problem inserting into recursive BST

This is a discussion on problem inserting into recursive BST within the C Programming forums, part of the General Programming Boards category; hey guys, Im new to C and having a bit of problems inserting into a recursively defined BST. it seems ...

  1. #1
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    Question problem inserting into recursive BST

    hey guys,
    Im new to C and having a bit of problems inserting into a recursively defined BST. it seems to work ok inserting 3 or 4 items but any more than that and it starts to make duplicates and do wierd things, i have a feeling it could be a very simple obvious mistake but I just cannot find it. I have included the insertion code and how the tree is defined below:

    Code:
    struct bstnode{
       char *key;
       bst left;
       bst right;
    };
    
    bst bst_insert(bst b, char *s){
       if( b == NULL ){
          bst result = emalloc(sizeof * result);
          result->key = emalloc((strlen(s) + 1) * sizeof s[0]);
          strcpy(result->key, s);
          return result;
       } else{
          if( strcmp(b->key, s) == 0 ){
             return b;
          } else if( strcmp(b->key, s) < 0){
             b->right = bst_insert(b->right, s);
          } else if( strcmp(b->key, s) > 0){
             b->left = bst_insert(b->left, s);
          }
       }
    }
    Thanks in advance for the help!!

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    b->right = bst_insert(b->right, s) will kill the right-hand side of the tree, if it already existed -- probably, since it's not clear what bst_insert will return when we get to that part of the code.

  3. #3
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    how else can it be done?
    in the main method insert is simply called as b = bst_insert(b, "foo");
    if i do it like that inside the insert method the tree could never grow bigger than one node, so surely i have to use b->left = bst_insert(b->left, "string"). i really cant get my head around it? this recursive business takes some time to get used to... please let me know if you need more information..

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    Between my mentioning it, and all those lines that your compiler spits out that say: warning: end of non-void function reached (or something similar), you should figure out that the function ALWAYS needs to return something. When you go into b->right = bst_instert(b->right, s), the funciton still needs to return something. If you don't want the rest of the tree to go away, you need to add the line "return b;" so that the correct element gets passed back up the chain.

  5. #5
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    Thanks for the help, the new code seems to work.

    Code:
    bst bst_insert(bst b, char *s){
       if( b == NULL ){
          bst result = emalloc(sizeof * result);
          result->key = emalloc((strlen(s) + 1) * sizeof s[0]);
          strcpy(result->key, s);
          return result;
       } else{
          if( strcmp(b->key, s) == 0 ){
             return b;
          } else if( strcmp(b->key, s) < 0){
             b->right = bst_insert(b->right, s);
          } else if( strcmp(b->key, s) > 0){
             b->left = bst_insert(b->left, s);
          }
       }
       return b;
    }

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