Help with reading from a file.

raptor1770 · 07-02-2009, 04:02 PM

Hello,

I need to prompt the user for a filename and then open and read from that file. Also I need to be able to take the contents of the file I'm reading from and store them in a vector or stack.

This is the code that I have so far but I don't know if this is the most efficient way to do it or why it doesn't seem to be working.

Code:

        string file_name;

	cout<<"What is the name of the maze input file?"<<endl;
	getline(cin, file_name);

	char c_file[20];

	int i = 0;

	while(file_name[i]!=0) //I convert the string into an array of chars because 
                                            //the open function only accepts char arrays as a  
	{                                  //paremeter
		c_file[i]=file_name[i];
		i++;
	}

	fstream read_file;

	read_file.open(c_file);

The file to be opened is a simple text file containing a series of integers in this fashion.

Code:

From here I can't figure out how to actually pull information out of the file efficiently.
Thank you for any help.

tabstop · 07-02-2009, 04:05 PM

If you have a string, you can use .c_str() to get a C-style string out of it without having to do the conversions yourself.

You can use >> with files just as you can with anything else. (You'll need to be a little bit careful here; when you say a series of integers, do you mean you want to read "888818888" as one number, or as a sequence of characters? It will make a difference, both how you read and how you store.)

raptor1770 · 07-02-2009, 04:14 PM

Quote:

Originally Posted by tabstop

If you have a string, you can use .c_str() to get a C-style string out of it without having to do the conversions yourself.

You can use >> with files just as you can with anything else. (You'll need to be a little bit careful here; when you say a series of integers, do you mean you want to read "888818888" as one number, or as a sequence of characters? It will make a difference, both how you read and how you store.)

Thank you for the prompt response.

What I meant is that I need to pull the integers out one at a time so that I can store them individually in a two dimensional array or vector. Imagine that the integers in the file will become a two dimensional grid.

So the value of [0][0] == 9
[0][-1] == 8
[1][-1] == 8 and so on.

tabstop · 07-02-2009, 04:19 PM

Quote:

Originally Posted by raptor1770

Thank you for the prompt response.

What I meant is that I need to pull the integers out one at a time so that I can store them individually in a two dimensional array or vector. Imagine that the integers in the file will become a two dimensional grid.

So the value of [0][0] == 9
[0][-1] == 8
[1][-1] == 8 and so on.

So that means you need to not be reading integers, but instead be reading characters. (In other words, if your maze suddenly was made out of . and X and S and F, that wouldn't change how you read it in, right?) So you need an two-dimensional array (or vector of vectors I suppose) of characters.

(Note also that most people consider the number after "0" to be "1", not "-1".)

raptor1770 · 07-02-2009, 04:26 PM

Quote:

Originally Posted by tabstop

So that means you need to not be reading integers, but instead be reading characters. (In other words, if your maze suddenly was made out of . and X and S and F, that wouldn't change how you read it in, right?) So you need an two-dimensional array (or vector of vectors I suppose) of characters.

Ok, I understand that thank you. So to read characters from the file I should just read into a two dimensional array of characters somewhat like this?

Code:

char_array[0][0] << file_name;

In some sort of loop?

Quote:

(Note also that most people consider the number after "0" to be "1", not "-1".)

Lol I know but its just easier for me to think of the two dimensional array as a grid with X and Y coordinates with its origin at the first character. So moving down a line is moving -1 in the Y coordinate and moving to the right is +1 in the X coordinate.

Thanks again for your help.

tabstop · 07-02-2009, 04:31 PM

Quote:

Originally Posted by raptor1770

Ok, I understand that thank you. So to read characters from the file I should just read into a two dimensional array of characters somewhat like this?

Code:

char_array[0][0] << file_name;

In some sort of loop?

The file is always first, whether it's in or out.

raptor1770 · 07-02-2009, 04:33 PM

I just realized that instead of

Code:

char_array[0][0] << file_name;

it should be

Code:

file_name>>char_array[0][0];

and it seems to be working just fine.

Thank you so much for your help, this saved me tons of time.

brewbuck · 07-02-2009, 06:02 PM

Quote:

Originally Posted by tabstop

If you have a string, you can use .c_str() to get a C-style string out of it without having to do the conversions yourself.

The manual conversion isn't even correct, because nowhere does it say that std::string is null-terminated (although in practice it probably is, in order to make the .c_str() function efficient)

07-02-2009, 04:02 PM	#1
raptor1770 Registered User Join Date: Nov 2008 Posts: 12	Help with reading from a file. Hello, I need to prompt the user for a filename and then open and read from that file. Also I need to be able to take the contents of the file I'm reading from and store them in a vector or stack. This is the code that I have so far but I don't know if this is the most efficient way to do it or why it doesn't seem to be working. Code: string file_name; cout<<"What is the name of the maze input file?"<<endl; getline(cin, file_name); char c_file[20]; int i = 0; while(file_name[i]!=0) //I convert the string into an array of chars because //the open function only accepts char arrays as a { //paremeter c_file[i]=file_name[i]; i++; } fstream read_file; read_file.open(c_file); The file to be opened is a simple text file containing a series of integers in this fashion. Code: 9 890000088 888888088 888888088 800088088 808008088 808000088 808800888 000008888 888818888 From here I can't figure out how to actually pull information out of the file efficiently. Thank you for any help.
raptor1770 is offline

07-02-2009, 04:05 PM	#2
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	If you have a string, you can use .c_str() to get a C-style string out of it without having to do the conversions yourself. You can use >> with files just as you can with anything else. (You'll need to be a little bit careful here; when you say a series of integers, do you mean you want to read "888818888" as one number, or as a sequence of characters? It will make a difference, both how you read and how you store.)
tabstop is offline

07-02-2009, 04:33 PM	#7
raptor1770 Registered User Join Date: Nov 2008 Posts: 12	I just realized that instead of Code: char_array[0][0] << file_name; it should be Code: file_name>>char_array[0][0]; and it seems to be working just fine. Thank you so much for your help, this saved me tons of time.
raptor1770 is offline

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