Value pushed to the stack is not what I pop out C++

I'm new to c++, I'm trying to read values from a text file and push only the integers to a stack. The issue I'm having is that when I do a pop() the value that comes out of the stack is different.

For example if I push a 4, when I do a pop it comes out as 52.

What am I doing wrong and how can I fix it?

IntegerStack.cpp

#include "IntegerStack.h"
#include <cstdlib>

using namespace std;

IntegerStack::IntegerStack()
{
    used = 0;
}

void IntegerStack::push(int entry)
{
    data[used] = entry;
    ++used;
}

int IntegerStack::pop()
{
    --used;
    return data[used];

}

int IntegerStack::peek() const
{
    return data[used-1];
}

IntegerStack.h

#ifndef INTEGERSTACK_H
#define INTEGERSTACK_H

#include <cstdlib>    // Provides the type size_t.

using namespace std;

class IntegerStack
{
    public:
        // MEMBER CONSTANT
        static const std::size_t CAPACITY = 100;

        // DEFAULT CONSTRUCTOR
        IntegerStack( );                       // Inline

        // MODIFICATION MEMBER FUNCTIONS
        void push ( int entry );
        int pop ( );

        // CONSTANT MEMBER FUNCTIONS
        std::size_t size ( ) const { return used; }    // Inline
        bool is_empty ( ) const { return used == 0; }  // Inline
        int peek ( ) const;

    private:
      // DATA MEMBERS
      int data[CAPACITY];
      std::size_t used;
};

#endif // INTEGERSTACK_H

main.cpp

#include <fstream>
#include <iostream>
#include "IntegerStack.h"

using namespace std;

int main()
{
    string content;
    ifstream inputFile;
    cout << "Enter input file name: ";
    cin >> content;

    IntegerStack operandStack;

    // Open file
    inputFile.open(content.c_str());

    if(inputFile)
    {
        // Place values in the stack
        while(getline(inputFile,content))
        {
            cout << "Expression: " << content << endl;
            for (int i = 0; i < content.size(); i++)
            {
                if(isdigit(content[i]))
                {
                    cout << "Adding " << content[i] << " to operandStack" << endl;
                    operandStack.push(content[i]);

                    int number = operandStack.pop();
                    cout << "The integer we just pushed: " << number << endl;
                }
                else
                {
                    // add it to operatorStack
                }
            }
        }

    }

    // Close file
    inputFile.close();

    return 0;
}

inix.dat

8 + 4 / 2 
( 7 * 4 ) - 2

OUTPUT

Enter input file name: infix.dat
Expression: 8 + 4 / 2
Adding 8 to operandStack
The integer we just pushed: 56
Adding 4 to operandStack
The integer we just pushed: 52
Adding 2 to operandStack
The integer we just pushed: 50
Expression: ( 7 * 4 ) - 2
Adding 7 to operandStack
The integer we just pushed: 55
Adding 4 to operandStack
The integer we just pushed: 52
Adding 2 to operandStack
The integer we just pushed: 50

Process returned 0 (0x0)   execution time : 4.762 s
Press any key to continue.