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Converting between types

This chapter covers how to convert between strings and numbers.

Introduction

This chapter will:

  • Introduce uses cases where data conversion makes sense.
  • Showcase how to use strconv library.

Why convert between types

There are different data types and a need to convert between them. For example, we often need to convert between text and numbers for presentational and other reasons. We also need to convert between numbers and decimals without losing information in the process.

The main package for dealing with conversions in Go is strconv.

Use case - command-line arguments

Let's show a common case where you start off with strings and you need to make it into numbers, command-line arguments. To use command-line arguments in a program, you need the os package.

os.Args points to an array representing your command line arguments. To access a specific argument, you would use the index operator [] like so:

arg := os.Args[1]

You can then start your program like so:

go run main.go 1

The 1 would then be stored in arg.

Finding the type

What type is arg in our code above? There are some ways to find out:

  • IDE, if you use for example Visual Studio Code and the Go plugin, hovering over the code, it will tell you that os.Args is a string array, string[].

  • PrintF() and %T. One of the easiest ways to find the type is typing like so:

    Printf("%T", os.Args[1])
Printf("%T", 1)

    You would get an output like so:

    string
int

  • Type coercion, you could try to modify that code and coerce it to be an integer like so, now what?

    var no int = os.Args[1]

    You get an error:

    cannot use os.Args[1] (type string) as type int in assignment

  • Use reflection. Another way to find the above is by using the reflect package like so:

    package main

import (
  "reflect"
  "fmt"
  "os"
)

func main () {
  arg := os.Args[1]
  fmt.Println(reflect.TypeOf(arg))
}

    Now, the program will print "string" as the type.

Addressing the problem with strconv

Ok, so we know what type something is, what if we need to use these command-line arguments, which are of type string, and feed them into let's say a calculator program?

Consider the below code, that at present WOULDN'T compile:

package main

import (
  "fmt"
  "os"
)

func add(first int, second int) int {
  return first + second
}

func main() {
  add(os.Args[1], os.Args[2]) // this would NOT compile
}

The reason is that the values on os.Args[1] and os.Args[2] are string not int. To fix this issue, we need to use the conversion package strconv.

Convert from string to int with strconv

To convert strings to integers, we need to use strconv and call the Atoi() (stands for Ascii to integer) function like so:

package main

import (
 "fmt"
 "os"
 "strconv"
)

func add(first int, second int) int {
 return first + second
}

func main() {
 no1, _ := strconv.Atoi(os.Args[1])
 no2, _ := strconv.Atoi(os.Args[2])
 var sum = add(no1, no2) 
 fmt.Println(sum)
}

Note _, this is a don't care symbol. What happens when you call Atoi() is that it returns two things, the number and an error if it fails.

Handling conversion error

To handle an error, we need to store it in a variable, err and inspect it. If it's not nil, then we have an error.

Here's how we could encode that behaviour below:

package main

import (
 "fmt"
 "os"
 "strconv"
)

func main() {
 no, err := strconv.Atoi(os.Args[1])
 fmt.Println(no)
 if err != nil {
  fmt.Println(err)
  fmt.Println("Couldn't convert: " + os.Args[1])

 } else {
  fmt.Println(no)
 }
}

Try to compile the above program and run it like so:

main 1 # 1
main hi # trconv.Atoi: parsing "hi": invalid syntax, Couldn't convert: hi

Parse string to int

There's another way to convert a string to an int. That's by using the ParseInt() method. It does more than converting though, it does two things in fact:

  • base, you can select according to what base to interpret the number as.
  • size, bit size, from 0 to 64.

The syntax for the method looks like so:

ParseInt(<s string>, <base int>, <bit int>) (int64, error)

Here's some examples:

package main

import (
 "fmt"
 "reflect"
 "strconv"
)

func main() {
 var no int = 100
 fmt.Println(reflect.TypeOf(no))

 var intStr string = "100"
 fourBaseEightBitInt, _ := strconv.ParseInt(intStr, 4, 8)    // becomes no 16 and int64
 tenBaseSixteenBitInt, _ := strconv.ParseInt(intStr, 10, 16) // no 100,  and int64
 fmt.Println(reflect.TypeOf(fourBaseEightBitInt))
 fmt.Println(reflect.TypeOf(tenBaseSixteenBitInt))
}

Integer to string

You might be dealing with the opposite; you have an integer, and you want it to be a string. In this case, you can use the Itoa() function, integer to ascii. Here's an example:

var noOfPlayers = 8
str := strconv.Itoa(noOfPlayers)

Additional parsing

The strconv library is what you want if you start with a string, and you want to convert to and from another format. Learn more about strconv library here

Assignment

Create an app that adds two numbers together. The values should come from the command line. Here's how the program should run:

go run main.go 2 4
6

Solution

package main

import (
 "fmt"
 "os"
 "strconv"
)

func add(no int, secondNumber int) int {
 return no + secondNumber
}

func main() {
 no1, _ := strconv.Atoi(os.Args[1])
 no2, _ := strconv.Atoi(os.Args[2])
 var sum = add(no1, no2)
 fmt.Println(sum)
}

🚀 Challenge

What happens if run the program like so?

go run main.go one two

Handle any conversion error and call panic() if there's a conversion error.