String Formatting

In this tutorial, we will learn about string formatting or sometimes also known as templating.

fmt package contains lots of functions. So to save time, we will discuss the most frequently used functions. Let's start with fmt.Print inside our main function.

...

fmt.Print("What", "is", "your", "name?")
fmt.Print("My", "name", "is", "golang")
...

$ go run main.go
Whatisyourname?Mynameisgolang

As we can see, Print does not format anything, it simply takes a string and prints it.

Next, we have Println which is the same as Print but it adds a new line at the end and also inserts space between the arguments.

...

fmt.Println("What", "is", "your", "name?")
fmt.Println("My", "name", "is", "golang")
...

$ go run main.go
What is your name?
My name is golang

That's much better!

Next, we have Printf also known as "Print Formatter", which allows us to format numbers, strings, booleans, and much more.

Let's look at an example.

...
name := "golang"

fmt.Println("What is your name?")
fmt.Printf("My name is %s", name)
...

$ go run main.go
What is your name?
My name is golang

As we can see that %s was substituted with our name variable.

But the question is what is %s and what does it mean?

So, these are called annotation verbs and they tell the function how to format the arguments. We can control things like width, types, and precision with these and there are lots of them. Here's a cheatsheet.

Now, let's quickly look at some more examples. Here we will try to calculate a percentage and print it to the console.

...
percent := (7.0 / 9) * 100
fmt.Printf("%f", percent)
...

$ go run main.go
77.777778

Let's say we want just 77.78 which is 2 points precision, we can do that as well by using .2f.

Also, to add an actual percent sign, we will need to escape it.

...
percent := (7.0 / 9) * 100
fmt.Printf("%.2f %%", percent)
...

$ go run main.go
77.78 %

This brings us to Sprint, Sprintln, and Sprintf. These are basically the same as the print functions, the only difference being they return the string instead of printing it.

Let's take a look at an example.

...
s := fmt.Sprintf("hex:%x bin:%b", 10 ,10)
fmt.Println(s)
...

$ go run main.go
hex:a bin:1010

So, as we can see Sprintf formats our integer as hex or binary and returns it as a string.

Lastly, we have multiline string literals, which can be used like this.

...
msg := `
Hello from
multiline
`

fmt.Println(msg)
...

Great! But this is just the tip of the iceberg...so make sure to check out the go doc for fmt package.

For those who are coming from C/C++ background, this should feel natural, but if you're coming from, let's say Python or JavaScript, this might be a little strange at first. But it is very powerful and you'll see this functionality used quite extensively.