Go - Functions

A function is a group of statements that together perform a task. Every Go program has at least one function, which is main(), and all the most trivial programs can define additional functions.

You can divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically the division usually is so each function performs a specific task.
A function declaration tells the compiler about a function's name, return type, and parameters. A function definition provides the actual body of the function.
The Go standard library provides numerous built-in functions that your program can call. For example, function len() takes arguments of various types and return the length of the type. For example, if a string is passed to it, it will return length of the string in bytes and if an array is passed to it, it will return the array length as number of elements it have.
A function is known with various names like a method or a sub-routine or a procedure, etc.

Defining a Function:

The general form of a function definition in Go programming language is as follows:

func function_name( [parameter list] ) [return_types]
{
   body of the function
}

A function definition in Go programming language consists of a function header and a function body. Here are all the parts of a function:

• func func starts the declaration of a function.
• Function Name: This is the actual name of the function. The function name and the parameter list together constitute the function signature.
• Parameters: A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
• Return Type: A function may return a list of values. The return_types is the list of data types of the values the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the not required.
• Function Body: The function body contains a collection of statements that define what the function does.

Example:

Following is the source code for a function called max(). This function takes two parameters num1 and num2 and returns the maximum between the two:

/* function returning the max between two numbers */
func max(num1, num2 int) int
{
   /* local variable declaration */
   result int

   if (num1 > num2) {
      result = num1
   } else {
      result = num2
   }
   return result 
}

Calling a Function:

While creating a Go function, you give a definition of what the function has to do. To use a function, you will have to call that function to perform the defined task.
When a program calls a function, program control is transferred to the called function. A called function performs defined task and when its return statement is executed or when its function-ending closing brace is reached, it returns program control back to the main program.
To call a function, you simply need to pass the required parameters along with function name, and if function returns a value, then you can store returned value. For example:

package main

import "fmt"

func main() {
   /* local variable definition */
   var a int = 100
   var b int = 200
   var ret int

   /* calling a function to get max value */
   ret = max(a, b)

   fmt.Printf( "Max value is : %d\n", ret )
}

/* function returning the max between two numbers */
func max(num1, num2 int) int {
   /* local variable declaration */
   var result int

   if (num1 > num2) {
      result = num1
   } else {
      result = num2
   }
   return result 
}

I kept max() function along with main() function and compiled the source code. While running final executable, it would produce the following result:

Max value is : 200

Returning multiple values from Function

A Go function can return multiple values. For example:

package main

import "fmt"

func swap(x, y string) (string, string) {
   return y, x
}

func main() {
   a, b := swap("Mahesh", "Kumar")
   fmt.Println(a, b)
}

When the above code is compiled and executed, it produces the following result:

Kumar Mahesh

Function Arguments:

If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function.
The formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit.
While calling a function, there are two ways that arguments can be passed to a function:

Call Type	Description
Call by value	This method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.
Call by reference	This method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.

By default, Go uses call by value to pass arguments. In general, this means that code within a function cannot alter the arguments used to call the function and above mentioned example while calling max() function used the same method.

Function Usage:

Function Usage	Description
Function as Value	Functions can be created on the fly and can be used as values.
Function Closures	Functions closure are anonymous functions and can be used in dynamic programming.
Method	Method are special functions with a receiver.