Standard Packages in Go

Like other languages, Go offers a lot of useful standard packages. You will look at:

• fmt
• strconv
• errors

fmt

fmt (format) implements are formatted I/O - you covered a good number of its verbs in module 1. Now you will see a range of additional useful verbs:

• %% for the percent sign
• %#v for the Go-syntax representation
• %t for a boolean; it formats a value as true or false
• %b for an integer; it formats the integer to its binary representation
• %c for an integer; it formats the integer to its corresponding Unicode character

If you use an invalid verb, you will get a string beginning with "%!" and a description.

To write to stdout:

1. func Print(a ...interface{}) (n int, err error) formats with default formats.
2. func Println(a ...interface{}) (n int, err error) formats with default formats and appends a newline.
3. func Printf(format string, a ...interface{}) (n int, err error) formats with given format.

These three functions return the number of bytes written and any applicable error.

To read from stdin:

1. func Scan(a ...interface{}) (n int, err error) parses using default formats. Values are space-separated. Newlines count as spaces.
2. func Scanln(a ...interface{}) (n int, err error) parses using default formats and stops at a newline.
3. func Scanf(format string, a ...interface{}) (n int, err error) parses using the given format. The verb %c matches every rune (space, tab, newline, etc.).

These three functions return the number of items scanned and an error if the count of items parsed is smaller than the count of arguments.

Perhaps you noticed a kind of function declaration we did not speak about: the variadic function can be called with any number of arguments. You can iterate over the arguments as follows:

Copy func variadicFunction(a ...interface{}) { for _,s:= range a { // do something } }

There are formatting functions on io.Writer and io.Reader, so we also have:

1. func Fprint(w io.Writer, a...interface{}) (n int, err error): same as Print but writes to w.
2. func Fprintln(w io.Writer, a...interface{}) (n int, err error): same as Println but writes to w.
3. func Fprintf(w io.Writer, a...interface{}) (n int, err error): same as Printf but writes to w.
4. func Fscan(r io.Reader, a...interface{}) (n int, err error): same as Fscan but scans from r.
5. func Fscanln(r io.Reader, a...interface{}) (n int, err error): same as Fscanln but scans from r.
6. func Fscanf(r io.Reader, a...interface{}) (n int, err error): same as Fscanf but scans from r.

io.Writer is the interface that declares the Write method. io.Reader is also an interface and it declares the Read method.

The functions Sprint, Sprintln, and Sprintf are similar to Print, Println, and Printf, with the difference that they return a string instead of writing to stdout.

The functions Sscan, Scanln, and Sscanf are similar to Fscan, Fscanln, and Fscanf with the difference that they scan from a string given as an argument.

The function Errorf formats according to a format and returns it as an error.

Further readings

• String formatting example (opens new window)
• Go walkthrough fmt (opens new window)
• Regular expressions example (opens new window)
• JSON example (opens new window)
• Io writer interface (opens new window)

strconv

The package strconv offers conversions to and from strings of basic data types.

Convert from string

Start with an example for using strconv (opens new window):

Copy package main import ( "fmt" "strconv" ) func main() { v32 := "-354634382" if s, err := strconv.ParseInt(v32, 10, 32); err == nil { fmt.Printf("%T, %v\n", s, s) } if s, err := strconv.ParseInt(v32, 16, 32); err == nil { fmt.Printf("%T, %v\n", s, s) } v64 := "-3546343826724305832" if s, err := strconv.ParseInt(v64, 10, 64); err == nil { fmt.Printf("%T, %v\n", s, s) } if s, err := strconv.ParseInt(v64, 16, 64); err == nil { fmt.Printf("%T, %v\n", s, s) } }

Test it online (opens new window).

• The function ParseInt(s string, base int, bitSize int) (i int64, err error) interprets s as an integer in base base, from 2 to 36; and with type bitSize, whereby 0 means int, 8 means int8, 16 means int16, 32 means int32, and 64 means int64.
• ParseUint is similar, with the difference that it returns an unsigned integer.
• The func Atoi(s string) (int, error) returns ParseInt(s, 10, 0) as type int.

Convert to string

1. func FormatBool(b bool) string returns "true" or "false" according to b.
2. func FormatFloat(f float64, fmt vyte, prec, bitSize int) string returns a string representation of f with format fmt and with precision prec.
3. func FormatInt(i int64, base int) string returns the string representation of i in base base.
4. func FormatUint(i uint, base int) string is the same as FormatInt, but it takes a unsigned integer.
5. func Itoa(i int) string is shorthand for FormatInt(int64(i), 10), so it gives a decimal representation of i as a string.

Further reading:

• Go walkthrough - includes also string operations (opens new window)

Errors

In Go, errors are values. The convention is that the last return value of a function is the error. This is the entire code of errors package:

Copy // Copyright 2011 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. // Package errors implements functions to manipulate errors. package errors // New returns an error that formats as the given text. func New(text string) error { return &errorString{text} } // errorString is a trivial implementation of error. type errorString struct { s string } func (e *errorString) Error() string { return e.s }

You use func New(text string) error to create an error. An example is as follows:

Copy package main import ( "errors" "fmt" ) func div(a, b float64) (float64, error) { if b == 0 { return -1, errors.New("can't perform division by zero") } return a / b, nil } func main() { for i := 2.; i >= -2; i = i - 0.5 { if x, err := div(3, i); err != nil { fmt.Println(err) } else { fmt.Printf("3/%v=%v\n", i, x) } } }

Test it online (opens new window).

In this case, nil means no error. For best practice, you should always check for errors. However, take the time to review the following list, because Go error handling differs from other languages.

Further reading:

• A tour of Go (opens new window)

synopsis

To summarize, this section has explored:

• The fmt (format) standard package in increased detail, including a range of useful verbs and function declarations.
• The strconv package, which offers conversions to and from strings of basic data types.
• Error values, which by convention are the last return value of a function.