Programming Forums > Application Development > C

nnxion · Apr 11th, 2006, 6:21 AM

I would like to read and write to one file, that can be done by setting the mode to rb+ in fopen.

What I want to achieve is putting a space before every line.
I thought it would work like:

(Toggle Plain Text)

#include <stdio.h>

main(int argc, char *argv[])
{
	FILE *fp;
	void insertspaces(FILE *, FILE *);

	argc = 2;
	argv[1] = "C:\\main.cpp";

	if (argc == 1)
	{
		printf("insertspaces: no file(s) specified\n", *argv);
		return 1;
	}

	else
		while(--argc > 0)

			if ((fp = fopen(*++argv, "rb+")) == NULL)
			{
				printf("insertspaces: can't open %s\n", *argv);
				return 1;
			}

			else
			{
				insertspaces(fp, fp);
				fclose(fp);
			}

	return 0;
}

/* insertspaces: insert spaces in file ifp to file ofp */
void insertspaces(FILE *ifp, FILE *ofp)
{
	int c;
	putc(' ', ofp);
	while ((c = getc(ifp)) != EOF)
	{
		putc(c, ofp);
		if(c == '\n')
			putc(' ', ofp);
	}
}

I think that were I'm going wrong is not reading the file fully but writing to the file while I'm still reading. Is it not possible? Or do I need to do things differently?

mackenga · Apr 11th, 2006, 8:02 AM

I find this slightly bizarre, but I've never actually tried to do this before. I believe the problem is that the characters you're writing to the file are overwriting the first couple of bytes of the next line rather than being inserted before them. Like I say, I've never really tried to insert data into a file in this way; I'd generally write this sort of program as a filter and have it read standard input and write the data with space inserted (or whatever processing done) to standard output.

I suspect to insert the data in place you'd have to do a more torturous sort of job, moving the data forward before writing spaces where it had been. This sounds like agony to me, which is probably why I prefer the filter approach.

Hope this helps (or that someone else has something more helpful to volunteer!)

DaWei · Apr 11th, 2006, 8:22 AM

I didn't look at your code, Ruben, just some comments. Since you aren't dealing with the medium at a low level, when you use the C/C++ libraries, you should view your file as if it were mag tape: one long string of bytes laid end to end. If you wish to make something longer than it currently is, as an in-place modification, you have to read ahead to get data out of the way so you can add bytes where it was. We discussed this half to death on That Other Forum. It can be done, but there's no point unless you're pushed for storage resources. Read, modify, write to temp file. When done rename temp file as original file. If you want to do it purely as a learning exercise, think fifo and bookeeping. You'll need seek and tell. That means you'll need to operate in binary mode if you're on Windows.

jim mcnamara · Apr 11th, 2006, 6:10 PM

This does not write to a single file but uses files already open by C.

(Toggle Plain Text)

/* unix quick version qk.c*/
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
	char tmp[4096]={0x0};
	while( fgets(tmp, sizeof(tmp), stdin)!=NULL)
	    fprintf(stdout, " %s", tmp);
	return 0;    
}

usage:
./qk < oldfile > newfile

This also works in DOS

Apr 11th, 2006, 6:21 AM	#1
nnxion Programming Guru Join Date: Jun 2005 Location: elemental plane Posts: 1,429 Rep Power: 5	Read and write to one file I would like to read and write to one file, that can be done by setting the mode to rb+ in fopen. What I want to achieve is putting a space before every line. I thought it would work like: (Toggle Plain Text) #include <stdio.h> main(int argc, char argv[]) { FILE fp; void insertspaces(FILE , FILE ); argc = 2; argv[1] = "C:\\main.cpp"; if (argc == 1) { printf("insertspaces: no file(s) specified\n", argv); return 1; } else while(--argc > 0) if ((fp = fopen(++argv, "rb+")) == NULL) { printf("insertspaces: can't open %s\n", argv); return 1; } else { insertspaces(fp, fp); fclose(fp); } return 0; } / insertspaces: insert spaces in file ifp to file ofp / void insertspaces(FILE ifp, FILE ofp) { int c; putc(' ', ofp); while ((c = getc(ifp)) != EOF) { putc(c, ofp); if(c == '\n') putc(' ', ofp); } } #include <stdio.h> main(int argc, char argv[]) { FILE fp; void insertspaces(FILE , FILE ); argc = 2; argv[1] = "C:\\main.cpp"; if (argc == 1) { printf("insertspaces: no file(s) specified\n", argv); return 1; } else while(--argc > 0) if ((fp = fopen(++argv, "rb+")) == NULL) { printf("insertspaces: can't open %s\n", argv); return 1; } else { insertspaces(fp, fp); fclose(fp); } return 0; } /* insertspaces: insert spaces in file ifp to file ofp / void insertspaces(FILE ifp, FILE *ofp) { int c; putc(' ', ofp); while ((c = getc(ifp)) != EOF) { putc(c, ofp); if(c == '\n') putc(' ', ofp); } } I think that were I'm going wrong is not reading the file fully but writing to the file while I'm still reading. Is it not possible? Or do I need to do things differently? __________________ "Employ your time in improving yourself by other men's writings, so that you shall gain easily what others have labored hard for." -- Socrates

Apr 11th, 2006, 8:02 AM	#2
mackenga Professional Programmer Join Date: Mar 2005 Location: Glasgow, Scotland Posts: 321 Rep Power: 4	I find this slightly bizarre, but I've never actually tried to do this before. I believe the problem is that the characters you're writing to the file are overwriting the first couple of bytes of the next line rather than being inserted before them. Like I say, I've never really tried to insert data into a file in this way; I'd generally write this sort of program as a filter and have it read standard input and write the data with space inserted (or whatever processing done) to standard output. I suspect to insert the data in place you'd have to do a more torturous sort of job, moving the data forward before writing spaces where it had been. This sounds like agony to me, which is probably why I prefer the filter approach. Hope this helps (or that someone else has something more helpful to volunteer!) __________________ "I'm not a genius. Why do I have to suffer?"

Apr 11th, 2006, 8:22 AM	#3
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	I didn't look at your code, Ruben, just some comments. Since you aren't dealing with the medium at a low level, when you use the C/C++ libraries, you should view your file as if it were mag tape: one long string of bytes laid end to end. If you wish to make something longer than it currently is, as an in-place modification, you have to read ahead to get data out of the way so you can add bytes where it was. We discussed this half to death on That Other Forum. It can be done, but there's no point unless you're pushed for storage resources. Read, modify, write to temp file. When done rename temp file as original file. If you want to do it purely as a learning exercise, think fifo and bookeeping. You'll need seek and tell. That means you'll need to operate in binary mode if you're on Windows. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Apr 11th, 2006, 6:10 PM	#4
jim mcnamara Hobbyist Programmer Join Date: Jun 2005 Location: New Mexico Posts: 228 Rep Power: 4	This does not write to a single file but uses files already open by C. (Toggle Plain Text) /* unix quick version qk.c/ #include <stdio.h> #include <stdlib.h> int main(int argc, char argv[]) { char tmp[4096]={0x0}; while( fgets(tmp, sizeof(tmp), stdin)!=NULL) fprintf(stdout, " %s", tmp); return 0; } /* unix quick version qk.c/ #include <stdio.h> #include <stdlib.h> int main(int argc, char argv[]) { char tmp[4096]={0x0}; while( fgets(tmp, sizeof(tmp), stdin)!=NULL) fprintf(stdout, " %s", tmp); return 0; } usage: ./qk < oldfile > newfile This also works in DOS

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Programming Forums > Application Development > C

Read and write to one file