Healty iteration of string in C

This is a discussion on Healty iteration of string in C within the C++ Programming forums, part of the General Programming Boards category; I have a function in which I iterate through strings. It seem fairly ok, but sometimes I run into segmentation ...

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    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?

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    Registered User rogster001's Avatar
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    std::string has iterator types of various flavours which you might explore. for your example of every fourth character you might consider using modulus of i
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    and that would be i%4 in C++?

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    The superhaterodyne twomers's Avatar
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    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.

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

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    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)

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    Registered User whiteflags's Avatar
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    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.

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    Just kidding.... fnoyan's Avatar
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    Quote Originally Posted by überfuzz View Post
    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

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    Algorithm Dissector iMalc's Avatar
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    I was going to say, you haven't shown the code where the real problem is.
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    Quote Originally Posted by whiteflags View Post
    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.

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    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.

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    Just kidding.... fnoyan's Avatar
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    Quote Originally Posted by überfuzz View Post
    .....
    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.

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    Quote Originally Posted by überfuzz View Post
    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.


    Quote Originally Posted by EVOEx View Post
    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.

  14. #14
    Algorithm Dissector iMalc's Avatar
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    The cast and the less-than comparison with the iterator looks dodgy to me. I'd remove the cast and use !=
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