Fmt, printing

In Go programs, output needs to be printed and reported. We have the "fmt" package, and many helpful funcs like Printf and Println.

Console programs

In Go we can use Printf with a special format code. This determines how a string or integer is formatted. Println does not require a format string.

Printf, V format

To start, with Printf many format codes are available. But to make programs simpler, we can just use the "%v" format code to insert values.

package main

import "fmt"

func main() {
    result := true
    name := "Spark"
    size := 2000
    // Print line with v format codes.
    fmt.Printf("Result = %v, Name = %v, Size = %v",
        result, name, size)
}
Result = true, Name = Spark, Size = 2000

Other formats

We can pass various format strings to Printf. Codes like "%s" and "%d" indicate insertion points for values. Those values are also passed as arguments.

package main

import "fmt"

func main() {
    name := "Mittens"
    weight := 12

    // Use %s to mean string.
    // ... Use an explicit newline.
    fmt.Printf("The cat is named %s.\n", name)
    // Use %d to mean integer.
    fmt.Printf("Its weight is %d.\n", weight)
}
The cat is named Mittens.
Its weight is 12.

Println

To continue, we examine the Println method. This method is one of the easiest and simplest ones in fmt. We first must use an import statement with the argument "fmt."

package main

import "fmt"

func main() {
    // The Println method can handle one or more arguments.
    fmt.Println("cat")
    fmt.Println("cat", 900)

    // Use Println on a slice.
    items := []int{10, 20, 30}
    fmt.Println(items)
}
cat
cat 900
[10 20 30]

Print, for-loop

The Println always inserts a newline after we use it. But Print does not: it just writes the data to the console with no trailing newline.

package main

import "fmt"

func main() {
    elements := []int{999, 99, 9}

    // Loop over the int slice and Print its elements.
    // ... No newline is inserted after Print.
    for i := 0; i < len(elements); i++ {
        fmt.Print(elements[i] + 1)
        fmt.Print(" ")
    }
    fmt.Println("... DONE!")
}
1000 100 10 ... DONE!

Print, println

Print and println are universal functions. We can call these without referencing the "fmt" package. But their functionality is not identical to the fmt methods.

package main

func main() {
    value := 10

    // Use println.
    println(value)

    // Use print.
    // ... Use println with no arguments to write a newline.
    print(value)
    println()

    // Done.
    println("DONE")
}
10
10
DONE

Println differences

The println universal function has different output than the fmt.Println method. They are not the interchangeable.

package main

import "fmt"

func main() {
    items := []int{10, 20}
    // These two println methods have different output.
    fmt.Println(items)
    println(items)
}
[10 20]
[2/2]0xc08200a250

Sprintf

This method is a string-based form of Printf. The "S" stands for "string." So we use Printf and it returns a string—the value is not written to the console.

import "fmt"

func main() {
    value1 := "a tree";
    value2 := "the park";

    // Use format string to generate string.
    result := fmt.Sprintf("I saw %v at %v.\n", value1, value2)

    // Write length of string, and string itself.
    fmt.Println("Length:", len(result))
    fmt.Println(result)
}
Length: 26
I saw a tree at the park.

Sprintln

This func takes any number or arguments—it works the same way as fmt.Println, but returns a string. It does not support format codes.

package main

import "fmt"

func main() {
    // Use Sprintln, no format strings are supported.
    // ... A newline is added.
    //     A string is returned.
    result := fmt.Sprintln("Hey friend", 100)
    fmt.Print("[" + result + "]")
}
[Hey friend 100
]

Fprint, Fprintf

These methods are just like fmt.Print but target a file in the first argument. They write to files—nothing is written to the console.

Sscan, Sscanf

We can parse in space-separated values with fmt.Sscan. And with Sscanf we use a format string to parse in values. Sometimes this is easier than using split() or fields().

Padding

We can apply padding with fmt.Printf to ensure strings are a certain number of characters long. Spaces are added before or after the value.

A review

With methods prefixed by s, we can output the results to a string. We can write those strings to a file, or use them in any way we can use a string.