Format a Go string without printing

Question

Is there a simple way to format a string in Go without printing the string   I can do   bar     bar  fmt Printf  foo   s   bar    But I want the formatted string returned rather than printed so I can manipulate it further   I could also do something like   s     foo      bar   But this becomes difficult to read when the format string is complex  and cumbersome when one or many of the parts aren t strings and have to be converted first  like  i    25 s     foo      strconv Itoa i    Is there a simpler way to do this

User · Answer

I came to this page specifically looking for a way to format an error string. So if someone needs help with the same, you want to use the fmt.Errorf() function.

The method signature is func Errorf(format string, a ...interface{}) error. It returns the formatted string as a value that satisfies the error interface.

You can look up more details in the documentation - https://golang.org/pkg/fmt/#Errorf.

User · Answer

1  Simple strings  For  simple  strings  typically what fits into a line  the simplest solution is using fmt Sprintf   and friends  fmt Sprint    fmt Sprintln     These are analogous to the functions without the starter S letter  but these Sxxx   variants return the result as a string instead of printing them to the standard output   For example   s    fmt Sprintf  Hi  my name is  s and I m  d years old     Bob   23    The variable s will be initialized with the value   Hi  my name is Bob and I m 23 years old    Tip  If you just want to concatenate values of different types  you may not automatically need to use Sprintf    which requires a format string  as Sprint   does exactly this  See this example   i    23 s    fmt Sprint   age    i          s will be   age 23     For concatenating only strings  you may also use strings Join   where you can specify a custom separator string  to be placed between the strings to join    Try these on the Go Playground   2  Complex strings  documents   If the string you re trying to create is more complex  e g  a multi-line email message   fmt Sprintf   becomes less readable and less efficient  especially if you have to do this many times    For this the standard library provides the packages text template and html template  These packages implement data-driven templates for generating textual output  html template is for generating HTML output safe against code injection  It provides the same interface as package text template and should be used instead of text template whenever the output is HTML   Using the template packages basically requires you to provide a static template in the form of a string value  which may be originating from a file in which case you only provide the file name  which may contain static text  and actions which are processed and executed when the engine processes the template and generates the output   You may provide parameters which are included substituted in the static template and which may control the output generation process  Typical form of such parameters are structs and map values which may be nested   Example   For example let s say you want to generate email messages that look like this   Hi  name    Your account is ready  your user name is   user-name   You have the following roles assigned   role 1    role 2        role n    To generate email message bodies like this  you could use the following static template   const emailTmpl    Hi    Name     Your account is ready  your user name is     UserName    You have the following roles assigned    range  i   r     Roles    if  i      end         end       And provide data like this for executing it   data    map string interface         Name        Bob        UserName    bob92        Roles        string  dbteam    uiteam    tester        Normally output of templates are written to an io Writer  so if you want the result as a string  create and write to a bytes Buffer  which implements io Writer   Executing the template and getting the result as string   t    template Must template New  email   Parse emailTmpl   buf     amp bytes Buffer   if err    t Execute buf  data   err    nil       panic err    s    buf String     This will result in the expected output   Hi Bob   Your account is ready  your user name is  bob92  You have the following roles assigned  dbteam  uiteam  tester   Try it on the Go Playground   Also note that since Go 1 10  a newer  faster  more specialized alternative is available to bytes Buffer which is  strings Builder  Usage is very similar   builder     amp strings Builder   if err    t Execute builder  data   err    nil       panic err    s    builder String     Try this one on the Go Playground   Note  you may also display the result of a template execution if you provide os Stdout as the target  which also implements io Writer    t    template Must template New  email   Parse emailTmpl   if err    t Execute os Stdout  data   err    nil       panic err      This will write the result directly to os Stdout  Try this on the Go Playground

User · Answer

We can custom A new String type via define new Type with Format support   package main  import        fmt       text template       strings     type String string func  s String  Format data map string interface     out string  err error        t    template Must template New     Parse string s        builder     amp strings Builder       if err   t Execute builder  data   err    nil           return           out   builder String       return     func main         const tmpl    Hi    Name       range  i   r     Roles    if  i      end         end        data    map string interface             Name        Bob            Roles        string  dbteam    uiteam    tester               s      String tmpl  Format data      fmt Println s

User · Answer

Sprintf is what you are looking for   Example  fmt Sprintf  foo   s   bar    You can also see it in use in the Errors example as part of  A Tour of Go    return fmt Sprintf  at  v   s   e When  e What

User · Answer

In your case  you need to use Sprintf   for format string   func Sprintf format string  a    interface    string  Sprintf formats according to a format specifier and returns the resulting string   s    fmt Sprintf  Good Morning  This is  s and I m living here from last  d years     John   20   Your output will be       Good Morning  This is John and I m living here from last 20 years

User · Answer

fmt SprintF function returns a string and you can format the string the very same way you would have with fmt PrintF

User · Answer

I ve created go project for string formatting from template  it allow to format strings in C  or Python style  just first version for very simple cases   you could find it here https   github com Wissance stringFormatter it works in following manner   func TestStrFormat t  testing T        strFormatResult  err    Format  quot Hello i am  0   my age is  1  and i am waiting for  2   because i am  0   quot                                  quot Michael Ushakov  Evillord666  quot    quot 34 quot    quot   quot Great Success  quot  quot       assert Nil t  err      assert Equal t   quot Hello i am Michael Ushakov  Evillord666   my age is 34 and i am waiting for   quot Great Success  quot   because i am Michael Ushakov  Evillord666   quot   strFormatResult       strFormatResult  err   Format  quot We are wondering if these values would be replaced    5    4    0  quot    quot one quot    quot two quot    quot three quot       assert Nil t  err      assert Equal t   quot We are wondering if these values would be replaced    5    4   one quot   strFormatResult       strFormatResult  err   Format  quot No args        0    1    2  quot       assert Nil t  err      assert Equal t   quot No args        0    1    2  quot   strFormatResult     func TestStrFormatComplex t  testing T        strFormatResult  err    FormatComplex  quot Hello  user  what are you doing here  app    quot   map string string  quot user quot   quot vpupkin quot    quot app quot   quot mn console quot        assert Nil t  err      assert Equal t   quot Hello vpupkin what are you doing here mn console   quot   strFormatResult

[string] Format a Go string without printing?

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