problem appending

Near the top of my code I have

Code:

---

ifstream accfile;

---

And I have been using accfile to open, read and close files several times in the code. However, I am not receiving an error with the following code

Code:

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          else
            {
              accfile.open ("loans.txt");
              accfile << name << ";";
            }
        }
      cout << "enter the ID number of the book." << endl;
      string book;
      accfile << book << endl;
      accfile.close();
      break;
    case 3:

---

My errors (line 179 - accfile << name << ";";)

testfile.cc:179: no match for `ifstream & << string &'
/usr/local/depot/gcc-2.95.3/lib/gcc-lib/sparc-sun-solaris2.8/2.95.3/../../../../include/g++-3/std/bastring.cc:469: candidates are: class ostream & operator <<<char, string_char_traits<char>, __default_alloc_template<false,0> >(ostream &, const basic_string<char,string_char_traits<char>,__defau lt_alloc_template<false,0> > &)
testfile.cc:184: no match for `ifstream & << string &'
/usr/local/depot/gcc-2.95.3/lib/gcc-lib/sparc-sun-solaris2.8/2.95.3/../../../../include/g++-3/std/bastring.cc:469: candidates are: class ostream & operator <<<char, string_char_traits<char>, __default_alloc_template<false,0> >(ostream &, const basic_string<char,string_char_traits<char>,__defau lt_alloc_template<false,0> > &)
testfile.cc:186: break statement not within loop or switch
testfile.cc:187: case label `3' not within a switch statement
testfile.cc:183: warning: destructor needed for `class string book'
testfile.cc:187: warning: where case label appears here
testfile.cc:187: warning: (enclose actions of previous case statements requiring
testfile.cc:187: warning: destructors in their own binding contours.)
testfile.cc:187: confused by earlier errors, bailing out

I suggest that you post the smallest and simplest program that demonstrates the problem. The snippet you gave is a little too incomplete.

But shouldn't this (below) work on it's own anyway?

Code:

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ifstream accfile;
accfile.open ("loans.txt");
string book;
cin >> book;
accfile << book << endl;
accfile.close();

---

Sorry, I had forgotten to write cin >> book before, but I still get the same error.

The reason I'm trying not to put large amounts of code here is because this code is for a coursework and I'm not sure how the system they use works, but I don't want to have to explain afterwards that I haven't plagerised anything.

Quote:

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Originally Posted by Furious5k

But shouldn't this (below) work on it's own anyway?

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Yes, but in your case it is not on its own.

Quote:

---

Originally Posted by Furious5k

The reason I'm trying not to put large amounts of code here is because this code is for a coursework and I'm not sure how the system they use works, but I don't want to have to explain afterwards that I haven't plagerised anything.

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That's another reason why your should narrow down the code.

Read the compiler errors:

Quote:

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testfile.cc:186: break statement not within loop or switch
testfile.cc:187: case label `3' not within a switch statement

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These two errors clearly say that the switch has ended earlier. Check your braces.

Quote:

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string book;

---

If this were in a switch case, the block using book would need to have braces around it (you can't declare variables within a switch unless they are in braces, since the scope of a variable declared in one case would be otherwise till the end of the switch, meaning that execution might reach a place where a variable is in scope, yet jump over the declaration / constructor call).

Would this do?

Code:

---

  int select;
  cin >> select;

  switch (select)
    {
    case 1:
      //code here
      break;
    case 2:
      cout << "message here." << endl;
      string name;
      cin >> name;

      int loancount;
      for (int i = 0; i < (BOOK_MAX+1); ++i)
        {
          if (name == Loans[i].User)
          {
            ++loancount;
          }
        }

          if (loancount > 3)
            {
              cout << "message here." << endl;
            }
          else
            {
              accfile.open ("loans.txt");
              accfile << name << ";";
            }
        }
      cout << "message here." << endl;
      string book;
      cin >> book;
      accfile << book << endl;
      accfile.close();
      break;
    case 3:
      //code here
      break;

---

Indent your code properly, e.g.,

Code:

---

int select;
cin >> select;

switch (select)
{
case 1:
    //code here
    break;
case 2:
    cout << "message here." << endl;
    string name;
    cin >> name;

    int loancount;
    for (int i = 0; i < (BOOK_MAX+1); ++i)
    {
        if (name == Loans[i].User)
        {
            ++loancount;
        }
    }

    if (loancount > 3)
    {
        cout << "message here." << endl;
    }
    else
    {
        accfile.open ("loans.txt");
        accfile << name << ";";
    }
}
cout << "message here." << endl;
string book;
cin >> book;
accfile << book << endl;
accfile.close();
break;
case 3:
//code here
break;

---

Of course, anon has spoilt my fun, but you can see that anon is correct.

