Strange behaviour of do while - c

Question

I was trying to implement a stack. I came up with this. All the other functions work as expected except when is try to push. When i try to push 4 some thing strange happens.
#include <stdio.h>
#include <stdlib.h>
#define MAX 10
typedef struct
{
int a[MAX];
int top;
}stack;
void init(stack *p)
{
p->top=-1;
}
int full(stack *p)
{
if(p->top==MAX)
return 1;
else
return 0;
}
int empty(stack *p)
{
if(p->top==-1)
{
init(p);
return 1;
}
else
return 0;
}
void display(stack *p)
{
if(!empty(p))
{
printf("Stack is::\n");
for(int i=0;i<=p->top;++i)
printf("%d\t",p->a[i]);
printf("\n");
}
else
{
printf("Stack is empty.\n");
init(p);
}
}
void push(stack *p, int x)
{
if(!full(p)) /*full() returns 1 is top==max, 0 otherwise*/
{
p->a[p->top++]=x;
printf("%d pushed.\n",x);
}
else
printf("Stack is full.\n");
}
void pop(stack *p)
{
if(!empty(p))
printf("%d popped.\n",p->a[p->top--]);
else
{
printf("Stack is empty.\n");
init(p);
}
}
int main()
{
stack p;
int ch,x;
printf("Hello world!\n");
init(&p);
printf("*****MENU*****\n");
do{
printf("1.Push\n2.Pop\n3.Display\n4.Exit\n");
printf("Enter your choice:: ");
scanf("%d",&ch);
switch(ch)
{
case 1:
printf("Enter element to push:: ");
scanf("%d",&x);
push(&p,x);
break;
case 2:
pop(&p);
break;
case 3:
display(&p);
break;
case 4:
exit(1);
}
}while(ch!=4);
return 0;
}
The program terminates.
I am testing the while loop with ch(=1) and not x(=4). So why is this happening??

Answer 1

This function
void push(stack *p, int x)
{
if(!full(p)) /*full() returns 1 is top==max, 0 otherwise*/
{
p->a[p->top++]=x;
printf("%d pushed.\n",x);
}
else
printf("Stack is full.\n");
}
is wrong. The initial value of top is -1 so in this statement
p->a[p->top++]=x;
you are trying to store the value in the element of the array with the index equal to -1.
Thus the program has undefined behaviour.
The function could look like
void push( stack *p, int x )
{
if ( !full( p ) ) /*full() returns 1 if top + 1 == max, 0 otherwise*/
{
p->a[++p->top]=x;
^^^^^^^^
printf("%d pushed.\n",x);
}
else
{
printf("Stack is full.\n");
}
}
Take into account that in this case function full should look like
int full( const stack *p )
{
return p->top + 1 == MAX;
^^^^^^^^^^
}
Function empty also can be written simpler
int empty( const stack *p )
{
return p->top == -1;
}
And a stack usually is printed in the reverse order relative to the order of entering elements
void display( const stack *p )
{
if ( !empty( p ) )
{
printf( "Stack is::\n" );
for ( int i = p->top; i != -1; --i )
printf( "%d\t", p->a[i] );
printf( "\n" );
}
else
{
printf( "Stack is empty.\n" );
}
}