# 2.3 Control statements and functions

In this section, we are going to talk about control statements and function operations in Go.

## Control statement

The greatest invention in programming is flow control. Because of them, you are able to use simple control statements that can be used to represent complex logic. There are three categories of flow control: conditional, cycle control and unconditional jump.

### if

`if` will most likely be the most common keyword in your programs. If it meets the conditions, then it does something and it does something else if not.

`if` doesn't need parentheses in Go.

	if x > 10 {
    	fmt.Println("x is greater than 10")
	} else {
    	fmt.Println("x is less than 10")
	} 
	
The most useful thing concerning `if` in Go is that it can have one initialization statement before the conditional statement. The scope of the variables defined in this initialization statement are only available inside the block of the defining `if`.

	// 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)
	
Use if-else for multiple conditions.

	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

`for` is the most powerful control logic in Go. It can read data in loops and iterative operations, just like `while`.

	for expression1; expression2; expression3 {
    	//...
	}
	
`expression1`, `expression2` and `expression3` are all expressions, where `expression1` and `expression3` are variable definitions or return values from functions, and `expression2` is a conditional statement. `expression1` will be executed before every loop, and `expression3` will be executed after.

Examples are more useful than words.

	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

Sometimes we need multiple assignments, but Go doesn't have the `,` operator, so we use parallel assignment like `i, j = i + 1, j - 1`.

We can omit `expression1` and `expression3` if they are not necessary.

	sum := 1
	for ; sum < 1000;  {
    	sum += sum
	}
	
Omit `;` as well. Feel familiar? Yes, it's identical to `while`.

	sum := 1
	for sum < 1000 {
    	sum += sum
	}
	
There are two important operations in loops which are `break` and `continue`. `break` jumps out of the loop, and `continue` skips the current loop and starts the next one. If you have nested loops, use `break` along with labels.

	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, you may want to use `_` to discard certain return values. 

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

Sometimes you may find that you are using too many `if-else` statements to implement some logic, which may make it difficult to read and maitain in the future. This is the perfect time to use the `switch` statement to solve this problem.

	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. `switch` is very flexible. Conditions don't have to be constants and it executes from top to bottom until it matches conditions. If there is no statement after the keyword `switch`, then it matches `true`.

	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")
	}
	
In the fifth line, we put many values in one `case`, and we don't need to add the `break` keyword at the end of `case`'s body. It will jump out of the switch body once it matched any case. If you want to continue to matching more cases, you need to use 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

Use the `func` keyword to define a function.

	func funcName(input1 type1, input2 type2) (output1 type1, output2 type2) {
    	// function body
    	// multi-value return
    	return value1, value2
	}
	
We can extrapolate the following information from the example above.

- Use keyword `func` to define a function `funcName`.
- Functions 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. 

Let's see one practical example. (calculate maximum value)

	package main
	import "fmt"

	// return 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 the above example, there are two arguments in the function `max`, their types are both `int` so the first type can be omitted. For instance, `a, b int` instead of `a int, b int`. The same rules apply for additional arguments. Notice here that `max` only has one return value, so we only need to write the type of its return value -this is the short form of writing it.

### Multi-value return

One thing that Go is better at than C is that it supports multi-value returns.

We'll use the following example here.

	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)
	}
	
The above example returns two values without names -you have the option of naming them also. If we named the return values, we would just need to use `return` to return the values since they are initialized in the function automatically. Notice that if your functions are going to be used outside of the package, which means your function names start with a capital letter, you'd better write complete statements for `return`; it makes your code more readable.

	func SumAndProduct(A, B int) (add int, Multiplied int) {
    	add = A+B
    	Multiplied = A*B
    	return
	}

### Variable arguments

Go supports variable arguments, which means you can give an uncertain numbers of argument to functions. 

	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

When we pass an argument to the function that was called, that function actually gets the copy of our variables so any change will not affect to the original variable.

Let's see one example in order to prove what i'm saying.

	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"
	}
	
Can you see that? Even though we called `add1` with `x`, the origin value of `x` doesn't change.

The reason is very simple: when we called `add1`, we gave a copy of `x` to it, not the `x` itself.

Now you may ask how I can pass the real `x` to the function.

We need use pointers here. We know variables are stored in memory and they have some memory addresses. 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"
	}
	
Now we can change the value of `x` in the functions. Why do we use pointers? What are the advantages?

- 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`, `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

Go has a well designed keyword called `defer`. You can have many `defer` statements in one function; they will execute in reverse order when the program executes to the end of functions. 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 `defer`s, 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 [, ...])
	
What's the advantage of this feature? The answer is that it allows us to pass functions as values.

	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 function type, and return values and arguments of `filter` are the same as `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 `goroutine`s 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` function and `init` function

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.

![](images/2.3.init.png?raw=true)

Figure 2.6 Flow of programs initialization in Go

### import

We use `import` very often in Go programs as follows.

	import(
    	"fmt"
	)
	
Then we use functions in that package 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.

1. Dot operator.
	Sometime 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")`.
2. 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")`.
3. `_` 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.
	
## Links

- [Directory](preface.md)
- Previous section: [Go foundation](02.2.md)
- Next section: [struct](02.4.md)