C++ Reversing a stack using two additional stacks

Question

I'm trying to reverse a stack (S) using two other stacks (S1 and S2). This is the code I'm trying:

#include <iostream>
#include <stack>

using namespace std;

int main()
{
    stack<int> S, S1, S2;
    S.push(1), S.push(2), S.push(3);

    cout << "The top element of S is: " << S.top() << endl;

    while (!S.empty()) {
        S1.push(S.pop()); 
    }
    while (!S1.empty()) 
        S2.push(S1.pop()); 
    while (!S2.empty()) 
        S.push(S2.pop()); 

    cout << "The top element of S is now: " << S.top() << endl;

    return 0;
}

This is the error I'm getting (x3) for each time I call pop from inside push. - stack.cc:14:11: error: reference to type 'const value_type' (aka 'const int') could not bind to an rvalue of type 'void' S1.push(S.pop());

I've tried assigning the popped value to a variable and then calling push with that variable, but it was unsuccessful also.

Any help would be appreciated!

You might want to read some reference documentation before using any library functions: [`std::stack::pop()`](http://en.cppreference.com/w/cpp/container/stack/pop) returns `void`. Another thing is that you don't really need two stacks to reverse a stack. Also, a thing that might go weird is the misuse of the [comma operator](http://stackoverflow.com/questions/54142/how-does-the-comma-operator-work). Make sure you are ready to proceed to the coding by taking a C++ refresher from a [verified source](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — Ivan Aksamentov - Drop, Apr 10 '16 at 03:26

score 4 · Accepted Answer · answered Apr 10 '16 at 03:25

4

You should write

 S1.push(S.top());
 S.pop();

std::stack::pop() has a void return type.

answered Apr 10 '16 at 03:25

πάντα ῥεῖ

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Thanks! Do you know why this would be creating a segmentation fault? – Brittany Apr 10 '16 at 03:42
@Brittany No, that's a different issue, and you need to ask a separate question about this. Don't miss to provide a [MCVE]. – πάντα ῥεῖ Apr 10 '16 at 03:47

score 1 · Answer 2 · answered Apr 10 '16 at 03:25

1

pop() does not return the value popped from the stack. pop() returns a void. pop() removes the value from the stack, and does nothing else.

You need to read the value at the top of the stack, using top(), and then call pop().

answered Apr 10 '16 at 03:25

Sam Varshavchik

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C++ Reversing a stack using two additional stacks

2 Answers2