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  1. #1
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    [RESOLVED] Wrong output with pointers

    Hi everyone! I am trying to write a function that compares the original word with its reversed. My main problem, and I cannot figure out why, I am getting always a "false" return in a boolean function that compares the two words. I have checked for spaces or strange word composition but it seems all ok. Here is my code:

    Code:
    int len(const char* s)
    {
        int n = 0;
        while (*s != '\0')
        {
            ++n;
            ++s;
        }
        return n;
    }
    
    char* reverse_word(const char* c)
    {
        int n = len(c);
        //    char* p2 = new char[n + 1];
        //    char* temp = &p2[n];
        char* p2 = new char[n + 1](); // *** default initialize
        char* temp = &p2[n - 1]; // ***
        
        while(*c != '\0') *temp-- = *c++;
        
        delete[] p2;
        
        return p2;
    }
    
    bool is_palindrome(const char* c, const char* p)
    {
        if (p == c) return true;
        return false;
    }
    
    
    int main(int argc, const char * argv[])
    {
        char ar[] = {"kayak"};
        char* p = &ar[0];
        char* p1 = nullptr;
        p1 = reverse_word(p);
        cout << p << " and " << p1 << " are ";
        if (!is_palindrome(p,p1)) cout << "not ";
        cout << "palindromes." << endl;
        keep_window_open();
        return 0;
    }
    Thank you very much

  2. #2
    VictorN's Avatar
    VictorN is offline Super Moderator Power Poster
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    Re: Wrong output with pointers

    Well, if it is your homework and you have to use pointers then learn how to debug your code and debug it to understand what where and why goes wrong.
    Otherwise use STL of MFC library to make your life easier!
    Victor Nijegorodov

  3. #3
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    Re: Wrong output with pointers

    Yep it is an homework. I have tried to debug it, checking scope by scope what is going on inside the code. I always try to avoid posting for help because of "no pain no gain"! But in this case I am very puzzled and I do not know what I did wrong!

  4. #4
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    Talking Re: Wrong output with pointers

    I have solved changing my bool function in something more verbose but more specific:

    Code:
    bool is_palindrome(const char* c, const char* p)
    {
        int n = len(c);
        for (int i = 0; i < n; ++i)
        {
            if (c[i] != p[i]) return false;
        }
        
        return true;
    }
    Now it seems to work!

  5. #5
    VictorN's Avatar
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    Re: Wrong output with pointers

    Quote Originally Posted by leourb View Post
    Now it seems to work!
    Good!
    Now the question: why did you implement your own function
    Code:
    int len(const char* s)
    What was wrong with the standard C-Runtime strlen?
    Victor Nijegorodov

  6. #6
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    Re: [RESOLVED] Wrong output with pointers

    You are perfectly right sir and I agree with you! I have no rancors with std functions! As I partially mentioned it is an exercise in my C++ book, written by Bjarne Stroustrup, where he forbade us to use std functions to let us appreciate them more when we can use them! So I had the need to write an ugly-customized copy of that!

  7. #7
    VictorN's Avatar
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    Re: [RESOLVED] Wrong output with pointers

    OK. understood!
    Victor Nijegorodov

  8. #8
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    Re: [RESOLVED] Wrong output with pointers

    Thanks again to have let me think deeper about sir!

  9. #9
    2kaud's Avatar
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    Re: [RESOLVED] Wrong output with pointers

    Code:
    if (p == c) return true;
    you are comparing pointers (ie the memory addresses) which in this case will always be different - rather than the data to which they point. If you want to compare the data at these memory addresses, use strcmp(). See http://www.cplusplus.com/reference/cstring/

    Your program also has other problems - eg reverse_word you are freeing memory and then returning a pointer to the freed memory!

    Why write a function len() when there is the standard strlen() function? https://msdn.microsoft.com/en-us/library/78zh94ax.aspx

    Why use c-style null terminated strings when you are using c++? why not use the c++ string class?

    [Posted before seen previous posts].
    Last edited by 2kaud; August 16th, 2015 at 01:30 PM.
    All advice is offered in good faith only. All my code is tested (unless stated explicitly otherwise) with the latest version of Microsoft Visual Studio (using the supported features of the latest standard) and is offered as examples only - not as production quality. I cannot offer advice regarding any other c/c++ compiler/IDE or incompatibilities with VS. You are ultimately responsible for the effects of your programs and the integrity of the machines they run on. Anything I post, code snippets, advice, etc is licensed as Public Domain https://creativecommons.org/publicdomain/zero/1.0/ and can be used without reference or acknowledgement. Also note that I only provide advice and guidance via the forums - and not via private messages!

    C++20 Compiler: Microsoft VS2022 (17.0.5)

  10. #10
    2kaud's Avatar
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    Re: Wrong output with pointers

    Quote Originally Posted by leourb View Post
    I have solved changing my bool function in something more verbose but more specific:

    Code:
    bool is_palindrome(const char* c, const char* p)
    {
        int n = len(c);
        for (int i = 0; i < n; ++i)
        {
            if (c[i] != p[i]) return false;
        }
        
        return true;
    }
    Now it seems to work!
    This assumes that the length of the strings pointed to by c and p are the same (or at least string length p is not less than string length c).
    All advice is offered in good faith only. All my code is tested (unless stated explicitly otherwise) with the latest version of Microsoft Visual Studio (using the supported features of the latest standard) and is offered as examples only - not as production quality. I cannot offer advice regarding any other c/c++ compiler/IDE or incompatibilities with VS. You are ultimately responsible for the effects of your programs and the integrity of the machines they run on. Anything I post, code snippets, advice, etc is licensed as Public Domain https://creativecommons.org/publicdomain/zero/1.0/ and can be used without reference or acknowledgement. Also note that I only provide advice and guidance via the forums - and not via private messages!

    C++20 Compiler: Microsoft VS2022 (17.0.5)

  11. #11
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    Re: [RESOLVED] Wrong output with pointers

    Thanks for replying 2kaud! I have already implemented code with a check in length:
    Code:
        int n = len(c);
        int m = len(p);
        if (n != m) error("Lenght of the two pointers mismatch.");
    I do not use std functions because of it is an homework and I cannot use them (and I would too trust me )

    For the issue regarding p2 it works anyway but you are right, I made a mistake! I have fixed it

  12. #12
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    Re: [RESOLVED] Wrong output with pointers

    Your code is quite inefficient. len() iterates over the string and then you again iterate over the string. Also accessing via a pointer is more efficient than accessing via an index (timings with MSVS2013 confirm this - previously posted to this forum).

    Consider
    Code:
    #include <iostream>
    using namespace std;
    
    size_t len(const char* s)
    {
    const char *ss { s };
    
    	while (*ss++);
    	return ss - s - 1;
    }
    
    char* reverse_word(const char* c)
    {
    size_t n { len(c) };
    
    char *p2 { new char[n + 1]() };
    
    	for (char *temp = p2 + n - 1; *c; *temp-- = *c++);
    	return p2;
    }
    
    bool is_palindrome(const char* c, const char* p)
    {
    	while (*c && *p)
    		if (*c++ != *p++)
    			return false;
    
    	return (*c == *p);
    }
    
    int main(int argc, const char * argv[])
    {
    const char ar[] { "kayak" };
    char *p1 { reverse_word(ar) };
    
    	cout << ar << " and " << p1 << " are " << (is_palindrome(ar, p1) ? "" : "not ") << "palindromes." << endl;
    	delete [] p1;
    	return 0;
    }
    All advice is offered in good faith only. All my code is tested (unless stated explicitly otherwise) with the latest version of Microsoft Visual Studio (using the supported features of the latest standard) and is offered as examples only - not as production quality. I cannot offer advice regarding any other c/c++ compiler/IDE or incompatibilities with VS. You are ultimately responsible for the effects of your programs and the integrity of the machines they run on. Anything I post, code snippets, advice, etc is licensed as Public Domain https://creativecommons.org/publicdomain/zero/1.0/ and can be used without reference or acknowledgement. Also note that I only provide advice and guidance via the forums - and not via private messages!

    C++20 Compiler: Microsoft VS2022 (17.0.5)

  13. #13
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    Re: [RESOLVED] Wrong output with pointers

    You can also undertake a palindrome check without first having to reverse the word - and by only checking half of the string! Consider
    Code:
    bool is_palindrome(const char* p)
    {
    const char *e { p };
    
    	for (; *e; ++e);
    
    	for (const char *s = p, *m = p + (e - p) / 2; s != m; ++s)
    		if (*s != *--e)
    			return false;
    
    	return true;
    }
    
    int main(int argc, const char * argv[])
    {
    const char ar[] { "cabbac" };
    
    	cout << ar << " is " << (is_palindrome(ar) ? "" : "not ") << "a palindrome." << endl;
    	return 0;
    }
    Note that if don't need argc and argv then main can be defined as
    Code:
    int main()
    {
    ...
    }
    Last edited by 2kaud; August 16th, 2015 at 04:56 PM.
    All advice is offered in good faith only. All my code is tested (unless stated explicitly otherwise) with the latest version of Microsoft Visual Studio (using the supported features of the latest standard) and is offered as examples only - not as production quality. I cannot offer advice regarding any other c/c++ compiler/IDE or incompatibilities with VS. You are ultimately responsible for the effects of your programs and the integrity of the machines they run on. Anything I post, code snippets, advice, etc is licensed as Public Domain https://creativecommons.org/publicdomain/zero/1.0/ and can be used without reference or acknowledgement. Also note that I only provide advice and guidance via the forums - and not via private messages!

    C++20 Compiler: Microsoft VS2022 (17.0.5)

  14. #14
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    Re: [RESOLVED] Wrong output with pointers

    Thank you very much 2kaud, really! I have a very performant code now and I learnt something new, thanks!!

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