Use of ** in BST insert( )

Question

The following is my code for BST insert function.Could someone explain why this gives a segmentation fault?

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

struct node{
    int value;
    struct node* right;
    struct node* left;
};

struct node* insert(struct node* n,int age){
    if (n==NULL){
        n = malloc(sizeof(struct node));   
        n->value = age;
        n->left = n->right = NULL; 
    }
    else if(age < n->value){ 
        n->left = insert(n->left, age);
    }
    else {
        n->right = insert(n->right, age);
    }
    return n;
}

void main(){
    int age;
    struct node* n=NULL;
    scanf("%d",&age);
    while (age!=-1){
        n=insert(n,age);    
        scanf("%d",&age);
    }
}

I referred to this and it suggests the use of ** (reference to pointer).

f( &px );
//...

void f( int **px )
{
    *px = malloc( sizeof( int ) );

    printf( "*px = %p\n", *px );
}

But why can't we avoid the use of ** by changing the return type from void to node*?

`insert` always crashes. Doesn't your compiler warn for this? — melpomene, Dec 11 '16 at 08:23
Don't use `scanf` for user input (but if you do, always check its return value). — melpomene, Dec 11 '16 at 08:24
`if (n=NULL){` --> `if (n==NULL){` and need `return n;` end of `insert` — BLUEPIXY, Dec 11 '16 at 08:29
@BLUEPIXY Does the position of return statement matter? I didn't get any compilation error.Yet,I made the changes and tried but didn't get the output. — Alekhya Vellanki, Dec 11 '16 at 08:52
@melpomene The compilation was clean.No errors or warnings. Why would `insert()` crash? — Alekhya Vellanki, Dec 11 '16 at 08:54
`insert ` must return `struct node*`. So, `return` is necessary. — BLUEPIXY, Dec 11 '16 at 08:56
BTW Do not change posting code without annotation. because matching with questions and answers is lost. — BLUEPIXY, Dec 11 '16 at 09:05
_didn't get the output_ Because you are not printing anything. — BLUEPIXY, Dec 11 '16 at 09:07
By not getting output i meant, the code doesn't even accept values and work the way it should.When I run the code, it gives `segmentation fault(core dumped)` and exits. — Alekhya Vellanki, Dec 11 '16 at 09:12
@AlekhyaVellanki Because `n=NULL` assigns `NULL` to `n` and then the following `n->value` dereferences a null pointer. What compiler are you using? — melpomene, Dec 11 '16 at 09:23

RoadRunner · Answer 1 · 2016-12-11T09:25:07.110

This seems to work for me. I didn't change much or your code, except for how you are using scanf(), which doesn't end when you enter 1.

It is better to just call scanf once, and make sure you allow continuous input, using while (scanf(.....) == 1, to make sure that one value is always read until terminated, in this case, until age is 1.

Unless I am missing something, this is the proposed code:

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

struct node{
    int value;
    struct node* right;
    struct node* left;
};

struct node* insert(struct node* n,int age){
    if (n==NULL){
        n = malloc(sizeof(struct node));   
        n->value = age;
        n->left = n->right = NULL; 
    }
    else if(age < n->value){ 
        n->left = insert(n->left, age);
    }
    else {
        n->right = insert(n->right, age);
    }
    return n;
}

void
print_tree(struct node *n) {
    if (n != NULL) {
        print_tree(n->left);
        printf("%d\n", n->value);
        print_tree(n->right);
    }
}

int main(){
    int age;
    struct node* n = NULL;

    printf("Enter some numbers(1 to stop): ");
    while (scanf("%d", &age) == 1 && age != 1) {
        n = insert(n, age);
    }

    printf("\nYour numbers inserted into BST:\n");
    print_tree(n);

    return 0;
}

I just changed my `void main()` to `int main()` and it works.Thanks!.But I fail to understand why `void` didn't work. — Alekhya Vellanki, Dec 11 '16 at 09:21
This answer does not seem to fix any segfault. The code in the question, as it now stands, seems to be correct. Using `scanf` in the way that @RoadRunner suggests is a good idea, but not necessary. — nickie, Dec 11 '16 at 09:22

Use of ** in BST insert( )

1 Answers1