programs will initialize the constants and variables within the imported packages, then execute the init function if it exists, and so on. After all the other packages are initialized, programs will initialize constants and variables in the main package, then execute the init function inside the package if it exists. The following figure shows the process.

Flow of programs initialization

import

import is very often used in Go programs.

import(
    "fmt"
)

Methods of fmt are called as follows.

fmt.Println("hello world")

fmt is from Go standard library, it is located within $GOROOT/pkg. Go supports third-party packages in two ways.

  1. Relative path import "./model" // load package in the same directory, I don't recommend this way.
  2. Absolute path import "shorturl/model" // load package in path "$GOPATH/pkg/shorturl/model"

There are some special operators when we import packages, and beginners are always confused by these operators.

Dot operator

Sometimes we see people use following way to import packages.

import(
    . "fmt"
)

The dot operator means you can omit the package name when you call functions inside of that package. Now fmt.Printf("Hello world") becomes to Printf("Hello world").

Alias operation

It changes the name of the package that we imported when we call functions that belong to that package.

import(
    f "fmt"
)

Now fmt.Printf("Hello world") becomes to f.Printf("Hello world").

_ operator

This is the operator that is difficult to understand without someone explaining it to you.

import (
    "database/sql"
    _ "github.com/ziutek/mymysql/godrv"
)

The _ operator actually means we just want to import that package and execute its init function, and we are not sure if want to use the functions belonging to that package.

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