Code:

---

        }
      cout << "message here." << endl;

---

Which brace does that match?

You also need an extra set of braces around case 2, since you declare variables in this case.

You're trying to write to an input file... that won't work ;)

Quote:

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Originally Posted by EVOEx

You're trying to write to an input file... that won't work

---

Oops, you're right.

Quote:

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Originally Posted by anon

If this were in a switch case, the block using book would need to have braces around it (you can't declare variables within a switch unless they are in braces, since the scope of a variable declared in one case would be otherwise till the end of the switch, meaning that execution might reach a place where a variable is in scope, yet jump over the declaration / constructor call).

---

the last 11 words just went *whoosh* over my head.
Using any kind of braces ( or { or [ around "string book;" gave me an undeclared error.

I changed the brace to correctly end switch statement.

Quote:

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Originally Posted by EVOEx

You're trying to write to an input file... that won't work ;)

---

Thank you!

To get an output file I should add this line?

Code:

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ofstream accfile

---

Quote:

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Originally Posted by Furious5k

Using any kind of braces ( or { or [ around "string book;" gave me an undeclared error.

---

As a simplified example, try to compile this:

Code:

---

int main()
{
    int x = 10;
    switch (x)
    {
    case 1:
        int y = 20;
        x = y;
        break;
    case 2:
        int z = 30;
        x = z * z;
        break;
    };
}

---

A correct version would be:

Code:

---

int main()
{
    int x = 10;
    switch (x)
    {
    case 1:
        {
            int y = 20;
            x = y;
        }
        break;
    case 2:
        {
            int z = 30;
            x = z * z;
        }
        break;
    };
}

---

Quote:

---

Originally Posted by Furious5k

To get an output file I should add this line?

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Not add, but change.

Yeah, I realised what he meant after the second reply. First time around I read "block using book would need to have braces around it" as "book would need to have braces around it".

Poor choice of words, I meant to say "use" this line.

And I'm not indenting anything myself, emacs is doing it.

Now I just get a single last error (accfile << book << endl;).
Is this because I have "ofstream accfile" and "accfile.open" inside an if/else statement and then trying to output to a file outside of that statement?

Code:

---

          else
            {
              ofstream accfile;
              accfile.open ("loans.txt");
              accfile << name << ";";
            }
      cout << "enter the ID number of the book." << endl;
      string book;
      cin >> book;
      accfile << book << endl;
      accfile.close();
      break;
      }

---

testfile.cc:186: no match for `ifstream & << string &'
/usr/local/depot/gcc-2.95.3/lib/gcc-lib/sparc-sun-solaris2.8/2.95.3/../../../../include/g++-3/std/bastring.cc:469: candidates are: class ostream & operator <<<char, string_char_traits<char>, __default_alloc_template<false,0> >(ostream &, const basic_string<char,string_char_traits<char>,__defau lt_alloc_template<false,0> > &)

Quote:

---

Originally Posted by Furious5k

Is this because I have "ofstream accfile" and "accfile.open" inside an if/else statement and then trying to output to a file outside of that statement?

---

Yes, now it looks like a problem of scope. If you only use accfile within that case, then declare it within that block. However, have you considered separating your code into functions?

It would appear, based on the error message, that you have accfile defined at the outer scope as being an ifstream, and you still can't write to an ifstream.

Anything that is declared after an opening brace has a lifetime that lasts until the corresponding closing brace and then goes "out of scope". So you cannot use accfile outside the scope where it was declared.

All the errors have been sorted.

I am now having a problem with only the latest pieces of data that I output being saved in the file. Whatever else was in there vanishes. From examples that I've looked at, it should be working.

Am I ouputting to the file wrong? How do I correctly append the file?

Quote:

---

Originally Posted by Furious5k

I am now having a problem with only the latest pieces of data that I output being saved in the file. Whatever else was in there vanishes. From examples that I've looked at, it should be working.

Am I ouputting to the file wrong? How do I correctly append the file?

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You should open the file with std::ios::app as the mode flag.

