do...while loop problem

Hi there,
I really can't understand why the do while loop doesn't work for me.
I have this simple program

Code:

//Calculation 1.3: this includes a do...while loop.
#include <iostream>
using std::cout;
using std::cin;
using std::endl;

class Operation
{
public:
void displayMessage()
	{
	cout <<"Welcome to Calculation 1.3. \nThis program contains a do...while loop.\n";
	}
void displayMessage1()
	{
	int a (0);
	int b (0);
	int sum;
	sum=a+b;
	
	do
		{
		cout <<"Insert your first number: \n";
	cin >> a;
	cout <<"Insert your second number: \n";
	cin >> b;
	cout << "The sum of your number is: "<< sum;
		}
		while (a>=5 && b<=10);
	}
};

int main()
{
Operation calculation;
calculation.displayMessage();
calculation.displayMessage1();
return 0;
}

The actual loop does work but it doesn't calculate the sum, it alway comes up with a sum of 0 and I can't understand why. I have been trying for hours, but I bet it is a silly error.
Any idea at all?
thanks

Code:

int a (0);
int b (0);
int sum;
sum=a+b;

This is why it always says sum is 0. You never update sum again after this.
sum stores the result of a+b. It doesn't store the formula itself. So every time you change a or b you have to update sum or it will be inaccurate.

uhm, I am not sure I understand this, sorry but I am fairly new to C++. Could you be a bit more specific? Is there a line of code I should use to update sum?

Something like

Code:

...
sum = a + b;
cout << "The sum of your number is: "<< sum;

You must add

Code:

sum=a+b;

inside your loop in order to change the sum value every time you insert the two numbers.

if you are new to C++ why are starting out with functions in classes?? You might wanna hold off on the object oriented side until you can get past functions and loops.

C++ is not a declarative language. You don't tell it how a variable is related to some other variables and then expect it to do this by itself if and when necessary. In C++ it does only what you tell it to, when you tell it to do it.
I.e. it only adds 'a' and 'b' before the loop because that's where you told it to.

Hi guys,
thank you for your help. I followed your instructions so that my loop now looks like this:

Code:

do
		{
		cout <<"Insert your first number: \n";
	cin >> a;
	cout <<"Insert your second number: \n";
	cin >> b;
	sum=a+b;
	cout << "The sum of your number is: "<< sum;
		}
		while (a>=5 && b<=10);
	}

I didn't know that I had to add

Code:

sum=a+b;

in the loop, I thought that being into a function was enough. SO basically, even if a loop is inside a function (in my case inside

Code:

void displayMessage1()

function) is a something separate, a different "entity" from the function. Is this for all the types of loops?So I don't even need to say

Code:

sum=a+b;

outside the function, do I?

On a different note:

if you are new to C++ why are starting out with functions in classes?? You might wanna hold off on the object oriented side until you can get past functions and loops.

I have been given as a manual to study C++, "Deitel and Deitel: C++ how to program" and they start with classes straight away. I found this book quite difficult to be honest and I am now learning using internet tutorials (and this website's), so I found myself with a very little knowledge of classes, so I thought to myself, might be worth using them :-)