problem sorting a dynamic list

This is a discussion on problem sorting a dynamic list within the C Programming forums, part of the General Programming Boards category; I have a list whose structure is the following: Code: typedef struct dats_s { char email[MAX_STR]; struct data_s *next; } ...

  1. #1
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    Exclamation problem sorting a dynamic list

    I have a list whose structure is the following:

    Code:
    typedef struct dats_s
    {
        char email[MAX_STR];
        struct data_s *next;
    } data;
    and I need to order the elements of the list alpabetically I've produced the following code but it seem not to work... the idea is to use two pointer and two while cycles so that I compare the first element of the list pointed by the first pointer while I scroll from the second to the last element of the list in serch of a smaller element; when I find it I simply exchange the two strings using a temporary string; when the second pointer reaches the end of the list I repeat this process starting from the second element and so on...

    this is the code involved in the sorting process:

    Code:
    p_corr1=p_head;
    p_corr2=(*p_corr1).next;
    
        while(p_corr1!=NULL)
        {
            while(p_corr2!=NULL)
            {
                if(strcmp((*p_corr1).email,(*p_corr2).email)>0)
                {
                    strcpy(temp_email,(*p_corr1).email);
                    strcpy((*p_corr1).email,(*p_corr2).email);
                    strcpy((*p_corr2).email,temp_email);
                }
                p_corr2=(*p_corr2).next;
            }
            p_corr1=(*p_corr1).next;
        }
    any suggestion is welcome!!
    Thank

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    Do you plan to move p_corr2 back to the front of the list so you can get the second element correct (and then the third, etc.)?

  3. #3
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    .... mmm.... so I have to add
    Code:
    p_corr2=(*p_corr1).next;
    at the end of the first while... this way:
    Code:
    p_corr1=p_head;
    p_corr2=(*p_corr1).next;
    
        while(p_corr1!=NULL)
        {
            while(p_corr2!=NULL)
            {
                if(strcmp((*p_corr1).email,(*p_corr2).email)>0)
                {
                    strcpy(temp_email,(*p_corr1).email);
                    strcpy((*p_corr1).email,(*p_corr2).email);
                    strcpy((*p_corr2).email,temp_email);
                }
                p_corr2=(*p_corr2).next;
            }
            p_corr1=(*p_corr1).next;
            p_corr2=(*p_corr1).next;
        }
    is this what you mean??
    Last edited by andryjohn; 09-05-2009 at 11:22 AM. Reason: grammar mistake :)

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    That would be what I mean.

    And as fun as it is to simulate for loops with while, you may want to consider actually using for loops.

  5. #5
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    ... the program this way crashes... because when p_corr1 reaches NULL I try to assign to p_corr2 something pointed by NULL!!!!

    i would fix it this way...

    Code:
    while(p_corr1!=NULL)
        {
            while(p_corr2!=NULL)
            {
                if(strcmp((*p_corr1).email,(*p_corr2).email)>0)
                {
                    strcpy(temp_email,(*p_corr1).email);
                    strcpy((*p_corr1).email,(*p_corr2).email);
                    strcpy((*p_corr2).email,temp_email);
                }
                p_corr2=(*p_corr2).next;
            }
            p_corr1=(*p_corr1).next;
            if(p_corr1!=NULL)
                p_corr2=(*p_corr1).next;
        }
    using for cycle...
    come on don't make me suffer... tell me how!!! :-)

  6. #6
    and the Hat of Guessing tabstop's Avatar
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    Quote Originally Posted by andryjohn View Post
    ... the program this way crashes... because when p_corr1 reaches NULL I try to assign to p_corr2 something pointed by NULL!!!!
    And that's why you have to be careful when you simulate for-loops with while. Think about what your for loops would like:
    Code:
    for (p_corr1 initialized; p_corr1 not NULL; p_corr1 moves forward) {
        for (p_corr2 initialized; p_corr2 not NULL; p_corr2 moves forward) {
    If p_corr1 is NULL the second loop doesn't even happen.

  7. #7
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    oooo yes!! now I see... that's way cleaner!!!

    this could be the code:
    Code:
    for (p_corr1=*p_dati; p_corr1!=NULL; p_corr1=(*p_corr1).next)
        {
            for (p_corr2=(*p_corr1).next; p_corr2!=NULL; p_corr2=(*p_corr2).next)
            {
                if(strcmp((*p_corr1).email,(*p_corr2).email)>0)
                {
                    strcpy(temp_email,(*p_corr1).email);
                    strcpy((*p_corr1).email,(*p_corr2).email);
                    strcpy((*p_corr2).email,temp_email);
                }
            }
        }
    and it works perfectly!!!

    THANK YOU SOOO MUCH!!! you're help was infinitely useful for me!!! thank's again tabstop!!

    Andryjohn

  8. #8
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by andryjohn
    this could be the code:
    I suggest that you rewrite it slightly:
    Code:
    for (p_corr1 = *p_dati; p_corr1 != NULL; p_corr1 = p_corr1->next)
    {
        for (p_corr2 = p_corr1->next; p_corr2 != NULL; p_corr2 = p_corr2->next)
        {
            if (strcmp(p_corr1->email, p_corr2->email) > 0)
            {
                strcpy(temp_email, p_corr1->email);
                strcpy(p_corr1->email, p_corr2->email);
                strcpy(p_corr2->email, temp_email);
            }
        }
    }
    The logic is entirely the same. I merely modified the formatting slightly to be more consistent, and used the notation a->b instead of the notation (*a).b.
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  9. #9
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    thanks laserlight but what do you mean with
    more consistent
    ?

  10. #10
    C++ Witch laserlight's Avatar
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    Notice that the indentation style of your outer for loop is different from that of your inner for loop.
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    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  11. #11
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    ok... I see... It may have occurred when pasting the code any way thanks!!

  12. #12
    CSharpener vart's Avatar
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    the nice thing about list is that you can just play with pointers to change the order of the links - instead of copying the whole nodes...

    you are working with the list as if it is array

    Code:
    Head -> A -> B -> C
    
    
    To swap A and C you are doing
    
    Head -> C -> B -> A
    
    While you can
    
    Head  A <- B <- C
      |             ^
      --------------|
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  13. #13
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    I'm not a big fan of pointers.... they require my only neuron to work overtime!!!

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