Thread: homework assignment problem

OK, I'm glad you guys are there. Here's my problem, I have been working with Sue Ballew in St. Louis. I have this stack program that is supposed to do a bunch of stuff, push, pop, count, print list, and "delete". I can't get the delete function to work, I took the code right from my King textbook, p 378 but it doesn't work. I've tried different forms for the function, e.g.,

void delete( int input_value)
{
struct node *cur, *prev;

for (cur = first, prev = NULL;
cur != NULL && cur->value != input_value;
prev = cur, cur = cur->next)
;

if (cur == NULL)
printf("value not found\n");
return;
if (prev == NULL)
first = first->next;
else
prev->next = cur->next;
free (cur);
return;
}
but that didn't work either.

I am including the whole program here, if any one can figure out what I am sure is some dumb mistake on my part, I will be very grateful. I am 56 and much too old for this ****.

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

struct node {
int value;
struct node *next;
};

struct node *first = NULL;
struct node *new_node;
void push( int input_value);
void pop();
/* struct node *delete(struct node *first, int input_value);
*/
void delete(struct node *list, int input_value);
void instructions(void);
void printlist();
main()
{
int input, value, number, lv;

instructions();
printf("?");
scanf("%d", &input);
while (input != 0) {
switch (input) {
case 1:
printf("Enter an integer: ");
scanf("\n%d", &number);
push(number);
printlist();
break;
case 2:
pop();
printlist();
break;
case 3:
printf("Enter value to be removed: ");
scanf("\n%d", &number);
delete(first, number);
printlist();
break;
default:
printf("Invalid choice.\n\n");
break;
}
printf("?");
scanf("%d", &input);
}
printf("End of run.\n");
return 0;
}

void push( int input_value)
{
new_node = malloc(sizeof(struct node));
new_node->value = input_value;
new_node->next = first;
first = new_node;
}

void pop()
{
int lv;
if (first == NULL)
return;
lv = first->value;
printf(" %d returned\n", lv);
first = first->next;
return;
}

void delete(struct node *list,int input_value)
{
struct node *cur, *prev, *q;
for (cur = list, prev = NULL;
cur != NULL && cur->value != input_value;
prev = cur, cur = cur->next)
;

if (cur == NULL)
printf("value not found\n");
return;
if (prev == NULL) {
list = list->next;

printf(" First value removed"); }
else
prev->next = cur->next;
free (cur);

return;
}

void instructions (void)
{
printf("Enter your choice:\n"
" 0 to end program.\n"
" 1 to add a value to the list.\n"
" 2 to remove a value from the list.\n"
" 3 to delete a value from the list.\n");
}

void printlist()
{
struct node *p;
if (first == NULL)
printf("List is empty.\n\n");
else {
printf("The list is:\n");
p = first;
while (p != NULL) {

printf("%d --> ", p->value);
p = p->next;
}


printf("NULL\n\n");
}
}

OK, I'm glad you guys are there. Here's my problem, I have been working with Sue Ballew in St. Louis. I have this stack program that is supposed to do a bunch of stuff, push, pop, count, print list, and "delete". I can't get the delete function to work, I took the code right from my King textbook, p 378 but it doesn't work. I've tried different forms for the function, e.g.,

void delete( int input_value)
{
struct node *cur, *prev;

for (cur = first, prev = NULL;
cur != NULL && cur->value != input_value;
prev = cur, cur = cur->next)
;

if (cur == NULL)
printf("value not found\n");
return;
if (prev == NULL)
first = first->next;
else
prev->next = cur->next;
free (cur);
return;
}
but that didn't work either.

I am including the whole program here, if any one can figure out what I am sur#include <stdio.h>
#include <stdlib.h>

struct node {
int value;
struct node *next;
};

struct node *first = NULL;
struct node *new_node;
void push( int input_value);
void pop();
/* struct node *delete(struct node *first, int input_value);
*/
void delete(struct node *list, int input_value);
void instructions(void);
void printlist();
main()
{
int input, value, number, lv;

instructions();
printf("?");
scanf("%d", &input);
while (input != 0) {
switch (input) {
case 1:
printf("Enter an integer: ");
scanf("\n%d", &number);
push(number);
printlist();
break;
case 2:
pop();
printlist();
break;
case 3:
printf("Enter value to be removed: ");
scanf("\n%d", &number);
delete(first, number);
printlist();
break;
default:
printf("Invalid choice.\n\n");
break;
}
printf("?");
scanf("%d", &input);
}
printf("End of run.\n");
return 0;
}

void push( int input_value)
{
new_node = malloc(sizeof(struct node));
new_node->value = input_value;
new_node->next = first;
first = new_node;
}

void pop()
{
int lv;
if (first == NULL)
return;
lv = first->value;
printf(" %d returned\n", lv);
first = first->next;
return;
}

void delete(struct node *list,int input_value)
{
struct node *cur, *prev, *q;
for (cur = list, prev = NULL;
cur != NULL && cur->value != input_value;
prev = cur, cur = cur->next)
;

if (cur == NULL)
printf("value not found\n");
return;
if (prev == NULL) {
list = list->next;

printf(" First value removed"); }
else
prev->next = cur->next;
free (cur);

return;
}

void instructions (void)
{
printf("Enter your choice:\n"
" 0 to end program.\n"
" 1 to add a value to the list.\n"
" 2 to remove a value from the list.\n"
" 3 to delete a value from the list.\n");
}

void printlist()
{
struct node *p;
if (first == NULL)
printf("List is empty.\n\n");
else {
printf("The list is:\n");
p = first;
while (p != NULL) {

printf("%d --> ", p->value);
p = p->next;
}


printf("NULL\n\n");
}
}e is some dumb mistake on my part, I will be very grateful. I am 56 and much too old for this ****.

void delete( int input_value)
{
...

if (cur == NULL)
printf("value not found\n");
return;
if (prev == NULL)
first = first->next;
else
prev->next = cur->next;
free (cur);
return;
}

Just looking that this I can see that it will always display: value not found or else nothing, it will return instead. You need to use braces { ... } so that you form code blocks. Without code blocks conditions con only execute one line of code following the test.

Overall this code is semi okay. You probably have a poor textbook. It's not totally wrong, but it's not too good either.

Thanks so much for your reply, but there was only one area where I could have added a block after the else statement at the end, I tried it but it didn't help. I have tried everything I could think of and it just doesn't work. As I said most of the code comes right from the text in a delete a node discussion. They make a big deal of the liberal use of comma operators and the empty body of the for statement.

void delete(struct node *list,int input_value)
{
struct node *cur, *prev, *q;
for (cur = list, prev = NULL;
cur != NULL && cur->value != input_value;
prev = cur, cur = cur->next)
;

if (cur == NULL)
printf("value not found\n");
return;
if (prev == NULL) {
list = list->next;

printf(" First value removed"); }
else
prev->next = cur->next;
free (cur);

return;
}

You have not used code blocks yet. See there is a difference between these statements:

Code:

if ( 10 > 9)  x = 4; //one statement executes if true

if ( 10 > 9) 
{
    x = 4;    //two or more statements execute
    y = 10;  //if the condition is true
}

If you can understand this than you will see your error.