C Program to see if two files are the same

The Crab · 10-11-2011 3:09pm #1

Hi, another problem. This is a program to see if two files are the same or different. It compiles but regardless of whether the files are the same or different, it says the files are different (i.e. if two files contain the same information). Am I completely off beam, or is it something simple that my program is missing?

As before, I'm really grateful for any help.

#include<stdio.h> 
int main()
{
	FILE *fp;
	int letter;
	int num_asterix1=0;
	int num_asterix2=0;
	int num_quotes=0;
	char fname[80];
	char fname1[80];
	printf("Please enter the name of the first file you wish to check: ");
	gets(fname);
	printf("Please enter the name of the second file you wish to check: ");
	gets(fname1);
	fp=fopen (fname, "r");
	fp=fopen (fname1, "r");
		if(strcmp(fname, fname1)==0)
		printf("The files <%s> and <%s> are identical.", fname, fname1);
		else
		printf("The files <%s> and <%s> are different.", fname, fname1);
	
return 0;
}

Webmonkey · 10-11-2011 3:15pm

Now at a complete guess...I think I remember something about gets leaving the \n (newline) character in the input buffer. So the next time you use it, you'll end up getting that \n.

If you enter:
file1.txt
file1.txt

fname will be "file1.txt"
fname1 will be "\nfile1.txt"

Again this is a guess. In the output do you see anything strange when you print them. Is there a sudden line break in the middle of your sentence. "The files <%s> and <%s> are different."

yupyup7up · 10-11-2011 3:17pm

The Crab wrote: »
Hi, another problem. This is a program to see if two files are the same or different. It compiles but regardless of whether the files are the same or different, it says the files are different (i.e. if two files contain the same information). Am I completely off beam, or is it something simple that my program is missing?

As before, I'm really grateful for any help.
#include<stdio.h> 
int main()
{
	FILE *fp;
	int letter;
	int num_asterix1=0;
	int num_asterix2=0;
	int num_quotes=0;
	char fname[80];
	char fname1[80];
	printf("Please enter the name of the first file you wish to check: ");
	gets(fname);
	printf("Please enter the name of the second file you wish to check: ");
	gets(fname1);
	fp=fopen (fname, "r");
	fp=fopen (fname1, "r");
		if(strcmp(fname, fname1)==0)
		printf("The files <%s> and <%s> are identical.", fname, fname1);
		else
		printf("The files <%s> and <%s> are different.", fname, fname1);
	
return 0;
}

Haven't done C in a long time, but looks to me like you are comparing the filenames as opposed to the actual stream from the fopen. Also, you are assigning fp to both fopen calls so that it is just being overwritten.

You should have FILE *fp, *fp1; I reckon and compare fp and fp1 instead.

As I said I'm awfully rusty with C

carveone · 10-11-2011 3:18pm

Well. You've successfully compared the names of the two files not their contents.

Webmonkey · 10-11-2011 3:18pm

yupyup7up wrote: »

Haven't done C in a long time, but looks to me like you are comparing the filenames as opposed to the actual stream from the fopen. Also, you are assigning fp to both fopen calls so that it is just being overwritten.

You should have FILE *fp, *fp1; I reckon and compare fp and fp1 instead.

As I said I'm awfully rusty with C

I was thinking this also but assumed it was actually the filenames he was comparing?

Edit: Okay i read the complete post so yeah it appears he does want to compare the contents, not the names.

The Crab · 10-11-2011 3:19pm

Webmonkey wrote: »

Now at a complete guess...I think I remember something about gets leaving the \n (newline) character in the input buffer. So the next time you use it, you'll end up getting that \n.

If you enter:
file1.txt
file1.txt

fname will be "file1.txt"
fname1 will be "\nfile1.txt"

Again this is a guess. In the output do you see anything strange when you print them. Is there a sudden line break in the middle of your sentence. "The files <%s> and <%s> are different."

No, nothing strange. If I put in exactly the same file names then it says they are identical.

I wonder is the problem strcmp - is that comparing the names of the files rather than the files themselves?

Webmonkey · 10-11-2011 3:20pm

The Crab wrote: »

No, nothing strange. If I put in exactly the same file names then it says they are identical.

I wonder is the problem strcmp - is that comparing the names of the files rather than the files themselves?

It is most definitely. Think about the other thread you posted earlier today about reading the contents of the file, you'll need something along those lines.
You'll have to do something in parallel and once something isn't the same, you'll have proven the files arn't the same.

It's not much more work tbh.

The Crab · 10-11-2011 3:21pm

Forget the last message I put (about it not being strcmp that's the problem), I made a mistake as per that.

Hmm, I'm a little confused now. Is it that I need a while loop comparing the inner of the file?

carveone · 10-11-2011 3:21pm

The Crab wrote: »

I wonder is the problem strcmp - is that comparing the names of the files rather than the files themselves?

Yup. You pretty much have to rethink the whole thing

Open each file. In binary if you are using Windows. Read a block from each file, compare them, repeat until end of block.

Unless there's a CompareFileContents call... which probably isn't the point of the homework!

Webmonkey · 10-11-2011 3:22pm

The Crab wrote: »

Something really weird is happening. If I change the if condition to NOT EQUAL (at the moment it is at EQUAL) then the same problem happens in reverse, i.e. it says two completely different files are identical.

At least that says the problem isn't strcmp comparing the two file names.

