Implementation of some operations in doubly linked lists in c (push back, printing, substitution)

doubly_linked_list_operations.c

/**
 * Copyright 2019 Błażej Sewera
 * Licensed under MIT License
 */

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


typedef struct node {
  int data;
  struct node* next;
  struct node* prev;
} Node;


Node* push_back(Node** first_ref, Node** last_ref, int data) {
  // Func inserting a node at the end of the list
  Node* new = malloc(sizeof(Node)); // same as new Node(); in cpp

  new->data = data;
  Node* current;

  if (*first_ref == NULL) {
    // if the list is empty
    new->next = NULL;
    new->prev = NULL;
    *first_ref = new;
    *last_ref = new;
  } else {
    current = *first_ref;
    // Traverse to the end of the list
    while (current->next != NULL)
      current = current->next;
    current->next = new;
    new->prev = current;
    *last_ref = new;
  }
  
  return new;
}


void print_list(Node** first_ref, Node** last_ref) {
  Node* current = *first_ref;

  printf("List forward: [ ");
  while (current != NULL) {
    printf("%d, ", current->data);
    current = current->next;
  }
  printf("]\n");

  current = *last_ref;
  printf("List backwards: [ ");
  while (current != NULL) {
    printf("%d, ", current->data);
    current = current->prev;
  }
  printf("]\n");
}


void substitute(Node** first_ref, Node** last_ref, int n, int new_data) {
  Node* current = *first_ref;
  
  // if you have a `size` variable in list class,
  // you can check for index out of range error
  // before the for loop
  // Also, this is some kind of
  // get_node_ptr_at_index(int n)
  // function, so you can use it instead
  // if provided
  for (int i = 0; i < n; ++i) {
    if (current != NULL)
      current = current->next;
    else {
      printf("[E] Index number out of range\n");
      return;
    }
  }

  Node* new = malloc(sizeof(Node)); // = new Node();

  new->data = new_data;
  
  new->next = current->next;
  new->prev = current->prev;

  if (current->prev != NULL)
    current->prev->next = new;
  else
    *first_ref = new;

  if (current->next != NULL)
    current->next->prev = new;
  else
    *last_ref = new;
}


int main() {
  Node* first = NULL;
  Node* last = NULL;
  
  push_back(&first, &last, 0);
  push_back(&first, &last, 1);
  push_back(&first, &last, 2);
  push_back(&first, &last, 3);
  push_back(&first, &last, 4);
  push_back(&first, &last, 5);

  print_list(&first, &last);

  substitute(&first, &last, 2, 99);
  substitute(&first, &last, 4, 120);

  print_list(&first, &last);

  substitute(&first, &last, 0, 32);
  substitute(&first, &last, 5, 64);

  print_list(&first, &last);
}