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Sorting Room
Sorting Room

Sorting Room

Learning Exercise
OverviewCommunity Solutions

Introduction

Type Conversion

Go requires explicit conversion between different types. Converting between types (also known as type casting) is done via a function with the name of the type to convert to. For example, to convert an int to a float64 you would need to do the following:

var x int = 42 // x has type int
f := float64(x) // f has type float64 (ie. 42.0)

Converting between primitive types and strings

There is a strconv package for converting between primitive types (like int) and string.

import "strconv"

var intString string = "42"
var i, err = strconv.Atoi(intString)

var number int = 12
var s string = strconv.Itoa(number)

Type Assertions

Interfaces in Go can introduce ambiguity about the underlying type. A type assertion allows us to extract the interface value's underlying concrete value using this syntax: interfaceVariable.(concreteType).

For example:

var input interface{} = 12
number := input.(int)

NOTE: this will cause a panic if the interface variable does not hold a value of the concrete type.

We can test whether an interface value holds a specific concrete type by making use of both return values of the type assertion: the underlying value and a boolean value that reports whether the assertion succeeded. For example:

str, ok := input.(string) // no panic if input is not a string

If input holds a string, then str will be the underlying value and ok will be true. If input does not hold a string, then str will be the zero value of type string (ie. "" - the empty string) and ok will be false. No panic occurs in any case.

Type Switches

A type switch can perform several type assertions in series. It has the same syntax as a type assertion (interfaceVariable.(concreteType)), but the concreteType is replaced with the keyword type. Here is an example:

var i interface{} = 12 // try: 12.3, true, int64(12), []int{}, map[string]int{}

switch v := i.(type) {
case int:
    fmt.Printf("the integer %d\n", v)
case string:
    fmt.Printf("the string %s\n", v)
default:
    fmt.Printf("type, %T, not handled explicitly: %#v\n", v, v)
}

Instructions

Jen is working in the sorting room in a large factory. The sorting room needs to process anything that comes into it by categorizing it with a label. Jen is responsible for things that were pre-categorized as numbers and needs a program to help her with the sorting.

Most primitive values should get straight-forward labels. For numbers, she wants strings saying "This is the number 2.0" (if the number was 2). Jen wants the same output for integers and floats.

There are a few Box interfaces that need to be unwrapped to get their contents. For a NumberBox, she wants strings saying "This is a box containing the number 3.0" (if the Number() method returns 3). For a FancyNumberBox, she wants strings saying "This is a fancy box containing the number 4.0", but only if the type is a FancyNumber.

Anything unexpected should say "Return to sender" so Jen can send them back where they came from.

1. Describe simple numbers

Jen wants numbers to return strings like "This is the number 2.0" (including one digit after the decimal):

fmt.Println(DescribeNumber(-12.345))
// Output: This is the number -12.3

2. Describe a number box

Jen wants number boxes to return strings like "This is a box containing the number 2.0" (again, including one digit after the decimal):

fmt.Println(DescribeNumberBox(numberBoxContaining{12}))
// Output: This is a box containing the number 12.0

3. Implement a method extracting the number from a fancy number box

Jen needs a helper function to extract the number from a FancyNumberBox. If the FancyNumberBox is a FancyNumber, extract its value and convert it from a string to an int. Any other type of FancyNumberBox should return 0.

fmt.Println(ExtractFancyNumber(FancyNumber{"10"}))
// Output: 10
fmt.Println(ExtractFancyNumber(AnotherFancyNumber{"4"}))
// Output: 0

4. Describe a fancy number box

If the FancyNumberBox is a FancyNumber, Jen wants strings saying "This is a fancy box containing the number 4.0". Any other type of FancyNumberBox should say "This is a fancy box containing the number 0.0".

fmt.Println(DescribeFancyNumberBox(FancyNumber{"10"}))
// Output: This is a fancy box containing the number 10.0
fmt.Println(DescribeFancyNumberBox(AnotherFancyNumber{"4"}))
// Output: This is a fancy box containing the number 0.0

NOTE: we should use the ExtractFancyNumber function!

5. Implement DescribeAnything which uses them all

This is the main function Jen needs which takes any input (the empty interface means any value at all: interface{}). DescribeAnything should delegate to the other functions based on the type of the value passed in. More specifically:

  • int and float64 should both delegate to DescribeNumber
  • NumberBox should delegate to DescribeNumberBox
  • FancyNumberBox should delegate to DescribeFancyNumberBox
  • anything else should result in "Return to sender" 
fmt.Println(DescribeAnything(numberBoxContaining{12.345}))
// Output: This is a box containing the number 12.3
fmt.Println(DescribeAnything("some string"))
// Output: Return to sender
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