Question

I've just tried to implement a recursive linked list append in C, and for some reason l keeps being NULL no matter what I do, even though I malloc when encountering a NULL value.

The following code prints nothing. Any ideas?

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

typedef struct node {
  unsigned v;
  struct node* next;
} node;

void insert(unsigned x, node* l) {
  if (l == NULL){
    node* new_node = (node*)malloc(sizeof(node));
    new_node->v = x;
    new_node->next = NULL;
    l = new_node;
  }
  else {
    insert(x, l->next);
  }
}

void print(node *l) {
  if (l == NULL) return;
  printf("%d\n", l->v);
  print(l->next);
}

int main(){

  node* l = NULL;
  insert(1, l);
  insert(2, l);

  print(l);

  return 0;
}

Help much appreciated.

EDIT: Even if I initialize it, it's still the same.

Answer 1

First off, you're passing l to your function by value so you won't see any changes made to it. Currently, when you set l to new_node, you only change the local value of l in the method insert. You may want to modify your function to return l and set l in your main to the return value of this function.

node *insert(int x, node *l){
  //.... your method
  return l;
}

In main:

node *l = NULL;
l = insert(1, l);
l = insert(2, l);

Second, your recursive call will not append nodes to your LL for the same reason. Change that call to this:

 l->next = insert(x, l);

Answer 2

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

typedef struct node {
unsigned v;
struct node* next;
} node;

node* insert(unsigned x, node** ll) {
  if (*ll == NULL){
*ll = (node*)malloc(sizeof(node));
(*ll)->v = x;
(*ll)->next = NULL;

  }
  else {
    insert(x, &(*ll)->next);
}
}

void print(node *l) {if (l == NULL) return;
printf("%d\n", l->v);
print(l->next);
}

int main(){

node* l = NULL;
insert(1, &l);
insert(2, &l);
print(l);

return 0;
}

编译没有gcc没有开关

输出： 1 2

Recursive linked list in C

2 个答案: