Thread: Healty iteration of string in C

I have a function in which I iterate through strings. It seem fairly ok, but sometimes I run into segmentation fault. Is there a way of iterating in C++ that's safer?..

Code:

for(int i = offset; i < (str.size()/4); i = i+4){
..		// do stuff at every fourth character, index(4*i);
..	..}

Or is there a better way of indexing i?

and that would be i%4 in C++?

Code:

for(int i = offset; i < (str.size()/4); i = i+4)

Why are you dividing str.size() by 4? Removing that division will make it work as expected.

Code:

for( int i=offset; i<str.size(); i+=4 )

You can replace i = i+4 by i+=4, as a shorter hand notation which accomplishes the same thing, if you want.

Thanks for the input! It turns out I have, or had, some gnarly elsewhere.

Imagine a three character string, "abc".
i = 0, so i < str.size() (= 3). The loop body will execute and assume there are 4 characters, reading past the end of the buffer. Instead, you'll want:

Code:

for(int i = offset; i < str.size() - 3; i += 4)

Or something similar. But even with this you have to be careful: str.size() is unsigned, to str.size() - 3 may underflow if str.size() < 3. So either catch that exception, or use something along the lines of:

Code:

for(int i = offset; i + 3 < str.size(); i += 4)

Imagine a three character string, "abc".
i = 0, so i < str.size() (= 3). The loop body will execute and assume there are 4 characters, reading past the end of the buffer. Instead, you'll want:

No problems here, the normal progression of the loop would be
i = 0, i < 3, repeat.
i = 4, i < 3, stop.
Even if there were a situation where i = 3, 3 is not < 3 so the loop stops anyway.

Originally Posted by überfuzz

I have a function in which I iterate through strings. It seem fairly ok, but sometimes I run into segmentation fault. Is there a way of iterating in C++ that's safer?..

Code:

for(int i = offset; i < (str.size()/4); i = i+4){
..		// do stuff at every fourth character, index(4*i);
..	..}

Or is there a better way of indexing i?

As twomers indicated, you can use +4 instead of dividing the str.size() by 4. Make sure that you examine the length of string before your loop to eliminate the cases where str.size()<4.

What is the "stuff" you are doing in your loop. Are you sure that the SIGSEGV is because of the for loop or some other assignment/modification in the loop

Originally Posted by whiteflags

No problems here, the normal progression of the loop would be
i = 0, i < 3, repeat.
i = 4, i < 3, stop.
Even if there were a situation where i = 3, 3 is not < 3 so the loop stops anyway.

Ah, I misread the OP. I thought he acted upon groups of 4 characters, ie. in the first loop iteration read/write the character with indices 0 through 3. I see now that there is little reason to assume this is the case.

I read about iteration an it seems an iterator should do the trick. This is what I ended up with.

Code:

	string compstr;
	string::iterator it;
	int index = offset;

..for ( it = str.begin() ; it < str.end(); ++it){
		if (index%4 == 0){
..  		compstr += (char)map.find(str.substr(index,4))->second;
   	}
  		index++;
   }

	return compstr;

Don't hesitate to comment if I've got it wrong in some way.

As I wrote in my last post, there's still popping up segmentation fault. It seems to be some issue in a file to string function I use. If I hard code the string directly in my code everything is fine and dandy. But if I use my file2string function I sometimes get segfault.

Originally Posted by überfuzz

.....
As I wrote in my last post, there's still popping up segmentation fault. It seems to be some issue in a file to string function I use. If I hard code the string directly in my code everything is fine and dandy. But if I use my file2string function I sometimes get segfault.

Well, without having file2string code, all the commets will be nothing but a stab in the dark.

I would try running the code in a debugger and examine the code snippet where SIGSEGV occurs.

Originally Posted by überfuzz

As I wrote in my last post, there's still popping up segmentation fault. It seems to be some issue in a file to string function I use. If I hard code the string directly in my code everything is fine and dandy. But if I use my file2string function I sometimes get segfault.

One possibility is that map.find(...) returns map.end(). map.end()->second is not valid.

Originally Posted by EVOEx

Ah, I misread the OP. I thought he acted upon groups of 4 characters, ie. in the first loop iteration read/write the character with indices 0 through 3. I see now that there is little reason to assume this is the case.

Actually, I take that back. Apparently he *was* acting on 4 characters at a time. Though to be fair, not in any way that would overflow.