Code:

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#include <iostream>
using namespace std;

---

If I'm using those, I don't need to write the "std::ios::" part?

Do I write this?

Code:

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accfile.setf(app);

---

Quote:

---

Originally Posted by Furious5k

If I'm using those, I don't need to write the "std::ios::" part?

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You would still need to qualify with ios.

Quote:

---

Originally Posted by Furious5k

Do I write this?

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More like:

Code:

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ofstream accfile(filename, ios::app);

---

or:

Code:

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accfile.open(filename, ios::app);

---

Could anyone point me in the right direction, I'm struggling to figure out/find example of how I can erase a line of text from a txt file?

You can't. You can copy the file or the parts you want to keep.

Alright, how would I go about doing that?

Would I just read everything into an array and then write it back skipping the unwanted bits? The only thing I found in tutorials is "ios::trunc", would I write the following before writing everything (minus unwanted bits) to the file to get an updated version?

Code:

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accfile.open(filename, ios::trunc, ios::app);

---

You do not want ios::trunc, ios::app - you either want to append, or you want to truncate (which makes the file zero length) - and if you want to combine such attributes, they should be combined with | not a comma.

Yes, to remove a something in a file, you have two choices:

• Read all the file into memory (an array or some such), create a new file with the same name [do not crash at this point, or all data is lost!], write back out the bits you actually want to keep [of course, you can "not store them in memory" if that is easy to determine when you are reading the file into memory].
• Create a temporary file, read from the input file, write the data (except the bits you want to remove) to the temporary file. Remove the old file, then rename the temporary file to the original name.

You can obviously do the first variant by writing the new data to a temporary file, then removing the original and renaming the temporary file, which avoids the "all is lost if you crash here".

--
Mats

Is the code for creating/removing/renaming files difficult? (working on a 10" netbook during holidays frustrated me into giving up until I got home, so I'm a bit tight on the deadlines now)

Quote:

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Originally Posted by Furious5k

Is the code for creating/removing/renaming files difficult? (working on a 10" netbook during holidays frustrated me into giving up until I got home, so I'm a bit tight on the deadlines now)

---

Look up the furnctions remove() and rename() - they are not very hard to use.

--
Mats

Code:

---

      ofstream accfile;
      accfile.open ("loans.txt", ios::trunc | ios::app);

      string book;
      cin >> book;

      for (int i = 0; i < (BOOK_MAX+1); i++)
        {
          if (book == Loans[i].Book)
            {
              accfile << Loans[i].User << ";" << Loans[i].Book;
            }
        }
      accfile.close();

---

I'm getting a "Segmentation fault", which is odd since I'm outputting much the same as I did before. (I made a copy of the file for crashes, thanks for the pointer).

I agree with matsp: ios::trunc | ios::app does not make sense. In The C++ Standard Library: A Tutorial and Reference Josuttis states on page 632 that "other combinations not listed in the table, such as trunc|app, not allowed".

Quote:

---

Originally Posted by laserlight

I agree with matsp: ios::trunc | ios::app does not make sense. In The C++ Standard Library: A Tutorial and Reference Josuttis states on page 632 that "other combinations not listed in the table, such as trunc|app, not allowed".

---

But he said:

Quote:

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and if you want to combine such attributes, they should be combined with | not a comma.

---

Yes, if you want to combine such attributes (e.g., ios::in | ios::out), but you cannot combine those two.

I tried to seperate them, like so, but it still didn't work (same segmentation fault).

Code:

---

      ofstream accfile;
      accfile.open ("loans.txt", ios::trunc);
      accfile.close();

      accfile.open ("loans.txt", ios::app);

---

Quote:

---

Originally Posted by Furious5k

But he said:

---

Yes, I said that - but I also said "Do not combine app with trunc" as you are at the same time saying "truncate the file" and "append to the file" - which makes absolutely no sense - and what you actually get from it is completely undefined.

What I wanted to tell is that IF you have valid combinations that you want to pass in, you need to combine them using the bitwise or operator.

Did you actually test if accfile is valid before writing to it? Such as

Code:

---

if (!accFile) {
    cout << "Could not open the file... " << endl;
    return;
}

---

It may well be that the iostream implementation actually detects the bad combination and the opening of the file fails, perhaps?

I am also a bit suspicious about this one:

Code:

---

      for (int i = 0; i < (BOOK_MAX+1); i++)

---

Why is there a +1 there?

