Thread: Help with "cin" in C++

When you declare a variable inside a function, once the function ends, the variable is destroyed and no longer exists, so you can't refer to the variable outside the function, i.e. in main().

You either have to declare the variable in main() and pass it to the function, or return a value from the function which you store in another variable.

Here is a very simple function:

Code:

#include <iostream>
using namespace std;

void showNumber()
{
   cout<<100<<endl;
}

int main ()
{
   showNumber();

   return 0;
}

Functions must specify a 'return type', and the return type precedes the function name. In the function above, there is no return value, and that is designated by 'void'. The function is 'called' in main() by this line:

showNumber();

That says, "please go find a function named showNumber, and execute the lines of code contained therein."

Here is another example:

Code:

#include <iostream>
using namespace std;

void showNumber(int a)
{
   cout<<a<<endl;
}

int main ()
{
   showNumber(100);
   showNumber(25);

   return 0;
}

Now, instead of always displaying the same number, you can send the function a specific number, and the function will display whatever number you send it. Once again, the function does not return a value, so its return type is 'void'. You should note that the type of the number you send to the function must match the function 'parameter'. The function parameter above is "int a", which means the function is expecting an int type to be 'sent' to it. You send a value to a function like this:

showNumber(100);

To send a value to a function, you just put the value between the parentheses after the function name. The function parameter "int a" creates an integer variable named "a" which stores the value you send to the function--that's why the type of the value you send has to match the function parameter. Then, inside the function you can use the variable "a" by referring to it by name:

cout<<a<<endl;

Here is another example:

Code:

#include <iostream>
using namespace std;

int addTen(int a)
{
   int answer = a + 10;
   return answer;
}

int main ()
{
   int result1 = addTen(2);
   int result2 = addTen(5);

   cout<<"Here are the results: "<<result1<<" "<<result2<<endl;
   
   return 0;
}

This time the function returns a value. The return type preceding the function name must match the type of the value in the 'return statement'. The return statement replaces the function call in main(), e.g. addTen(2), with the return value. So, result1 is assigned 12, and result2 is assigned 15.

To summarize:
1) A function must list a return type before its name.
2) The return type must match the type of the value in the return statement--if there is no return statement, then the type is 'void'.
3) When you call a function, the value you send it, must match the function parameter.
4) The function call in main() is replaced by the function return value.
5) Sometimes if a function doesn't have any parameters, like in the first example, "void" will be used for the parameter. For instance you might see the function in the first example written like this:

Code:

void showNumber(void)
{
   cout<<100<<endl;
}

When you get a handle on 7studs reply then you can learn the other way to accomplish transfer of information between functions---passing by reference (using either a pointer or a reference) as opposed to passing by value and using a return value to accomplish the transfer (which is what 7stud told you about, and which you should understand very well before moving on, IMO).

C++ is a standardized language. That means there is a governing body that decides what can and can't be part of the C++ language. Therefore, things like what parameters a function has are published as part of the documentation of the C++ language. What compiler you use is irrelevant.

However, for various reasons, the compilers the sofware companies distribute are not always in complete compliance with the C++ standards. But, until you know otherwise, you can assume your compiler follows the standards(and it will in almost all cases).

There is a site called www.google.com that can help you complete todays homework assignment.

Do not call main. Use a loop instead.