Sorting two stacks by using structs - sorting

Question

There are 3 stacks - A, B, C
Stacks A and B are sorted (the number on the top of the stack is the biggest). Stack C is Empty Only 5 operation are allowed:
push,
pop,
top,
is_empty,
create
We need to write a function that receives the stacks A and B, moves all the numbers in stacks A and B to stack C and stack C must be sorted (biggest Number is on top).
I have the algorithm :
Compare top of A with top of B
Pop the least element and push to stack C
Repeat step 2 until any of the stack ( A or B) becomes empty
Move remaining elements from non-empty stack to C. Now you have all the elements in C but in ascending order. (That is least element at top).
Move all the elements from C to A. (Contents in A are in descending order)
Move all the elements from A to B. (Contents in B are in ascending order)
Move all the elements from B to C.
and i started to write the code but there are errors and i don't know why !
the code :
#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#define MAX_MEMBERS 10
typedef struct
{
int num;
}ITEM;
typedef struct
{
ITEM a[MAX_MEMBERS];
int top;
}STACK;
void create_stack(STACK *s)
{
s->top=-1;
}
int is_empty(STACK *s)
{
return s->top==-1;
}
int is_full(STACK *s)
{
return s->top==MAX_MEMBERS-1;
}
ITEM pop(STACK *s)
{
return s->a[s->top--];
}
void (STACK *s,ITEM *item)
{
s->a[++s->top]=*item;
}
ITEM top(STACK *s)
{
return s->a[s->top];
}
void sort (STACK *a,STACK *b,STACK *c)
{
while(!is_empty(&a)||!is_empty(&b))
if(top(&a)>top(&b))
push(&c,pop(&b));
if(!is_empty(&a))
{
while(!is_empty(&a))
push(&c,pop(&a));
}
else
{
while(!is_empty(&b))
push(&c,pop(&b));
}
while(!is_empty(&c))
push(&a,pop(&c));
while(!is_empty(&a))
push(&b,pop(&a));
while(!is_empty(&b))
push(&c,pop(&b));
}
void main(void)
{
STACK a,b,c;
create_stack(&a);
create_stack(&b);
create_stack(&c);
sort(&a,&b,&c);
}

Answer 1

You should pop the highest element and push it to C.
To push all elements onto C in reversed order you need the highest element at the bottom of C (this value must be pushed first). If a > b Then: pop a, push c.
Anyway, your code doesn't seem to be very safe. Have a look at the following:
while(!is_empty(&a)||!is_empty(&b))
if(top(&a)>top(&b))
let's assume that a is empty and b is not so that a.top equals to -1 and b.top is in range 0 to MAX_MEMBERS - 1. Since one of the stacks is not empty the condition is true.
By calling top(&a) following code is executed:
return a[-1]; //Error: Index out of bounds
This is how your code is resolved.
is_empty(&a) returns true
!is_empty(&a) returns false
(false || true) returns true
while(false || true)
if( top(&a) > top(&b) )
Anyway, i doubt that your code does even compile.
void (STACK *s,ITEM *item)
{
s->a[++s->top]=*item;
}
This should be that:
void push(STACK *s,ITEM *item)
{
s->a[++s->top]=*item;
}
Also consider that:
void main(void)
{
STACK a,b,c;
create_stack(&a); //stack a is created, contains no elements
create_stack(&b); //stack b is created, contains no elements
create_stack(&c); //stack c is created, contains no elements
sort(&a,&b,&c); //sort which elements of the stack?
}