Question

I don’t know how to pass a pointer to functions and change the values that pointer pointing to,

I did it like this:

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

struct node{
    struct node* lchild;
    struct node* rchild;
    struct node* p;
    int noChild;
    char* data;
    int height;
};

typedef struct node text_t;

text_t * create_text(){
    text_t * txt = NULL;
    return txt;
}

int length_text(text_t *txt){
    if(txt == NULL)return 0;
    return txt->noChild+1;
}

char * get_line(text_t * txt, int index){
     if(index>txt->noChild+1) return NULL;

     text_t * current = txt;
     while((current->noChild+1)!=index-1){
        if(index-1>current->noChild+1){
           current = current->rchild;
           }else{
             current = current->lchild;
             }                                                     
     }
     return current->data;
}

void append_line(text_t *txt, char * new_line){     
      text_t * temp;

      if (txt == NULL){

       txt = (text_t *)malloc(sizeof(text_t));
       txt->lchild = NULL;
       txt->rchild = NULL;
       txt->noChild = 0;
       txt->p = NULL;
       txt->data = new_line;
       txt->height = 1;
       printf(txt->data);
     }else{
       text_t * current = txt;

       while(current->rchild!=NULL){
       current = current->rchild;
       }

       temp = (text_t *)malloc(sizeof(text_t));
       temp->lchild = NULL;
       temp->rchild = NULL;
       temp->noChild = 0;
       temp->data = new_line;
       temp->height = 1;
       temp->p = current;

}}    







int main()
{  int i, tmp; text_t *txt1, *txt2; char *c;
   printf("starting \n");
   txt1 = create_text();//1
   txt2 = create_text();

   append_line(txt1, "line one" );//2
 ...

I want to create an object of such a struct in C and use it to build a balanced tree.

However, after I did append_line(), nothing changes with txt1, it’s still NULL.
Why?

Thanks

Answer 1

It’s because c is pass by value.

 void append_line(text_t *txt, char * new_line){ 
   // ....

   txt = (text_t *)malloc(sizeof(text_t));

   // .....
 }

 int main() {
   // ...
   append_line(txt1, "line one" );
   // ...
 }

txt1 in main isn’t affected with the malloc statement in the appen_line to txt. If you want to affect the txt1 in main then change the function text_t parameter to pointer to a pointer.

void append_line(text_t **txt, char * new_line);

Now pass the address of txt1 from main –

append_line(&txt1, "line one" );