There are situations when you want to execute a code based on some condition such as fetching data from server only when connection has been established.
Golang permits this using its if statement.

if statement is followed by a condition and code surrounded between curly braces.
This code is executed only when the condition after if evaluates to a bool true. Note that the condition should be such that it should result in either true or false, otherwise you get a compiler error.

if syntax
Syntax to write an if statement is given below.

if condition {
 // conditional code
}

It consists of an if keyword followed by a condition. This condition may be an expression that should result in true or false.
Curly braces after if statement is called if block. There may be any number of statements inside an if block.
if example
Below is an example of using if statement to check whether age is greater than 18 or not.

package main

import "fmt"

func main() {

  age := 19
  if age > 18 {
    fmt.Println("You are allowed to vote")
  }
}

Logical operators with if
Often there may be multiple conditions to check in a single if statement before executing the code.
Example, before starting a laptop, you might need to test that it is connected to a power source and charging.

With if, you can add multiple conditions using logical AND and logical OR operators as shown below.

package main

import "fmt"

func main() {

  connected := true
  charging := true
  if connected && charging {
    fmt.Println("Laptop is ready to use")
  }
}

Likewise, you can use comparison operators( >, <, >=, <=, != ), equality operator( == ) with along with logical operators with if statement .
Below code example checks if a number lies between a range(18 to 24),

package main

import "fmt"

func main() {

  number := 21
  if number >=18 && number <=24 {
    fmt.Println("Number is betweeen 18 and 24")
  }
}

Nested if
An if statement inside another if statement is called nested if statement. Inner if statement is executed only when the outer statement evaluates to true.

Nested if statements are used to represent a logical AND condition. Thus, above code example may also be written as

package main

import "fmt"

func main() {

  number := 21
  if number >=18 {
          // inner if
    if number <=24 {
      fmt.Println("Number is betweeen 18 and 24")
    }
  }
}

There is no limit on the number of levels that you can nest if statements.
if-else
There are scenarios when you want to execute a code block when the if statement evaluates to false.
An example of this case would be checking for a database connection in if block.
If it is connected, then fetch the data otherwise try to connect again in else block.

Note that else block executes only when the if condition is false. This means that either if block is executed or else, not both at the same time.

Syntax of an if-else construct in Go is

if condition {
  // executes when condition evaluates to true  
} else {
  // executes when condition evaluates to false
}

Example of an if-else block is

package main

import "fmt"

func main() {

  number := 15
  if number >= 18 {
    fmt.Println("Number is greater than equal 18")
  } else {
    fmt.Println("Number is lesser than 18")

  }
}

if…else if
When multiple conditions need to be tested, then if...else if construct is used. Both if and else if statements are followed by a condition and curly braces with the code to be executed.
There may be multiple else if blocks after the first if block.

Syntax of if...else if construct is

if condition1 {
  // executes when condition1 is true
} else if condition2 {
  // executes when condition2 is true
} else if condition3 {
  // executes when condition3 is true
}

As soon as a condition evaluates to true, the corresponding block is executed and all others are skipped. Remember that only one block out of all blocks is executed.
Example of if...else if is given below

package main

import "fmt"

func main() {

  fruit := "juicy"
  if fruit == "pulpy" {
    fmt.Println("May be apple, papaya or guava")
  } else if fruit == "juicy" {
    fmt.Println("May be orange, pomegranate or watermelon ")

  }
}

if…else if…else
This is the same as if...else if but with a default code block that will execute when none of the conditions evaluates to true.
The default code block is written as an else block. This construct should be used when there is one block that should be executed in any case.
Example is given below.

package main

import "fmt"

func main() {

  number := -4
  if number > 0 {
    fmt.Println("Number is positive")
  } else if number < 0 {
    fmt.Println("Number is negative")
  } else {
    fmt.Println("Number is zero")
  }
}

Above code checks if a number is positive, negative or 0.
if statement checks the number to be greater than 0, else if statement checks the number to be lesser than 0.
When both the conditions evaluate to false, this implies that the number will be 0 and hence put in the else block.

Inline Initialization
In all the above examples, we defined a variable before if and checked it in the if condition. But, it is also possible to define the variable in the same line as if statement.

Variable definition and condition should be separated by a semi-colon as shown below.

package main

import "fmt"

func main() {

  if age := 19; age > 18 {
    fmt.Println("You are allowed to vote")
  }
}

Though the syntax looks concise but it is difficult to understand for someone reading the code, so it is better to define variables before the if or if-else blocks.

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