How to insert data in linked structure

Question

My insertion worked, so a new question, How do I set a data to NULL in C? Pls see the last part for illustration.!

I have defined a structure type

typedef struct node {
    char* data;
    int weight;
    bool  end_of_key;
    struct node * left;
    struct node * equal;
    struct node * right;
} node_t;



int main(int argc, char *argv[]){
node_t* root=NULL;
int weight;
int i = 1;

insert(root,"cat",3);
insert(root,"at",2);
insert(root,"cute",4);
.....
return 0 ;}

This is my insert function

node_t*  insert(node_t* pNode,char* word, int weight) {

    /**
    * Create a new pNode, and save a character from word
    */
  pNode = (node_t*)malloc(sizeof(node_t));

if(*pNode->data == NULL){   
        pNode->left = NULL;
        pNode->equal = NULL;
        pNode->right = NULL;
        pNode->data = word;
  }



if (word[0] < *(pNode->data)) {
    /**
    * Insert the character on the left branch
    */
    pNode->left = insert(pNode, word, weight);
}

else if (word[0] == *(pNode->data)) {
    if ((word[1]) == '\0') {
        /**
        *set pNode end_of_key_flag to true and assign weight
        */
        pNode->end_of_key = true;
        pNode->weight = weight;
    //  printf("%c", *(pNode->data++));
    }
    else {
        /**
        * If the word contains more characters, try to insert them
        * under the equal branch
        */
    //  printf("%c", *(pNode->data++));
        pNode->equal = insert(pNode,word + 1, weight);

    }
}

else {
    /**
    * If current char in word is greater than char in pData
    * Insert the character on the right branch
    */
    pNode->right = insert(pNode,word, weight);
}

return pNode;}

this code is trying to do this

So my insertion finally worked but it appears that it can only insert one thing,I am wondering how do I set data to NULL in C?

if(*pNode->data == NULL){   
        pNode->left = NULL;
        pNode->equal = NULL;
        pNode->right = NULL;
        pNode->data = word;
  }

I want to run this four lines of code when *pNode->data is empty but it apparently did not work the way I wanted it to.

while you're at it, can you show us the exact line where the SEGV occurs (congrats for actually using a debugger in a SO question, that said) — Jean-François Fabre
– Jean-François Fabre ♦, Commented Sep 4, 2017 at 8:55
also I don't get why you're trying to insert c by passing "cat" — Jean-François Fabre
– Jean-François Fabre ♦, Commented Sep 4, 2017 at 8:59
2 hours ago you asked this: initializing linked structure node. Your problems originating from the fact that you have to get a basic understanding of C, especially pointers, C-strings and arrays. I don't want to offend you take the following as an advice: Learn C properly instead of asking about each problem, you will benefit from it and gain understanding. You will not learn if we fix your code and you don't understand. Learn and fix yourself and try to understand. — Andre Kampling
– Andre Kampling, Commented Sep 4, 2017 at 9:02
There are many issues within your code. You should learn how to use a debugger. Problems spotted: if ((word + 1) == '\0')-> if ((word[1]) == '\0'), insert(root, xxx, 3); -> root = insert(root, xxx, 3);, insert won't modify the root parameter. And you don't initialize weight and end_of_key in all cases. There are probably more problems. — Jabberwocky
– Jabberwocky, Commented Sep 4, 2017 at 9:54

vadim_hr · Accepted Answer · 2017-09-05 08:08:21Z

Some improvements of your code

insert() must have first parameter to be node_t ** (see comments)
char *data must be char data, because every node contains only one char
weight can be calculated when the list is filled up

Here is corrected version of your code. Function get() is used to find the key in the filled up list (for testing).

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct node {
    char data;
    int weight;
    struct node * left;
    struct node * equal;
    struct node * right;
} node_t;

void insert(node_t** pNode, char* word, int weight)
{
    char data;
    node_t **pNext;
    node_t *pCurrent;

    if (word == NULL)
    {
        return ;
    }

    data = word[weight];

    if (*pNode == NULL)
    {
        *pNode = malloc(sizeof(node_t));
        pCurrent = *pNode;
        memset(pCurrent, 0, sizeof(node_t));
        pCurrent->data = data;
    }
    else
    {
        pCurrent = *pNode;
    }

    if (data == pCurrent->data)
    {
        weight ++;
        if (strlen(word) == weight)
        {
            pCurrent->weight = weight;
            return;
        }

        pNext = &pCurrent->equal;
    }
    else if (data > pCurrent->data)
    {
        pNext = &pCurrent->right;
    }
    else
    {
        pNext = &pCurrent->left;
    }

    insert(pNext, word, weight);
}

int get(node_t** pNode, char *word, int weight)
{
    char data;
    node_t **pNext;
    node_t *pCurrent;

    if (word == NULL)
    {
        return 0;
    }

    data = word[weight];

    if (*pNode == NULL)
    {
        return 0; // not found
    }

    pCurrent = *pNode;

    if (data == pCurrent->data)
    {
        weight ++;

        if (strlen(word) == weight)
        {
            return pCurrent->weight;
        }

        pNext = &pCurrent->equal;
    }
    else if (data > pCurrent->data)
    {
        pNext = &pCurrent->right;
    }
    else
    {
        pNext = &pCurrent->left;
    }

    return get(pNext, word, weight);
}

int main()
{
    node_t * root = NULL;

    insert(&root, "cat", 0);
    insert(&root, "at", 0);
    insert(&root, "cute", 0);

    printf("cat=%d\n",get(&root,"cat",0)); // cat=3
    printf("at=%d\n",get(&root,"at",0)); // at=2
    printf("cute=%d\n",get(&root,"cute",0)); // cute=4

    // todo: free memory

    return 0;
}

The code is tested except freeing the memory.

Gilles Gouaillardet · Accepted Answer · 2017-09-04 13:16:42Z

0

First, there is something wrong with your insert() signature (as already pointed by @MichaelWalz )

you'd rather

node_t* insert(node_t** pNode, char* word, int weight);

and then

insert(&root,"cute",4);

why don't you start fixing this and edit your post ?

edited Sep 4, 2017 at 13:16

answered Sep 4, 2017 at 11:04

Gilles Gouaillardet

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