"file_in.getline", compiler error?

Hey guys, thanks in advance for the help.

I'm fairly new to C++, so the code should look very simple to the guru crowd. When I try to compile the short program below, I get the error that "struct std:: ofstream has no member function 'getline'". I've checked around a bit and this line is used fine elsewhere, as well as in the book I'm learning from. Here's the code, with the problem area bolded:

Code:

---

#include<iostream>
#include<fstream>
using namespace std;

int main() {
    int c,i;
    char filename[81], input_line[101];

    cout << "Enter a filename and press enter: ";          //prompt for filename
    cin.getline(filename, 80);
    ofstream file_in(filename);

    if (!file_in) {                                            //avoiding errors
        cout << "File " << filename << "could not be opened.";
        return -1;
    }

    while (1) {                                              //prints five lines of the selected file at a time
        for (i = 1; i < 5 && !file_in.eof(); i++) {
            file_in.getline(input_line, 100);
            cout << input_line << "\n";
        }

        if (file_in.eof()) {                                  //exits if end of file is reached
            break;
        }

        cout << "More? (Press 'Q' and ENTER to quit): "; //ask user if they want more printed
        cin.getline(input_line, 80);
        c = input_line[0];
        if (c == 'Q' || c == 'q') {
            break;
        }
    }

    cin.get();
    return 0;
}

---

You want to read from file_in -> make it an ifstream.
Kurt

I suggest you use std::getline instead of ifstream.getline and use std::string instead of char.

Heh, I knew it would be a small stupid error like that. Thanks.

I also see you're using eof() as condition in the header for the loop. That's also bad. For details, you can see the FAQ. It's better to check if the read succeeds or not.

Thanks Elysia. As I was reading over the code a last time before posting, I thought of that line in your sig when I went over the eof() part, heh.

The original problem referred to by this thread was fixed, and since then a new problem arose.

Going along with my book, one of the exercises is to make the code that I posted in the first post print the designated file in all lowercase letters (the file reads AAAA\n BBBB\n CCCC\n etc). The problem code that I have now compiles fine, but when run, it ignores the requested number of lines to be printed and instead prints one line every time (the change from capital to lowercase works fine). Here's the code:

Code:

---

#include<iostream>
#include<fstream>
#include<stdlib.h>
#include<ctype.h>
using namespace std;

int main() {
    int c, i, lines;
    char filename[81], input_line[101];

    cout << "Enter a filename and press enter: ";
    cin.getline(filename, 80);
    ifstream file_in(filename);

    if (!file_in) {
        cout << "File " << filename << " could not be opened.";
        return -1;
    }

    while (1) {

        cout << "Enter number of lines to display, ";
        cout << "or press 'Q' and ENTER to quit: ";
        cin.getline(input_line, 80);
        lines = atoi(input_line);
        c = input_line[0];
        if (c == 'Q' || c == 'q') {
            break;
        }

        for (i = 0; i < lines; i++) {
            file_in.getline(input_line, 100);

            for (i = 0; i < (strlen(input_line)); i++) {
                input_line[i] = tolower(input_line[i]);
            }

            cout << input_line << endl;
        }

        if (file_in.eof()) {
            cout << "The end of the file has been reached.";
            break;
        }
    }

    cin.get();
    return 0;
}

---

Thanks in advance.

Code:

---

    for (i = 0; i < lines; i++) {
        file_in.getline(input_line, 100);

        for (i = 0; i < (strlen(input_line)); i++) {

---

Wonder what messes up loop counter?

Slap me hard. Please.

Thanks lol..

May I also point out that strlen is a bit expensive function to call a lot (it has to loop over the entire array to determine the length). In fact you needn't find out the length at all and simply loop converting characters to lowercase until you get to the null-terminator of input-line.

Dare I say it will be optimized by the compiler in release mode?
Then again, maybe not. I suppose the compiler cannot assume that strlen returns the same value each time.
But anyway, unless time is critical, it's not really necessary. So we could just say it's good practice.

Just for reference, how would I go about making it so strlen is only called once (seeing as the line of text that strlen is measuring is called within the 'for' loop) ?

Store it in a variable and use the variable in the loop condition.

Quote:

---

Dare I say it will be optimized by the compiler in release mode?

---

I just tried it with gcc 4.1 some time ago.

It only gets optimized on -O3, not even -O2.

To convert every character in a C-string to lowercase, the loop might look like this:

Code:

---

for (int i = 0; str[i] != '\0'; ++i) {
    str[i] = tolower(str[i]);
}

---

Of course, you might use pointer arithmetics instead of indexed access. However there is no need to call strlen for that at all. Both strlen and this loop work the same way: keep looping until the current character is '\0'.

-----------
And one more note on coding style. It might be wiser in C++ to declare variables at the smallest scope you are using them. In case of for-loops the loop counter can be declared as part of the for statement (see example above).

As to your mistake of using the same variable i for two nested loops - had you declared the two i's in the for statements a) the compiler could have warned you about problems with variable names, and b) if you ignored the warnings the code would have still done the right thing. (A variable declared in a nested scope shadows the variable of the same name in the outer scope.)

Quote:

---

Originally Posted by cyberfish

I just tried it with gcc 4.1 some time ago.

It only gets optimized on -O3, not even -O2.

---

I was unable to get it optimized away at all. I'm thinking the compiler doesn't know anything about strlen and thus cannot assume that it will return the same thing everytime you call it.

Quote:

---

I was unable to get it optimized away at all. I'm thinking the compiler doesn't know anything about strlen and thus cannot assume that it will return the same thing everytime you call it.

---

It only gets optimized away on the highest optimization level on gcc. I am guessing perhaps it's because strlen is inlined in -O3 (-O3 turns on heuristic function inlining). I have never examined the asm output, however.

Same for Visual Studio. The function was inlined, but not optimized away. I guess gcc was the better optimizer.