Writing and reading from a file sequentially using the same file object

Question

I am learning data file handling basics in c++ (and am working in the compiler turbo C++). So I wanted to create a text file , write some data onto it and then read it. So I wrote this: -

int main()
{
fstream fin;
fin.open("textfile.txt",ios::in|ios::out);
for(int i=0;i<3;i++)
{
char x;
cin>>x;
fin<<x;
}
fin.seekg(0,ios::beg); //I added this and also tried seekp() when I didn't get the desired output
                   //but to no use 
while(!fin.eof())
{
char v;
fin>>v;
cout<<v;
}
fin.close();
getch();
return 0;
}

But instead of outputting only the 3 characters which I input, it outputs 4 characters. I tried removing the loops and taking input and giving outputs one by one like this (among other things):

...
char x,y,z;
cin>>x>>y>>z;
fin<<x<<y<<z; 
fin.seekg(0,ios::beg);
char q,w,e;
fin>>q>>w>>e;
cout<<q<<w<<e;
...

But it still didn't work.

I think it has something to do with file pointers and their location but don''t know what. I tried finding a similar question on the net but to no avail.

So I want to know what is wrong with what I did and how to I improve this to actually write and read in a file sequentially using the same file object (if it is even possible). And is seekg() even necessary here?

Thanks.

`while(!fin.eof())` https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong — drescherjm, Jan 27 '18 at 15:59
Off-topic: your file input variable may want to be `finout` since it is both used for input and output. — Thomas Matthews, Jan 27 '18 at 16:06
Actually, you want to use meaningful names, such as "test_data_file" or "experiment_io_stream". Remember to code so that you'll be able to understand it after not looking at it for 6 months (in which you forget how your program operated or what it should do). — Thomas Matthews, Jan 27 '18 at 16:16
Your input should be: `while (fin >> n)`. Follow the link from drescherjm. — Thomas Matthews, Jan 27 '18 at 16:17
@drescherjm, so is that the only wrong thing I did? And was the seekg() actually necessary? — HeWhoMustBeNamed, Jan 27 '18 at 16:24
@Thomas, I did now but now the first character is not being displayed. — HeWhoMustBeNamed, Jan 27 '18 at 17:03
@ThomasMatthews, can you tell me whether seekg() is actually required here? Can't we use seekp() instead? Also I just used fin.get(n) with while(!fin.eof()) and it only displays 3 characters! Can you tell me why? — HeWhoMustBeNamed, Jan 27 '18 at 17:38
As an aside: your teachers are doing you quite a disservice by sticking to Turbo-anything. There are better development environments running in a browser these days than Turbo-C++. — Kuba hasn't forgotten Monica, Jun 29 '18 at 05:51

score 0 · Answer 1 · edited Jun 29 '18 at 05:27

The problem you face is a general problem. Your input code is right and there is no error in that. The problem is your ouput code and to be more specific the line while(!fin.eof()). eof(end-of-file) works on a end of file mark whose numeric value is generally -1. But this function goes false only when the end character is encountered and traversed. To remove this error just replace this statement with a read statement that is move this line fin>>v from loop statements to the conditional statements. In this false will be when it encounters a end character.

Writing and reading from a file sequentially using the same file object

1 Answers1