Read text file into string array and write

Question

The ability to read  and write  a text file into and out of a string array is I believe a fairly common requirement  It is also quite useful when starting with a language removing the need initially to access a database  Does one exist in Golang  e g    func ReadLines sFileName string  iMinLines int     string  bool      and     func WriteLines saBuff  string  sFilename string   bool       I would prefer to use an existing one rather than duplicate

User · Answer

func readToDisplayUsingFile1 f  os File       defer f Close       reader    bufio NewReader f      contents       ioutil ReadAll reader      lines    strings Split string contents     n       or   func readToDisplayUsingFile1 f  os File       defer f Close       slice    make   string 0       reader    bufio NewReader f       for       str  err    reader ReadString   n       if err    io EOF          break                slice   append slice  str

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If you don t care about loading the file into memory  as of Go 1 16  you can use the os ReadFile and bytes Count functions  package main  import        quot log quot       quot os quot       quot bytes quot     func main         data  err    os ReadFile  quot input txt quot       if err    nil           log Fatal err             n    bytes Count data    byte   n         fmt Printf  quot input txt has  d lines n quot   n

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As of Go1 1 release  there is a bufio Scanner API that can easily read lines from a file   Consider the following example from above  rewritten with Scanner   package main  import        bufio       fmt       log       os        readLines reads a whole file into memory    and returns a slice of its lines  func readLines path string     string  error        file  err    os Open path      if err    nil           return nil  err           defer file Close        var lines   string     scanner    bufio NewScanner file      for scanner Scan             lines   append lines  scanner Text              return lines  scanner Err         writeLines writes the lines to the given file  func writeLines lines   string  path string  error       file  err    os Create path      if err    nil           return err           defer file Close        w    bufio NewWriter file      for    line    range lines           fmt Fprintln w  line            return w Flush      func main         lines  err    readLines  foo in txt       if err    nil           log Fatalf  readLines   s   err            for i  line    range lines           fmt Println i  line             if err    writeLines lines   foo out txt    err    nil           log Fatalf  writeLines   s   err

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Note  ioutil is deprecated as of Go 1 16  If the file isn t too large  this can be done with the ioutil ReadFile and strings Split functions like so  content  err    ioutil ReadFile filename  if err    nil         Do something   lines    strings Split string content    quot  n quot    You can read the documentation on ioutil and strings packages

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Cannot update first answer  Anyway  after Go1 release  there are some breaking changes  so I updated as shown below   package main  import        os       bufio       bytes       io       fmt       strings        Read a whole file into the memory and store it as array of lines func readLines path string   lines   string  err error        var           file  os File         part   byte         prefix bool           if file  err   os Open path   err    nil           return           defer file Close        reader    bufio NewReader file      buffer    bytes NewBuffer make   byte  0       for           if part  prefix  err   reader ReadLine    err    nil               break                   buffer Write part          if  prefix               lines   append lines  buffer String                buffer Reset                       if err    io EOF           err   nil           return    func writeLines lines   string  path string   err error        var           file  os File            if file  err   os Create path   err    nil           return           defer file Close          writer    bufio NewWriter file      for   item    range lines             fmt Println item             err    file WriteString strings TrimSpace item      n               file Write   byte item             if err    nil                 fmt Println  debug               fmt Println err              break                       content    strings Join lines    n          err   writer WriteString content        return    func main         lines  err    readLines  foo txt       if err    nil           fmt Println  Error   s n   err          return           for    line    range lines           fmt Println line              array      string  7 0    8 5    9 1       err   writeLines lines   foo2 txt       fmt Println err

User · Answer

You can use os File  which implements the io Reader interface  with the bufio package for that  However  those packages are build with fixed memory usage in mind  no matter how large the file is  and are quite fast   Unfortunately this makes reading the whole file into the memory a bit more complicated  You can use a bytes Buffer to join the parts of the line if they exceed the line limit  Anyway  I recommend you to try to use the line reader directly in your project  especially if do not know how large the text file is    But if the file is small  the following example might be sufficient for you   package main  import        os       bufio       bytes       fmt        Read a whole file into the memory and store it as array of lines func readLines path string   lines   string  err os Error        var           file  os File         part   byte         prefix bool           if file  err   os Open path   err    nil           return           reader    bufio NewReader file      buffer    bytes NewBuffer make   byte  1024       for           if part  prefix  err   reader ReadLine    err    nil               break                   buffer Write part          if  prefix               lines   append lines  buffer String                buffer Reset                       if err    os EOF           err   nil           return    func main         lines  err    readLines  foo txt       if err    nil           fmt Println  Error   s n   err          return           for    line    range lines           fmt Println line            Another alternative might be to use io ioutil ReadAll to read in the complete file at once and do the slicing by line afterwards  I don t give you an explicit example of how to write the lines back to the file  but that s basically an os Create   followed by a loop similar to that one in the example  see main

[go] Read text file into string array (and write)

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