C Devoweller memory error - c

Question

I have almost completed my devowelling program for class but have encountered the memory access violation error when it reaches a while loop where I have tried to detect the absence of vowels from a linked list node. I realise that the way I have done this is ridiculously inefficient (many logical OR checks) but I was struggling with other ways to do this. Utterly confused. Not expecting too much help but any pointers (:S) would be greatly appreciated.
https://gist.github.com/3992412
Or, copy'n'paste:
#include <iostream>
#include <stdlib.h>
struct NODE {
char letter;
struct NODE *next;
};
int vowelcheck(struct NODE *llist, int num);
void addnode(struct NODE *llist, char c);
void showsentence(struct NODE *llist);
void devowel(struct NODE *llist);
int main(void) {
char charin;
int input = 1;
struct NODE *llist;
int nodeno = 0;
llist = (struct NODE *)malloc(sizeof(struct NODE));
llist->letter = 0;
llist->next = NULL;
while(input != 0) {
printf("\n\n --Disemvoweler--\n");
printf("(0) Quit\n");
printf("(1) Enter sentence\n");
printf("(2) Disemvowel\n");
printf("(3) Display parsed sentence\n");
scanf("%d", &input);
switch(input) {
case 0: //exit
default:
printf("Exiting\n");
break;
case 1: //sentence input
printf("\nEnter sentence, finish sentence with full stop (.) :\n");
do
{
charin=getchar();
addnode(llist, charin);
}
while (charin != '.');
break;
case 2: //remove vowels
printf("Your choice: `Disembvowel'\n");
while(llist->next != NULL) {
devowel(llist);
llist = llist->next;
}
printf("Disembvoweled!\n");
break;
case 3: //show sentence in memory (devoweled or not)
printf("\n Parsed sentence: \n");
showsentence(llist);
break;
}
}
free(llist);
return(0);
}
void showsentence(struct NODE *llist) {
while(llist->next != NULL) { //while not the last link (ie not full stop)
printf("%c ", llist->letter); //print letter
llist = llist->next; //move to next link
}
}
void addnode(struct NODE *llist, char charin) {
while(llist->next != NULL)
llist = llist->next;
llist->next = (struct NODE *)malloc(sizeof(struct NODE));
llist->next->letter = charin;
llist->next->next = NULL;
}
void devowel(struct NODE *llist) {
struct NODE *temp;
temp = (struct NODE *)malloc(sizeof(struct NODE));
if(llist->letter == 'A' || llist->letter == 'a' || llist->letter == 'E' || llist->letter == 'e' || llist->letter == 'I' || llist->letter == 'i' || llist->letter == 'O' || llist->letter == 'o' || llist->letter == 'U' || llist->letter == 'u')
{
/* remove the node */
temp = llist->next;
free(llist);
llist = temp;
} else {
while(llist->next->letter != 'A' || llist->next->letter != 'a' || llist->next->letter != 'E' || llist->next->letter != 'e' || llist->next->letter != 'I' || llist->next->letter != 'i' || llist->next->letter != 'O' || llist->next->letter != 'o' || llist->next->letter != 'U' || llist->next->letter != 'u')
llist = llist->next;
temp = llist->next->next;
free(llist->next);
llist->next = temp;
}
}

Answer 1

while(...HORRIBLE CONDITION DEREFERENCING llist->next SNIPPED...)
llist = llist->next;
temp = llist->next->next;
free(llist->next);
llist->next = temp;
}
This piece of code has at least two fatal problems. First of all, in C++ indentation does not determine blocks, {} do, and you are missing a pair. Second, you access contents of llist->next without checking if there is next element, or if list has ended.

Answer 2

You might have use for something like:
int character_is_vowel(char ch)
{
return strchr("AEIOUaeiou", ch) != NULL;
}
See what I did, there? I took a small part of the problem at hand (determine whether a character is a vowel in English) and broke it out into a standalone piece of the program.
Then, using standard library functions to cut down on the repetitive nature is of course another idea that's generally good to make code more readable.
Regarding your code, your linked-list code is severely broken in many places. You should consider if you really must implement this using linked lists. It's a string transform, and strings in C are not typically treated as linked lists. Of course, since this was for class, I guess your hands are tied.
Then, you should look hard at all the list operations you do, and analyze if they make sense. Think about memory validity, checks for NULL so you don't overstep the end of the list, and (again) if it's maybe possible to break these operations out into dedicated functions that you can write, think about, and test in isolation from each other and from the actual problem you're trying to solve.

Answer 3

Epigrams about mountains and molehills spring to mind.
Here's a working program that reads lines of input and then disemvowels those lines.
Sample run
$ ./disemvowel
What's the point of including vowels if you're going to strip 'em all?
Entered: What's the point of including vowels if you're going to strip 'em all?
Disemvowelled: Wht's th pnt f ncldng vwls f y'r gng t strp 'm ll?
$
Sample source
#include <string.h>
#include <stdio.h>
static int is_vowel(char c)
{
return(strchr("aeiouAEIOU", c) != 0);
}
int main(void)
{
char line[4096];
while (fgets(line, sizeof(line), stdin) != 0)
{
printf("Entered: %s", line);
char *dst = line;
char *src = line;
char c;
while ((c = *src++) != '\0')
{
if (!is_vowel(c))
*dst++ = c;
}
*dst = '\0';
printf("Disemvowelled: %s", line);
}
return(0);
}
If the string contains no vowels, it copies the string, byte-by-byte, over itself. However, adding a conditional in the loop to see if dst < src would complicate things for negligible benefit (and, in the long run, it would slow things down). If you want to speed this up, you'd use a table-driven is_vowel() function:
static int is_vowel(char c)
{
static vowels[256];
if (vowels['a'] == 0)
{
unsigned char *v = "aeiouAEIOU";
while (*v != '\0')
vowel[*v++] = 1;
}
return vowels[(unsigned char)c];
}
Note the coercion of possibly signed char c to unsigned char to ensure no problems.
The next level of performance would replace the is_vowel() function with a macro that accesses a global array vowels that is appropriately initialized before first use. That's harder to orchestrate (but it's what typically happens with the macros in <ctype.h> such as isalpha()).
Note, too, that the name is_vowel() steers clear of the names reserved by the <ctype.h> header:
§7.31.2 Character handling <ctype.h>
¶1 Function names that begin with either is or to, and a lowercase letter may be added to
the declarations in the <ctype.h> header.