Author: Saim Khalid

Pointers in Go are easy and fun to learn. Some Go programming tasks are performed more easily with pointers, and other tasks, such as call by reference, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect Go programmer.

As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which will print the address of the variables defined −

package main

import "fmt"

func main() {
   var a int = 10   
   fmt.Printf("Address of a variable: %x\n", &a  )
}

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

Address of a variable: 10328000

So you understood what is memory address and how to access it. Now let us see what pointers are.

What Are Pointers?

A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before you can use it to store any variable address. The general form of a pointer variable declaration is −

var var_name *var-type

Here, type is the pointer’s base type; it must be a valid C data type and var-name is the name of the pointer variable. The asterisk * you used to declare a pointer is the same asterisk that you use for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Following are the valid pointer declaration −

var ip *int        /* pointer to an integer */
var fp *float32    /* pointer to a float */

The actual data type of the value of all pointers, whether integer, float, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.

How to Use Pointers?

There are a few important operations, which we frequently perform with pointers: (a) we define pointer variables, (b) assign the address of a variable to a pointer, and (c) access the value at the address stored in the pointer variable.

All these operations are carried out using the unary operator * that returns the value of the variable located at the address specified by its operand. The following example demonstrates how to perform these operations −

Live Demo

package main

import "fmt"

func main() {
   var a int = 20   /* actual variable declaration */
   var ip *int      /* pointer variable declaration */

   ip = &a  /* store address of a in pointer variable*/

   fmt.Printf("Address of a variable: %x\n", &a  )

   /* address stored in pointer variable */
   fmt.Printf("Address stored in ip variable: %x\n", ip )

   /* access the value using the pointer */
   fmt.Printf("Value of *ip variable: %d\n", *ip )
}

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

Address of var variable: 10328000
Address stored in ip variable: 10328000
Value of *ip variable: 20

Nil Pointers in Go

Go compiler assign a Nil value to a pointer variable in case you do not have exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned nil is called a nil pointer.

The nil pointer is a constant with a value of zero defined in several standard libraries. Consider the following program −

package main

import "fmt"

func main() {
   var  ptr *int

   fmt.Printf("The value of ptr is : %x\n", ptr  )
}

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

The value of ptr is 0

On most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location. But by convention, if a pointer contains the nil (zero) value, it is assumed to point to nothing.

To check for a nil pointer you can use an if statement as follows −

if(ptr != nil)     /* succeeds if p is not nil */
if(ptr == nil)    /* succeeds if p is null */

Go Pointers in Detail

Pointers have many but easy concepts and they are very important to Go programming. The following concepts of pointers should be clear to a Go programmer −

Go programming language provides a data structure called the array, which can store a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

Instead of declaring individual variables, such as number0, number1, …, and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and …, numbers[99] to represent individual variables. A specific element in an array is accessed by an index.

All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element.

Declaring Arrays

To declare an array in Go, a programmer specifies the type of the elements and the number of elements required by an array as follows −

var variable_name [SIZE] variable_type

This is called a single-dimensional array. The arraySize must be an integer constant greater than zero and type can be any valid Go data type. For example, to declare a 10-element array called balance of type float32, use this statement −

var balance [10] float32

Here, balance is a variable array that can hold up to 10 float numbers.

Initializing Arrays

You can initialize array in Go either one by one or using a single statement as follows −

var balance = [5]float32{1000.0, 2.0, 3.4, 7.0, 50.0}

The number of values between braces { } can not be larger than the number of elements that we declare for the array between square brackets [ ].

If you omit the size of the array, an array just big enough to hold the initialization is created. Therefore, if you write −

var balance = []float32{1000.0, 2.0, 3.4, 7.0, 50.0}

You will create exactly the same array as you did in the previous example. Following is an example to assign a single element of the array −

balance[4] = 50.0

The above statement assigns element number 5^th in the array with a value of 50.0. All arrays have 0 as the index of their first element which is also called base index and last index of an array will be total size of the array minus 1. Following is the pictorial representation of the same array we discussed above −

Accessing Array Elements

An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array. For example −

float32 salary = balance[9]

The above statement will take 10^th element from the array and assign the value to salary variable. Following is an example which will use all the above mentioned three concepts viz. declaration, assignment and accessing arrays −

package main

import "fmt"

func main() {
   var n [10]int /* n is an array of 10 integers */
   var i,j int

   /* initialize elements of array n to 0 */         
   for i = 0; i < 10; i++ {
  n&#91;i] = i + 100 /* set element at location i to i + 100 */
   }
   
