Read file line by line using ifstream in C

Question

The contents of file txt are  5 3 6 4 7 1 10 5 11 6 12 3 12 4  Where 5 3 is a coordinate pair  How do I process this data line by line in C    I am able to get the first line  but how do I get the next line of the file  ifstream myfile  myfile open   quot file txt quot

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This is a general solution to loading data into a C   program  and uses the readline function  This could be modified for CSV files  but the delimiter is a space here    int n   5  p   2   int X n  p    ifstream myfile   myfile open  data txt     string line  string temp       int a   0     row index   while  getline myfile  line       while there is a line      int b   0     column index      for  int i   0  i  lt  line size    i         for each character in rowstring           if   isblank line i         if it is not blank  do this               string d 1  line i       convert character to string               temp append d      append the two strings           else                 X a  b    stod temp       convert string to double               temp          reset the capture               b       increment b cause we have a new number                    X a  b    stod temp     temp         a       onto next row

User · Answer

Expanding on the accepted answer  if the input is   1 NYC 2 ABQ       you will still be able to apply the same logic  like this    include  lt fstream gt   std  ifstream infile  thefile txt    if  infile is open          int number      std  string str      char c      while  infile  gt  gt  number  gt  gt  c  gt  gt  str  amp  amp  c                 std  cout  lt  lt  number  lt  lt       lt  lt  str  lt  lt    n     infile close

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Use ifstream to read data from a file   std  ifstream input   filename ext       If you really need to read line by line  then do this   for  std  string line  getline  input  line               for each line in input        But you probably just need to extract coordinate pairs   int x  y  input  gt  gt  x  gt  gt  y    Update   In your code you use ofstream myfile   however the o in ofstream stands for output  If you want to read from the file  input  use ifstream  If you want to both read and write use fstream

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First  make an ifstream    include  lt fstream gt  std  ifstream infile  thefile txt      The two standard methods are    Assume that every line consists of two numbers and read token by token   int a  b  while  infile  gt  gt  a  gt  gt  b           process pair  a b     Line-based parsing  using string streams    include  lt sstream gt   include  lt string gt   std  string line  while  std  getline infile  line         std  istringstream iss line       int a  b      if    iss  gt  gt  a  gt  gt  b     break       error         process pair  a b       You shouldn t mix  1  and  2   since the token-based parsing doesn t gobble up newlines  so you may end up with spurious empty lines if you use getline   after token-based extraction got you to the end of a line already

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Although there is no need to close the file manually but it is good idea to do so if the scope of the file variable is bigger       ifstream infile szFilePath        for  string line       getline infile  line                     do something with the line            if infile is open            infile close

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Reading a file line by line in C   can be done in some different ways    Fast  Loop with std  getline    The simplest approach is to open an std  ifstream and loop using std  getline   calls  The code is clean and easy to understand    include  lt fstream gt   std  ifstream file FILENAME   if  file is open          std  string line      while  std  getline file  line                using printf   in all tests for consistency         printf   s   line c str               file close         Fast  Use Boost s file description source  Another possibility is to use the Boost library  but the code gets a bit more verbose  The performance is quite similar to the code above  Loop with std  getline       include  lt boost iostreams device file descriptor hpp gt   include  lt boost iostreams stream hpp gt   include  lt fcntl h gt   namespace io   boost  iostreams   void readLineByLineBoost         int fdr   open FILENAME  O RDONLY       if  fdr  gt   0            io  file descriptor source fdDevice fdr  io  file descriptor flags  close handle           io  stream  lt io  file descriptor source gt  in fdDevice           if  fdDevice is open                  std  string line              while  std  getline in  line                        using printf   in all tests for consistency                 printf   s   line c str                               fdDevice close                         Fastest  Use C code  If performance is critical for your software  you may consider using the C language  This code can be 4-5 times faster than the C   versions above  see benchmark below  FILE  fp   fopen FILENAME   r    if  fp    NULL      exit EXIT FAILURE    char  line   NULL  size t len   0  while   getline  amp line   amp len  fp      -1           using printf   in all tests for consistency     printf   s   line     fclose fp   if  line      free line     Benchmark -- Which one is faster   I have done some performance benchmarks with the code above and the results are interesting  I have tested the code with ASCII files that contain 100 000 lines  1 000 000 lines and 10 000 000 lines of text  Each line of text contains 10 words in average  The program is compiled with -O3 optimization and its output is forwarded to  dev null in order to remove the logging time variable from the measurement  Last  but not least  each piece of code logs each line with the printf   function for consistency   The results show the time  in ms  that each piece of code took to read the files   The performance difference between the two C   approaches is minimal and shouldn t make any difference in practice  The performance of the C code is what makes the benchmark impressive and can be a game changer in terms of speed                                10K lines     100K lines     1000K lines Loop with std  getline           105ms          894ms          9773ms Boost code                       106ms          968ms          9561ms C code                            23ms          243ms          2397ms

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This answer is for visual studio 2017 and if you want to read from text file which location is relative to your compiled console application   first put your textfile  test txt in this case  into your solution folder  After compiling keep text file in same folder with applicationName exe  C  Users  username  source repos  solutionName   solutionName    include  lt iostream gt   include  lt fstream gt   using namespace std  int main         ifstream inFile         open the file stream     inFile open     test txt           check if opening a file failed     if  inFile fail              cerr  lt  lt   Error opeing a file   lt  lt  endl          inFile close            exit 1             string line      while  getline inFile  line                 cout  lt  lt  line  lt  lt  endl               close the file stream     inFile close

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Since your coordinates belong together as pairs  why not write a struct for them   struct CoordinatePair       int x      int y       Then you can write an overloaded extraction operator for istreams   std  istream amp  operator gt  gt  std  istream amp  is  CoordinatePair amp  coordinates        is  gt  gt  coordinates x  gt  gt  coordinates y       return is      And then you can read a file of coordinates straight into a vector like this    include  lt fstream gt   include  lt iterator gt   include  lt vector gt   int main         char filename      coordinates txt       std  vector lt CoordinatePair gt  v      std  ifstream ifs filename       if  ifs            std  copy std  istream iterator lt CoordinatePair gt  ifs                    std  istream iterator lt CoordinatePair gt                     std  back inserter v              else           std  cerr  lt  lt   Couldn t open    lt  lt  filename  lt  lt    for reading n                Now you can work with the contents of v

[c++] Read file line by line using ifstream in C++

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