Reading from text file until EOF repeats last line

Question

The following C   code uses a ifstream object to read integers from a text file  which has one number per line  until it hits EOF  Why does it read the integer on the last line twice  How to fix this   Code    include  lt iostream gt   include  lt fstream gt  using namespace std   int main         ifstream iFile  input txt          input txt has integers  one per line      while   iFile eof                  int x          iFile  gt  gt  x          cerr  lt  lt  x  lt  lt  endl             return 0      input txt   10   20   30   Output   10   20   30   30   Note  I ve skipped all error checking code to keep the code snippet small  The above behaviour is seen on Windows  Visual C     cygwin  gcc  and Linux  gcc

User · Answer

The EOF pattern needs a prime read to  bootstrap  the EOF checking process   Consider the empty file will not initially have its EOF set until the first read   The prime read will catch the EOF in this instance and properly skip the loop completely   What you need to remember here is that you don t get the EOF until the first attempt to read past the available data of the file   Reading the exact amount of data will not flag the EOF   I should point out if the file was empty your given code would have printed since the EOF will have prevented a value from being set to x on entry into the loop    0   So add a prime read and move the loop s read to the end   int x   iFile  gt  gt  x     prime read here while   iFile eof          cerr  lt  lt  x  lt  lt  endl      iFile  gt  gt  x

User · Answer

At the end of the last line  you have a new line character  which is not read by    operator and it is not an end of file  Please  make an experiment and delete the new line  thelast character in file  - you will not get the duplication  To have a flexible code and avoid unwanted effects just apply any solution given by other users

User · Answer

Just follow closely the chain of events    Grab 10 Grab 20 Grab 30 Grab EOF   Look at the second-to-last iteration   You grabbed 30  then carried on to check for EOF   You haven t reached EOF because the EOF mark hasn t been read yet   binarically  speaking  its conceptual location is just after the 30 line    Therefore you carry on to the next iteration   x is still 30 from previous iteration   Now you read from the stream and you get EOF   x remains 30 and the ios  eofbit is raised   You output to stderr x  which is 30  just like in the previous iteration    Next you check for EOF in the loop condition  and this time you re out of the loop   Try this   while  true        int x      iFile  gt  gt  x      if  iFile eof     break      cerr  lt  lt  x  lt  lt  endl      By the way  there is another bug in your code   Did you ever try to run it on an empty file   The behaviour you get is for the exact same reason

User · Answer

There s an alternative approach to this    include  lt iterator gt   include  lt algorithm gt               copy istream iterator lt int gt  iFile   istream iterator lt int gt              ostream iterator lt int gt  cerr    n

User · Answer

int x  ifile  gt  gt  x  while   iFile eof            cerr  lt  lt  x  lt  lt  endl              iFile  gt  gt  x

User · Answer

Without to much modifications of the original code  it could become    while   iFile eof            int x      iFile  gt  gt  x      if   iFile eof    break      cerr  lt  lt  x  lt  lt  endl      but I prefer the two other solutions above in general

User · Answer

I like this example  which for now  leaves out the check which you could add inside the while block   ifstream iFile  input txt              input txt has integers  one per line int x   while  iFile  gt  gt  x         cerr  lt  lt  x  lt  lt  endl      Not sure how safe it is

[c++] Reading from text file until EOF repeats last line

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