stack

#include <stdio.h>
int sp=-1;
void push(int a[],int,int);

void pop(int a[]);
void traverse(int a[]);
int main()
{
int p=1,a[20],n=10,data,i ,b;
while(p)
{
printf("\n Enter 1 to PUSH the stack");
printf("\n Enter 2 to POP the stack");
 printf("\n Enter 3 to display the "); 
printf("\n Enter 4 to exit loop");
scanf("%d",&b);
switch(b)
{
case 1:printf("Enter the data");
scanf("%d",&data);
push(a,n,data);
break;
case 2: pop(a);
break;
case 3 : traverse(a);
break;
case 4 : p=0;
break; 
}
}

return 0;
}

void push(int a[],int n,int data)
{
if(sp>=n-1)
{
printf("\n The stack is over flow");
}
else
{
sp=sp+1;
a[sp]=data;
}
}
void pop(int a[])
{
if ( sp==-1)
{
printf("\n The stack is underflow");
}
else 
{
sp=sp-1;
}
}
void traverse(int a[])
{
int i;
printf("\n The elements of the stack ..\n");
for(i=0;i<=sp;i++)
{
printf("\n Arr[%d]=[%d]",i,a[i]);
}
}