Base I/O

The most basic I/O operations are read and write on the console. We already know about console writing because I’ve used the fmt.Println() method from the beginning. But it does not allow formatting strings or numbers. Let’s see other fmt methods, such as: Printf() and Sprintf().

Reading console data basically returns strings. We can specify other data types, as the [bufio package] (https://golang.org/pkg/bufio/) specifies, but we basically use strings. Let’s see how this works. In the example program we read a name from the console:

package main

import (
	"bufio"
	"fmt"
	"os"
	"strings"
)

func main() {
	reader := bufio.NewReader(os.Stdin)
	fmt.Print("Your name: ")
	text, _ := reader.ReadString('\n')
    ...

It expects you to type a name and press . The resulting string will contain everything up to the character. The **ReadString()** method returns the read string and an error code, which only happens if the string does not contain the delimiter. In this case, we will read until we find an . We could use any character, such as a "$":

text, _ := reader.ReadString('$')

It will read everything until it finds a ‘$’ character. Note that I used single quotes because it is a rune or a 32-bit integer representing a UTF-8 character.

And we can show what was typed. If we do with fmt.Println() it will look like this:

fmt.Printf("Hello, %v how are you today?\n", text)
...
Hello, Cleuton
 how are you today?

Here I used the Printf() method that allows us to format data. You can refer to the valid formats on this page, but I will summarize here:

Format	Description
%v	Any value
%t	true or false
%d	base 10 integer
%f	Floating point number

Why this line break after the name? The reason is the character ‘\n’ you enter is part of the string. We can use the strings library and remove the leading and trailing spaces from the typed text:

newName := fmt.Sprintf("%v", strings.TrimSpace(text))
fmt.Printf("Now, %v, without the extra line\n", newName)
...
Now, Cleuton, without the extra line

What if we want to read numbers? For example, integers? It’s simple:

fmt.Print("Now, type an integer: ")
nints, _ := reader.ReadString('\n')
nint, errInt := strconv.ParseInt(strings.TrimSpace(nints), 10, 64)
if errInt != nil {
    log.Fatal("Invalid integer")
}
fmt.Printf("You typed: %d\n", nint)

We read a string, eliminate the whitespaces, and transform it into an integer 64-bit base 10 with the ParseInt() method. But we have to remember to test if there was an error as the user can type an invalid number. Remember log.Fatal()? It terminates execution with the error message.

How about floating point numbers?

// Floating point
fmt.Print("Now, type a float: ")
nfs, _ := reader.ReadString('\n')
nf, errf := strconv.ParseFloat(strings.TrimSpace(nfs), 64)
if errf != nil {
    log.Fatal("Invalid float number")
}
fmt.Printf("You typed: %f\n", nf)

i18n

i18n Go has no direct internationalization support, but there are packages that can help, such as [golang.org/x/text] (https://godoc.org/golang.org/x/text). You need to install it with:

go get -v golang.org/x/text

Text files

I will show the basic I/O using text files. First, let’s create a text file:

// Creating a text file
stringArr := []byte("Good morning.\nEat an apple.\n")
// Permission: -rw-r--r--
err := ioutil.WriteFile("/tmp/arq.txt", stringArr, 0644)
check(err)
...
cat /tmp/arq.txt
Good morning.
Eat an apple.

As you can see, the ioutil.WriteFile() method writes an array of bytes. As we use Unicode, it can record accents smoothly. The permission is the same as we would use on the chmod command from Linux. I created a check function that uses the panic command in case of problems. Personally, I prefer log.Fatal().

Now let’s read that file:

// Reading a text file
data, err := ioutil.ReadFile("/tmp/arq.txt")
check(err)
fmt.Printf("\nType: %T\n", data)
textContent := string(data)
fmt.Println(textContent)
...
Type: []uint8
Good morning.
Eat an apple.

When we read a file the data type of the variable is byte array (unsigned 8 bits). We can convert it to string and use normally.

Challenge

Create a program that reads from the console the coefficient values of a quadratic equation, calculates the roots, and displays it on the screen, writing a file with each calculation.