Type inference in Go

Introduction

Type inference lets Go figure out the type of a variable automatically so you don't have to write it. This makes your code shorter and easier to read.

When you want to declare a variable without writing its type explicitly.

When the value assigned clearly shows what type the variable should be.

When you want to keep your code clean and simple.

When you are working with short variable declarations inside functions.

When you want to avoid repeating the type name.

Syntax

Go

variableName := value

The := operator declares and initializes a variable with inferred type.

This syntax can only be used inside functions, not at package level.

Examples

Go infers x is an int because 10 is an integer.

Go

x := 10

Go infers name is a string because of the quoted text.

Go

name := "Alice"

Go infers pi is a float64 because 3.14 is a decimal number.

Go

pi := 3.14

Go infers flag is a bool because true is a boolean value.

Go

flag := true

Sample Program

This program shows how Go automatically figures out the type of each variable using :=. It prints their values.

Go

package main

import "fmt"

func main() {
    age := 25
    city := "Paris"
    temperature := 18.5
    isRaining := false

    fmt.Println("Age:", age)
    fmt.Println("City:", city)
    fmt.Println("Temperature:", temperature)
    fmt.Println("Is it raining?:", isRaining)
}

Important Notes

You cannot use := outside functions; use var with explicit type there.

Type inference works only when you assign a value at the same time as declaring.

If you want to change the variable type later, you must declare a new variable.

Summary

Type inference lets Go guess the variable type from the value you give it.

Use := inside functions to declare and assign variables quickly.

It makes your code shorter and easier to read without losing type safety.

Practice

1. What does type inference mean in Go when you write x := 10 inside a function?

easy

A. You must tell Go the type of x explicitly.

B. Go automatically figures out that x is an int.

C. Go treats x as a string by default.

D. The variable x has no type.

Solution

1. Step 1: Understand the := operator in Go

   The := operator declares a variable and assigns a value, letting Go guess the type.

2. Step 2: Check the assigned value type

   Since 10 is an integer, Go infers the type of x as int automatically.

3. Final Answer:

   Go automatically figures out that x is an int. -> Option B

4. Quick Check:

   Type inference means Go guesses type from value [OK]

Hint: := means declare and infer type from value [OK]

Common Mistakes:

• Thinking you must always write the type explicitly
• Assuming variables have no type
• Believing Go defaults to string type

2. Which of the following is the correct way to declare and assign a variable with type inference inside a function in Go?

easy

A. x := 5

B. x = 5

C. var x int = 5

D. int x = 5

Solution

1. Step 1: Recall Go syntax for type inference

   Inside functions, := declares and assigns a variable with inferred type.

2. Step 2: Check each option

   var x int = 5 declares with explicit type, not inference. x = 5 misses declaration. int x = 5 is invalid Go syntax.

3. Final Answer:

   x := 5 -> Option A

4. Quick Check:

   Use := to declare and infer type [OK]

Hint: Use := inside functions for type inference [OK]

Common Mistakes:

• Using C or D which are invalid Go syntax
• Forgetting to declare variable before assignment
• Confusing var declaration with :=

3. What is the output of this Go program?

package main
import "fmt"
func main() {
    x := 3.14
    fmt.Printf("%T", x)
}

medium

A. float64

B. float32

C. int

D. string

Solution

1. Step 1: Identify the type inferred for 3.14

   In Go, untyped floating-point constants default to float64 when assigned with :=.

2. Step 2: Understand fmt.Printf with %T

   The %T verb prints the type of the variable, so it will print "float64".

3. Final Answer:

   float64 -> Option A

4. Quick Check:

   Float literals default to float64 [OK]

Hint: Float literals default to float64 type [OK]

Common Mistakes:

• Assuming float32 is default
• Thinking 3.14 is int
• Confusing %T output with value

4. What is wrong with this Go code snippet?

func main() {
    var x := 10
    fmt.Println(x)
}

medium

A. The value 10 is not a valid integer.

B. x is not declared before use.

C. You cannot use := with var keyword.

D. fmt.Println is not imported.

Solution

1. Step 1: Understand variable declaration syntax

   In Go, := is shorthand for declaration and assignment without var keyword.

2. Step 2: Identify the syntax error

   Using var and := together is invalid syntax; you must use either var x = 10 or x := 10.

3. Final Answer:

   You cannot use := with var keyword. -> Option C

4. Quick Check:

   var and := cannot be combined [OK]

Hint: Don't combine var and := in one statement [OK]

Common Mistakes:

• Mixing var and := together
• Ignoring import errors (not shown here)
• Thinking := works outside functions

5. Given this Go code, what is the type of result after execution?

func main() {
    a := 5
    b := 2.0
    result := a * int(b)
    fmt.Printf("%T", result)
}

hard

A. float64

B. string

C. float32

D. int

Solution

1. Step 1: Analyze variable types and conversion

   a is int, b is float64 (default for 2.0). The expression converts b to int before multiplication.

2. Step 2: Determine result type

   Multiplying int (a) by int (converted b) results in int type for result.

3. Final Answer:

   int -> Option D

4. Quick Check:

   int * int = int type [OK]

Hint: Type conversion affects result type [OK]

Common Mistakes:

• Assuming result is float64 without conversion
• Forgetting to convert float to int
• Confusing multiplication type rules