--
Mats

--
Mats

Quote:

---

Originally Posted by Furious5k

I tried to seperate them, like so, but it still didn't work (same segmentation fault).

---

The segmentation fault might have some other cause. For example, how many elements does Loans have?

Note that separating them the way you did just makes the first opening of the file have no net effect. You basically opened, did nothing, then closed.

Quote:

---

Originally Posted by laserlight

Note that separating them the way you did just makes the first opening of the file have no net effect. You basically opened, did nothing, then closed.

---

Well, in the sequence as described here, it truncates the file, so it does indeed have an effect - it removes the conent in the file. Why you would THEN append to the file, I don't really understand, as there is no content to append onto - so the second file opening is absolutely meaningless waste of time [as long as the close is removed]. When the file is truncated, it has a zero length, the write (put) pointer is at the beginning of the file, and the file is opened for write - so what does the open with ios::app actually do at this point?

--
Mats

matsp - due to my lack of experience. I think it should be -1. An array[5] would be 0 to 4, yes?

Also, sorry for misreading.

laserlight - I was under the impression that opening the file with ios::trunc would erase its contents. How do I do this properly?


On another note, would this sort a "string myaray[NUMBER]" alphabetically?

Code:

---

#include <algorithm>
sort(myarray, myarray+NUMBER)

---

Does renaming a temporary file make it permanent?

Quote:

---

Originally Posted by Furious5k

laserlight - I was under the impression that opening the file with ios::trunc would erase its contents. How do I do this properly?

---

What are you trying to do?

I have read the file into an array earlier on in the code.
Now I want to erase the contents and re-enter them from the array skipping the location in the array which holds the same content as the one entered by user

The alphabetical sort is for another part of the code. I probably shouldn't have posted it here, sorry for the confusion.

Quote:

---

Originally Posted by Furious5k

matsp - due to my lack of experience. I think it should be -1. An array[5] would be 0 to 4, yes?

---

That is probably causing your segmentation fault then - and it probably should be

Code:

---

for (int i = 0; i < BOOK_MAX; i++)

---

No +1 or -1.

Quote:

---

Also, sorry for misreading.

laserlight - I was under the impression that opening the file with ios::trunc would erase its contents. How do I do this properly?

---

It does - but you do not then need to use ios::app on the file to add to it - since it is already empty.


On another note, would this sort a "string myaray[NUMBER]" alphabetically?

Code:

---

#include <algorithm>
sort(myarray, myarray+NUMBER)

---

[/QUOTE]

I suppose so.

--
Mats

Quote:

---

Originally Posted by Furious5k

Does renaming a temporary file make it permanent?

---

No - all files are always permanent. Renaming a file changes it's name from one name to another. When I said temporary file, I meant something like "tmp.dat", and then rename it to "books.dat" or whatever the "real" name of the file is.

--
Mats

It's working now. But when I cat the contents of the file I get the content I want plus a great many ";". I know which part of the code it is.

How would I tell the code to check for null value (never done it)?

Code:

---

if (Loans[i].Book = NULL)

---

That's how I understand it.


With the temporary files, I was referring to another site I looked at, which defined "tmpfile" as "Open a temporary file". I guess it just creates a file then, which is (usually) used temporarily?

Quote:

---

Originally Posted by Furious5k

It's working now. But when I cat the contents of the file I get the content I want plus a great many ";". I know which part of the code it is.

How would I tell the code to check for null value (never done it)?

Code:

---

if (Loans[i].Book = NULL)

---

That's how I understand it.

---

That is incorrect, it is checking

Code:

---

if ((Loans[i].Book = NULL) != 0)

---

, and since NULL == 0, it will never be true.

--
Mats

...so you need == instead of =

Nevermind, thank you C_ntua.

But I now receive an error " no match for `string & == int' "

Quote:

---

Originally Posted by Furious5k

Nevermind, thank you C_ntua.

But I now receive an error " no match for `string & == int' "

---

Perhaps because you are trying to compare a string with NULL? Which is not the right thing to do, I'd say. Perhaps comparing with "" is better?

--
Mats

Thank you.

Quote:

---

Originally Posted by Furious5k

Code:

---

if (Loans[i].Book == 0)

---

doesn't work either. I'm stumped as to how I can check if a string array has an empty location.

---

Yes, that's EXACTLY the same as == 0, as NULL is a macro that expands to 0.

See my previous post for a suggestion.

--
Mats