How do I read a specified line in a text file

Question

Given a text file  how would I go about reading an arbitrary line and nothing else in the file   Say  I have a file test txt   How would I go about reading line number 15 in the file   All I ve seen is stuff involving storing the entire text file as a String array and then using the value of the line number as the number of the String to use from the array    but there are some complications  The text file is enormously huge and the machine that the application I m coding isn t exactly a top-notch system  Speed isn t the top priority  but it is definitely a major issue   Are there any ways to ONLY read a specific line of a text file and store the result as a string   Thanks for your responses  The file is KINDA structured  It s got 25 lines of info and then X lines of numbers but line 17 of the first 25 has the value of X   But then  there s 1 blank line and it repeats itself all over as a second record in the file and X can have a different value for each record   What I want to do is read and store the first 25 lines as independent values and then store the next X  usually around 250  lines as an array  Then I m going to store it in an SQL database and repeat with the NEXT record until I reach the Yth record  the number of records in the file is in line 3   EDIT 2  Alright  I think I ve gotten to a solution based on a combination of your alls  responses   I m going to read the first 25 lines and store it as an array  I ll copy the pertinent contents of the array to local variables then I ll delete the first 25 lines  Then  I can use the info to store the next X lines  the value of item 13 in the array  as an array  serialize it  store it in a database then delete the lines that I just read   I could then repeat the process for each subsequent record   Of course  this relies on one assumption I m making  which to be honest  I m not sure is true  Is it possible to delete the first n lines from a text file from within C  without having to read the entire thing and re-write it without the first n lines

User · Answer

No unfortunately there is not   At the raw level files do not work on a line number basis   Instead they work at a position   offset basis   The root filesystem has no concept of lines   It s a concept added by higher level components   So there is no way to tell the operating system  please open file at line blah   Instead you have to open the file and skip around counting new lines until you ve passed the specified number   Then store the next set of bytes into an array until you hit the next new line

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While you can t seek an N th line directly in a non-symmetrical file without reading data in the file  because you need to count how many lines you have progressed into the file  you can count line breaks until you get to the line you want which takes the least amount of memory and probably has the best performance   This is going to be more memory efficient than reading everything to an array  since it will only read into the file until it hits the end of the file or the line number  whichever comes first   It is far from perfect  but will probably suit your needs   string line15   ReadLine   C  File csv   15    public string ReadLine string FilePath  int LineNumber       string result           try      if  File Exists FilePath             using  StreamReader  StreamReader   new StreamReader FilePath            for  int a   0  a  lt  LineNumber  a                  result    StreamReader ReadLine                                   catch       return result

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You could read line by line so you don t have to read the entire all at once  probably at all   int i 0 while  stream eof    amp  amp  i  lineNum      stream readLine       i   line   stream readLine

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Tried and tested  It s as simple as follows   string line   File ReadLines filePath  ElementAt actualLineNumber - 1     As long as you have a text file  this should work  Later  depending upon what data you expect to read  you can cast the string accordingly and use it

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Late Answer but worth it    You need to load the lines in array or list object where each line will be assign to an index  then simply call any range of lines by their index in for loop    Solution is pretty Good  but there is a memory consumption in between    give it try    Its worth it

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If your file contains lines with different lengths and you need to read lines often and you need to read it quickly you can make an index of the file by reading it once  saving position of each new line and then when you need to read a line  you just lookup the position of the line in your index  seek there and then you read the line   If you add new lines to the file you can just add index of new lines and you don t need to reindex it all  Though if your file changes somewhere in a line you have already indexed then you have to reindex

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A variation   Produces an error if line number is greater than number of lines   string GetLine string fileName  int lineNum        using  StreamReader sr   new StreamReader fileName                 string line          int count   1          while   line   sr ReadLine       null                        if count    lineNum                                return line                            count                        return  line number is bigger than number of lines

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NET 4 0 edit  Since  NET 4 0  it is possible to access a single line of a file directly  For instance  to access line 15   string line   File ReadLines FileName  Skip 14  Take 1  First      This will return only the line required    Since you can t predict the location  can you   of the i-th line in the file  you ll have to read all previous lines too  If the line number is small  this can be more efficient than the ReadAllLines method   string GetLine string fileName  int line       using  var sr   new StreamReader fileName            for  int i   1  i  lt  line  i              sr ReadLine           return sr ReadLine

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if  File Exists fpath                          var data   File ReadLines fpath               Console WriteLine data ToArray   14

User · Answer

If each line is a fixed length then you can open a Stream around it  seek  bytes per line    n into the file and read your line from there    using  Stream stream   File Open fileName  FileMode Open          stream Seek bytesPerLine    myLine - 1   SeekOrigin Begin       using  StreamReader reader   new StreamReader stream                  string line   reader ReadLine              Alternatively you could just use the StreamReader to read lines until you found the one you wanted  That way s slower but still an improvement over reading every single line   using  Stream stream   File Open fileName  FileMode Open          using  StreamReader reader   new StreamReader fileStream                  string line   null          for  int i   0  i  lt  myLineNumber    i                         line   reader ReadLine

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As Mehrdad said  you cannot just seek to the n-th line without reading the file  However  you don t need to store the entire file in memory - just discard the data you don t need   string line  using  StreamReader sr   new StreamReader path       for  int i   0  i lt 15  i                 line   sr ReadLine           if  line  null  break     there are less than 15 lines in the file

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Read five lines each time  just put your statement in if statement   thats it           String str1     C  Users TEMP Desktop StaN txt               System IO StreamReader file   new System IO StreamReader str1            line   file ReadLine             Int32 ctn 0           try                        while   line   file ReadLine       null                                     if  Counter    ctn                                                MessageBox Show  I am here                            ctn ctn 5                          continue                                            else                                               Counter                              MessageBox Show Counter ToString                             MessageBox Show line ToString                                                           file Close                      catch  Exception er

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If the lines are all of a fixed length you can use the Seek method of a stream to move to the correct starting positiion     If the lines are of a variable length your options are more limited     If this is a file you will be only using once and then discarding  then you are best off reading it in and working with it in memeory   If this is a file you will keeping and will be reading from more than writing to  you can create a custom index file that contains the starting positions of each line   Then use that index to get your Seek position   The process of creating the index file is resource intensive   Everytime you add a new line to the file you will need to update the index  so maintenance becomes a non-trivial issue

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Unless you have fixed sized lines  you need to read every line until you reach the line you want   Although  you don t need to store each line  just discard it if it s not the line you desire   Edit   As mentioned  it would also be possible to seek in the file if the line lengths were predictable -- that is to say you could apply some deterministic function to transform a line number into a file position

[c#] How do I read a specified line in a text file?

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