Your whole logic is wrong. You are not doing what you think you are doing. It's got nothing to do with strcmp. You shouldn't even need it!

Webmonkey · 10-11-2011 3:23pm

carveone wrote: »

Yup. You pretty much have to rethink the whole thing

Open each file. In binary if you are using Windows. Read a block from each file, compare them, repeat until end of block.

Unless there's a CompareFileContents call... which probably isn't the point of the homework!

They also had an assignment to read character by character each file so it's assumed text based so just simple character comparisons is all that's needed i'd say. OP re think the whole thing

yupyup7up · 10-11-2011 3:24pm

Yep, as said above, check the file stream character by character and exit if the character at the same index in both files doesnt match! Simples

The Crab · 10-11-2011 3:30pm

Webmonkey wrote: »

They also had an assignment to read character by character each file so it's assumed text based so just simple character comparisons is all that's needed i'd say. OP re think the whole thing

Would a while loop through the whole text like I did before help? Then put the if and else inside that?

C bamboozles me. Don't know why I found java easier

.

The Crab · 10-11-2011 3:31pm

I'm a bit confused over how to create a while loop which compares two files at the same time.

Webmonkey · 10-11-2011 3:31pm

Something along those lines. Read both files in parallel and then use your ifs on the characters you read from both streams. Remember the fgetc. If you get to the end of file in both cases at the same time without a character being wrong, then you have proven the file is equal. If any other case, if the EOF comes earlier in any file or if the characters don't match at any stage then the file is not equal.

The Crab · 10-11-2011 3:32pm

carveone wrote: »

Yup. You pretty much have to rethink the whole thing

Does that mean I should delete the file I currently have and start from scratch?

carveone · 10-11-2011 3:43pm

The Crab wrote: »

Does that mean I should delete the file I currently have and start from scratch?

Ah no. Just the last few lines. I'm thinking I'll let you think for a second but I'll drop some hints in a sec... (doing two things at once!)

carveone · 10-11-2011 3:48pm

Webmonkey wrote: »

Something along those lines. Read both files in parallel and then use your ifs on the characters you read from both streams. Remember the fgetc. If you get to the end of file in both cases at the same time without a character being wrong, then you have proven the file is equal. If any other case, if the EOF comes earlier in any file or if the characters don't match at any stage then the file is not equal.

That's it exactly. fgetc returns an "int" - so it's a character (0 to 255) or an EOF for end of file. EOF is -1.

open file1
open file2

int c1, c2;

do {
c1 = fgetc from file1
c2 = fgetc from file2

if (c1 != c2)
{
done -> files are not the same.
}

} while ((c1 != EOF) && (c2 != EOF))

done -> files are the same.

Something like that. Note that if they reach the end of the file at the same time c1 will equal c2 and that's ok.

The Crab · 10-11-2011 3:51pm

Thanks guys! You're a huge help! I have half this week's assignment done now and have tomorrow to play around with the rest, so expect me to be back annoying you tomorrow haha!

Webmonkey · 10-11-2011 3:57pm

Yep don't forget to do an additional check after to ensure that both are at EOF, otherwise one file ended earlier than the other and they are not even. It could happen that you get to the end of one file while the contents of both being the same up till then. So the loop will exit but you won't have got a chance to check the characters.

Continuing from above after the loop ends do another check

...
if (c1 != c2)
 // not the same.

Edit: Ignore the above. Day dreaming

carveone · 10-11-2011 3:59pm

Sure thing. C is pretty low level. Wait till you hit pointers. Muahahaha...

carveone · 10-11-2011 4:03pm

Webmonkey wrote: »

Yep don't forget to do an additional check after to ensure that both are at EOF, otherwise one file ended earlier than the other and they are not even.

Ah! But then c1 wouldn't be equal to c2 and the file wouldn't be same. Which is why I did a do {} while.

Where I said "done -> files are not the same" I did intend that the loop should be broken at that point, either using exit or break;

If you use break, you'll have to flag what happened. So you'd set flag_different = 0 before the loop and then go:

if (c1 != c2)
{
    flag_different = 1;
    break;
}

Webmonkey · 10-11-2011 4:05pm

Damn it sorry! I'm multi tasking all day here and seem to be making a lot of mistakes on boards today.

You are indeed right. Ignore my last post!

The Crab · 10-11-2011 4:09pm

carveone wrote: »

Sure thing. C is pretty low level. Wait till you hit pointers. Muahahaha...

What does it mean when people say low level and high level? Is it how java has loads of methods which makes it easier to do things (I've noticed even at this early stage that Java seems, although I could be completely wrong to have methods which you can call which does half the work for you???)

carveone · 10-11-2011 4:27pm

The Crab wrote: »

What does it mean when people say low level and high level? Is it how java has loads of methods which makes it easier to do things (I've noticed even at this early stage that Java seems, although I could be completely wrong to have methods which you can call which does half the work for you???)

Pretty much yes. The lowest level is assembly language which is what a compiler outputs. From there you have more and more abstraction from the machine. The advantage is less dependency on the machine and (hopefully) less work for the developer. The disadvantage is speed and memory usage.

C was created in the early 1970s for the PDP minicomputer - it's quite a low level language!

The Crab · 10-11-2011 9:48pm

Thanks again! That's good to know about java and C, that's why I'm finding java easier then.

C Program to see if two files are the same

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