Write calculation output in a txt file-CodePudding

This a very beginner question. I'm trying to create a simple c switch case based console calculator that creates a txt file and writes the output to it. I have very little experience with OOPS or C so I have no clue on how to make this work.

This snippet creates a text file and writes to it:

#include <iostream>
#include <fstream>
using namespace std;

int main()
{
    ofstream MyFile("filename.txt");

    MyFile << "Lorem ipsum dolor amet";

    MyFile.close();
}

When I try to add this to my calculator code, it gives me a bunch of errors:

#include <iostream>
#include <fstream>
using namespace std;

int main()
{
    char op;
    float num1, num2;

    cout << "Enter Operator:  , -, *, / : ";
    cin >> op;

    cout << "Enter two Operands: ";
    cin >> num1 >> num2;

    switch(op)
    {
        case ' ':
            cout << num1 << "   " << num2 << " = " << num1   num2;
            ofstream MyFile("filename.txt");
            MyFile << num1 << "   " << num2 << " = " << num1   num2;
            MyFile.close();
        break;

        case '-':
            cout << num1 << " - " << num2 << " = " << num1 - num2;
            ofstream MyFile("filename.txt");
            MyFile << num1 << " - " << num2 << " = " << num1 - num2;
            MyFile.close();
        break;
        
        case '*':
            cout << num1 << " * " << num2 << " = " << num1 * num2;
            ofstream MyFile("filename.txt");
            MyFile << num1 << " * " << num2 << " = " << num1 * num2;
            MyFile.close();
        break;
        
        case '/':
            cout << num1 << " / " << num2 << " = " << num1 / num2;
            ofstream MyFile("filename.txt");
            MyFile << num1 << " / " << num2 << " = " << num1 / num2;
            MyFile.close();
        break;

        default:
            cout << "Error! Operator is not correct";
        break;
    }
    return 0;
}

The error messages in the console:

calctext.cpp: In function 'int main()':
calctext.cpp:25:14: error: jump to case label
   25 |         case '-':
      |              ^~~
calctext.cpp:20:18: note:   crosses initialization of 'std::ofstream MyFile'      
   20 |         ofstream MyFile("filename.txt");
      |                  ^~~~~~
calctext.cpp:27:18: error: redeclaration of 'std::ofstream MyFile'
   27 |         ofstream MyFile("filename.txt");
      |                  ^~~~~~
calctext.cpp:20:18: note: 'std::ofstream MyFile' previously declared here
   20 |         ofstream MyFile("filename.txt");
      |                  ^~~~~~
calctext.cpp:32:14: error: jump to case label
   32 |         case '*':
      |              ^~~
calctext.cpp:20:18: note:   crosses initialization of 'std::ofstream MyFile'      
   20 |         ofstream MyFile("filename.txt");
      |                  ^~~~~~
calctext.cpp:34:18: error: redeclaration of 'std::ofstream MyFile'
   34 |         ofstream MyFile("filename.txt");
      |                  ^~~~~~
calctext.cpp:20:18: note: 'std::ofstream MyFile' previously declared here
   20 |         ofstream MyFile("filename.txt");
      |                  ^~~~~~
calctext.cpp:39:14: error: jump to case label
   39 |         case '/':
      |              ^~~
calctext.cpp:20:18: note:   crosses initialization of 'std::ofstream MyFile'      
   20 |         ofstream MyFile("filename.txt");
      |                  ^~~~~~
calctext.cpp:41:18: error: redeclaration of 'std::ofstream MyFile'
   41 |         ofstream MyFile("filename.txt");
      |                  ^~~~~~
calctext.cpp:20:18: note: 'std::ofstream MyFile' previously declared here
   20 |         ofstream MyFile("filename.txt");
      |                  ^~~~~~
calctext.cpp:46:9: error: jump to case label
   46 |         default:
      |         ^~~~~~~
calctext.cpp:20:18: note:   crosses initialization of 'std::ofstream MyFile'      
   20 |         ofstream MyFile("filename.txt");

CodePudding user response：

The scope of a variable declared inside a switch is the entire switch-block - a case is not a separate scope.
This leads to two problems:

Multiple declarations of the same variable name, and
Jumping across a variable initialization, which you're not allowed to do.

The trivial fix is to wrap each case in a pair of curly braces, adding a new scope:

case ' ':
{
    ofstream MyFile("filename.txt");
    cout << num1 << "   " << num2 << " = " << num1   num2;
    MyFile << num1 << "   " << num2 << " = " << num1   num2;
    // You don't need to explicitly close a file, its destruction takes care of it.
    break;
}

but I would remove the code duplication instead:

bool valid_op = true;
float result = 0;
switch(op)
{
    case ' ':
        result = num1   num2;
        break;
    case '-':
        result = num1 - num2;
        break;
    case '*':
        result = num1 * num2;
        break;
    case '/':
        result = num1 / num2;
        break;
    default:
        cout << "Error! Operator is not correct";
        valid_op = false;
        break;
}
if (valid_op)
{
    ofstream MyFile("filename.txt");
    cout << num1 << ' ' << op << ' ' << num2 << " = " << result;
    MyFile << num1 << ' ' << op << ' ' << num2 << " = " << result;
}

CodePudding user response：

The variables declared in a switch block have their scope as that switch block. So the MyFile object in case is visible in - case. So compiler thinks you have declared a variable with same name again.

You can possibly change the variable name for each case or, do something like this.

int main()
{
    char op;
    float num1, num2;

    cout << "Enter Operator:  , -, *, / : ";
    cin >> op;

    cout << "Enter two Operands: ";
    cin >> num1 >> num2;

    ofstream MyFile;
    switch(op)
    {
        case ' ':
        MyFile = ofstream("filename.txt");
        cout << num1 << "   " << num2 << " = " << num1   num2;
        MyFile << num1 << "   " << num2 << " = " << num1   num2;
        MyFile.close();
        break;

        case '-':
        MyFile = ofstream("filename.txt");
        cout << num1 << " - " << num2 << " = " << num1 - num2;
        MyFile << num1 << " - " << num2 << " = " << num1 - num2;
        MyFile.close();
        break;
        
        case '*':
        MyFile = ofstream("filename.txt");
        cout << num1 << " * " << num2 << " = " << num1 * num2;
        MyFile << num1 << " * " << num2 << " = " << num1 * num2;
        MyFile.close();
        break;
        
        case '/':
        MyFile = ofstream("filename.txt");
        cout << num1 << " / " << num2 << " = " << num1 / num2;
        MyFile << num1 << " / " << num2 << " = " << num1 / num2;
        MyFile.close();
        break;

        default:
        cout << "Error! Operator is not correct";
        break;
    }
    return 0;
}