How do I swap an element of a C++ stack - c++

Question

I have this task to do. Basically, I have to create a dynamic stack, then I have to choose an element and swap it with a new one which I inserted. The other elements remain the same and I have to display the new data. I've come to a dead end.
#include <iostream>
using namespace std;
struct elem
{
int key;
elem* next;
} *start = NULL, *p;
void push(int n);
int pop(int &n);
int main()
{
int num, counter = 0;
do
{
cout << "Input integers(!=0): ";
cin >> num;
if (num > 0 || num < 0)
{
push(num);
counter++;
}
}
while (num != 0);
cout << "\nStack: ";
while (pop(num))
cout << num << "\t";
cout << "\n";
int element, newValue;
cout << "Input element number: ";
cin >> element;
cout << "New value of element: ";
cin >> newValue;
here I struggle doing the swap
return 0;
}
void push(int n)
{
p = start;
start = new elem;
start->key = n;
start->next = p;
}
int pop(int &n)
{
if (start)
{
n = start->key;
p = start;
start = start->next;
delete p;
return 1;
}
else
return 0;
}

Answer 1

How do you swap a chosen one from a stack of (dinner) plates with the top one?
you pick the topmost, put it aside, then pick all the ones you don't want and put them on a new stack until you reach the chosen one. Pick it, exchange it with the original topmost one, put the chosen one aside.
Then re-stack, one by one, the plates from the new stack to the old one. Lastly, put the one you put aside on top.

Answer 2

Going through an intermediary stack like #Bathsheba suggested is the most straight forward solution here.
You can also save some execution time by simply modifying the "next" var of the previous
element to the element you're going the change.
#include <iostream>
using namespace std;
struct elem
{
int key;
elem* next;
}
*start = NULL, *p,*previous,*curr,*temp;
void push(int n);
int pop(int &n);
int main() {
int num, counter = 0;
do {
cout << "Input integers(!=0): ";
cin >> num;
if (num > 0 || num < 0) {
push(num);
counter++;
}
}
while (num != 0);
/* cout << "\nStack: ";
while (pop(num))
cout << num << "\t";
cout << "\n";
*/
int element, newValue;
cout << "Input element number: ";
cin >> element;
cout << "New value of element: ";
cin >> newValue;
curr = start ;
previous = NULL;
while(curr->next)
{
if(curr->key == element){
if(previous) {
temp = new elem;
temp->key = newValue;
temp->next = curr->next ;
previous->next = temp ;
break;
}
else {
temp = new elem;
temp->key = newValue;
temp->next = curr->next ;
start = temp ;
break;
}
}
else {
previous = curr ;
curr = curr->next;
}
}
while (pop(num))
cout << num << "\t";
cout << "\n";
return 0;
}
void push(int n) {
p = start;
start = new elem;
start->key = n;
start->next = p;
}
int pop(int &n) {
if (start) {
n = start->key;
p = start;
start = start->next;
delete p;
return 1;
}
else
return 0;
}