stack operations using switch case - c

Question

#include <stdio.h>
#define max 20
int stk[max];
int top=-1;
void push(int);
int pop();
void peep();
void display();
int isFull();
int isEmpty();
int main()
{
int ch,item;
do
{
printf("....Stack Operations....\n");
printf("Press 1 for Push\n");
printf("Press 2 for pop\n");
printf("Press 3 for peep\n");
printf("Press 4 for display\n");
printf("Enter your choice \n");
scanf("%d",&ch);
switch(ch)
{
case 1:
printf("Enter the item :- \n");
scanf("%d",&item);
push(item);
break;
case 2:
int k=pop();
printf("The poped element is %d\n",k);
break;
case 3:
peep();
break;
case 4:
display();
break;
default:
printf("Invalid Choice\n");
}
}while(ch>=1 && ch<=5);
void push(int item)
{
if(isFull())
printf("Stack Overflow\n");
else
{
top=top+1;
stk[top]=item;
}
}
int pop()
{
int s;
if(isEmpty())
printf("Stack Underflow\n");
else
{
s=stk[top];
top=top-1;
}
return s;
}
void peep()
{
if(isEmpty())
printf("Stack Underflow\n");
else
{
printf("Topmost Element of the stack is %d",stk[top]);
}
}
void display()
{
if(isEmpty())
printf("Stack Underflow\n");
else
{
for(int i=top;i>=0;i--)
printf("%d ",stk[i]);
}
}
int isFull()
{
if(top==max-1)
return 1;
else
return 0;
}
int isEmpty()
{
if(top==-1)
return 1;
else
return 0;
}
}
I wanted the program to run for all stack operations but I am getting this type of specific error:-
/usr/bin/ld: /tmp/ccHJ1ZkB.o: in function `main':
main.c:(.text+0xf7): undefined reference to `push'
/usr/bin/ld: main.c:(.text+0x103): undefined reference to `pop'
/usr/bin/ld: main.c:(.text+0x12b): undefined reference to `peep'
/usr/bin/ld: main.c:(.text+0x137): undefined reference to `display'
collect2: error: ld returned 1 exit status

Answer 1

It's not standard C to define functions within another function (main()) aka "nested function".
pop() returns an undefined value if the stack is empty. I changed it to return -1 but really you want to add the ability to tell caller of errors like this.
You need a block {} when introducing variables in a switch statement.
(not fixed) Avoid global variables and instead pass values to the functions that need them (functional style). Usually, you then group related data (stk and top) into a struct.
(not fixed) By convention constants, like max, should be upper case.
#include <stdio.h>
#define max 20
int stk[max];
int top=-1;
int isFull() {
return top==max-1;
}
int isEmpty() {
return top==-1;
}
void push(int item) {
if(isFull()) {
printf("Stack Overflow\n");
return;
}
stk[top++]=item;
}
int pop() {
if(isEmpty()) {
printf("Stack Underflow\n");
return -1;
}
return stk[top--];
}
void peep() {
if(isEmpty()) {
printf("Stack Underflow\n");
return;
}
printf("Topmost Element of the stack is %d",stk[top]);
}
void display() {
if(isEmpty()) {
printf("Stack Underflow\n");
return;
}
for(int i=top; i>=0; i--)
printf("%d ", stk[i]);
}
int main() {
int ch;
do {
printf(
"....Stack Operations....\n"
"Press 1 for Push\n"
"Press 2 for pop\n"
"Press 3 for peep\n"
"Press 4 for display\n"
"Enter your choice \n"
);
scanf("%d",&ch);
switch(ch) {
case 1: {
printf("Enter the item :- \n");
int item;
scanf("%d", &item);
push(item);
break;
}
case 2:
printf("The popped element is %d\n", pop());
break;
case 3:
peep();
break;
case 4:
display();
break;
default:
printf("Invalid Choice\n");
}
} while(ch>=1 && ch<=5);
}