System not declared in scope?

Question

I know its simple code, How do I fix "System not declared in scope" problem?

#include<iostream>
using namespace std;

int main(void)
{
    system ( "TITLE Calculator" );
    system ( "COLOR 2" );
    char cChar;
    double dfirstnumber;
    double dsecondnumber;
    char cDoagain;

    do
    {
        system("CLS");
        cout << "Please enter the first number you would like to use."<< endl;
        cin >> dfirstnumber;
        cout<< "Please enter the operation you would like to perform." << " (+,-,*,or /)" << endl;
        cin >> cChar;
        cout<< "Please enter the second number you would like to use." << endl;
        cin >> dsecondnumber;

        switch (cChar)
        {
            case '+':
                cout << "The answer is: " << dfirstnumber << "+" << dsecondnumber << "=" <<
                (dfirstnumber + dsecondnumber) << endl;
                break;
            case '-':
                cout << "The answer is: " << dfirstnumber << "-" << dsecondnumber << "=" <<
                (dfirstnumber - dsecondnumber) << endl;
                break;
            case '*':
                cout << "The answer is: " << dfirstnumber << "*" << dsecondnumber << "=" <<
                (dfirstnumber * dsecondnumber) << endl;
                break;
            case 'x':
                cout << "The answer is: " << dfirstnumber << "x" << dsecondnumber << "=" <<
                (dfirstnumber * dsecondnumber) << endl;
                break;
            case 'X':
                cout << "The answer is: " << dfirstnumber << "X" << dsecondnumber << "=" <<
                (dfirstnumber * dsecondnumber) << endl;
                break;
            case '/':
                if(dsecondnumber == 0){
                cout<< "That is an invalid operation." << endl;}
                else{
                cout << "The answer is: " << dfirstnumber << "/" << dsecondnumber << "=" <<
                (dfirstnumber / dsecondnumber) << endl;

        }
                break;
                default:
                    cout << "That is an invalid operation." << endl;
                    break;
    }
                cout << "Would you like to start again? (Y/N)" << endl;
                cin >>  cDoagain;
    }while (cDoagain == 'Y' or cDoagain == 'y');
    system("PAUSE");
    return 0;
}

Heres my end message:

C:\Documents and Settings\Nilo\My Documents\Work\Testing\main.cpp||In function 'int main()':| C:\Documents and Settings\Nilo\My Documents\Work\Testing\main.cpp|8|error: 'system' was not declared in this scope||

|=== Build finished: 1 errors, 0 warnings ===|

Answer 1

You need to add:

 #include <cstdlib>

in order for the compiler to see the prototype for system().

Answer 2

Chances are that you've not included the header file that declares system().

In order to be able to compile C++ code that uses functions which you don't (manually) declare yourself, you have to pull in the declarations. These declarations are normally stored in so-called header files that you pull into the current translation unit using the #include preprocessor directive. As the code does not #include the header file in which system() is declared, the compilation fails.

To fix this issue, find out which header file provides you with the declaration of system() and include that. As mentioned in several other answers, you most likely want to add #include <cstdlib>