   /* output each array element's value */
   for j = 0; j < 10; j++ {
  fmt.Printf("Element&#91;%d] = %d\n", j, n&#91;j] )
   }
}

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

Element[0] = 100
Element[1] = 101
Element[2] = 102
Element[3] = 103
Element[4] = 104
Element[5] = 105
Element[6] = 106
Element[7] = 107
Element[8] = 108
Element[9] = 109

Go Arrays in Detail

There are important concepts related to array which should be clear to a Go programmer −

A scope in any programming is a region of the program where a defined variable can exist and beyond that the variable cannot be accessed. There are three places where variables can be declared in Go programming language −

• Inside a function or a block (local variables)
• Outside of all functions (global variables)
• In the definition of function parameters (formal parameters)

Let us find out what are local and global variables and what are formal parameters.

Local Variables

Variables that are declared inside a function or a block are called local variables. They can be used only by statements that are inside that function or block of code. Local variables are not known to functions outside their own. The following example uses local variables. Here all the variables a, b, and c are local to the main() function.

Live Demo

package main

import "fmt"

func main() {
   /* local variable declaration */
   var a, b, c int 

   /* actual initialization */
   a = 10
   b = 20
   c = a + b

   fmt.Printf ("value of a = %d, b = %d and c = %d\n", a, b, c)
}

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

value of a = 10, b = 20 and c = 30

Global Variables

Global variables are defined outside of a function, usually on top of the program. Global variables hold their value throughout the lifetime of the program and they can be accessed inside any of the functions defined for the program.

A global variable can be accessed by any function. That is, a global variable is available for use throughout the program after its declaration. The following example uses both global and local variables −

Live Demo

package main

import "fmt"
 
/* global variable declaration */
var g int
 
func main() {
   /* local variable declaration */
   var a, b int

   /* actual initialization */
   a = 10
   b = 20
   g = a + b

   fmt.Printf("value of a = %d, b = %d and g = %d\n", a, b, g)
}

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

value of a = 10, b = 20 and g = 30

A program can have the same name for local and global variables but the value of the local variable inside a function takes preference. For example −

package main

import "fmt"
 
/* global variable declaration */
var g int = 20
 
func main() {
   /* local variable declaration */
   var g int = 10
 
   fmt.Printf ("value of g = %d\n",  g)
}

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

value of g = 10

Formal Parameters

Formal parameters are treated as local variables with-in that function and they take preference over the global variables. For example −

package main

import "fmt"
 
/* global variable declaration */
var a int = 20;
 
func main() {
   /* local variable declaration in main function */
   var a int = 10
   var b int = 20
   var c int = 0

   fmt.Printf("value of a in main() = %d\n",  a);
   c = sum( a, b);
   fmt.Printf("value of c in main() = %d\n",  c);
}
/* function to add two integers */
func sum(a, b int) int {
   fmt.Printf("value of a in sum() = %d\n",  a);
   fmt.Printf("value of b in sum() = %d\n",  b);

   return a + b;
}

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

value of a in main() = 10
value of a in sum() = 10
value of b in sum() = 20
value of c in main() = 30

Initializing Local and Global Variables

Local and global variables are initialized to their default value, which is 0; while pointers are initialized to nil.

There may be a situation, when you need to execute a block of code several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times and following is the general form of a loop statement in most of the programming languages −

Go programming language provides the following types of loop to handle looping requirements.

Loop Control Statements

Loop control statements change an execution from its normal sequence. When an execution leaves its scope, all automatic objects that were created in that scope are destroyed.

Go supports the following control statements −

The Infinite Loop

A loop becomes an infinite loop if its condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the for loop are required, you can make an endless loop by leaving the conditional expression empty or by passing true to it.

package main

import "fmt"

func main() {
   for true  {
   fmt.Printf("This loop will run forever.\n");
   }
}

When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but C programmers more commonly use the for(;;) construct to signify an infinite loop.

Note − You can terminate an infinite loop by pressing Ctrl + C keys.

Decision making structures require that the programmer specify one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.

Following is the general form of a typical decision making structure found in most of the programming languages −

Go programming language provides the following types of decision making statements. Click the following links to check their detail.