I dont get functions

**Elite** · 07-31-2003

Hi, I've been trying to get a simple fuction to work for ages, i understand what functions are and what they do i just dont know how to make the give and return values. I've read the chapter on functions over and over plus some tuts on the net but i still dont get it.
I know this is a bit much to ask but could someone please explain exactly how fuctions give and take data.

Heres a peice of something i'm trying to do

Code:

#include <iostream>

int half(int a);

int main()
{
	int a;
	std::cout<<"Enter a number: ";
	std::cin>>a;
	half(int a);
	std::cout<<"The number is now " <<a;
	return 0;
}

inline int half(int a)
{
	a=-5;
	return a;
}

this code gives me these errors

error C2144: syntax error : missing ')' before type 'int'
error C2660: 'half' : function does not take 0 parameters
error C2059: syntax error : ')'

i've tried just about everything i can think of but i just dont get it
Thanks

**...** · 07-31-2003

when you pass a variable to a function within the body of main, you dont have to define that variable again.

IE, instead of passing "int a" to half, just pass "a"

the function prototype and the function definitions just show what type of variable can be passed to that function. you could define your function like this:

int half ( int number );

and then pass it something else:

half( a );

but within the function body, you use whatever is in your function prototype:

int half ( int number )
{
number =number / 2;
return number;
}

**Elite** · 07-31-2003

Thanks ... , but that doest seem to work

example

Code:

#include <iostream>

int half(int letter);

int main()
{
	int a;
	std::cout<<"Enter a number: ";
	std::cin>>a;
	half(a);
	std::cout<<"The number is now " <<a;
	return 0;
}

inline int half(int letter)
{
	letter = / 2;
	return letter;
}

not only do i get this error
error C2059: syntax error : '/'

so i change the / to - just for simplicity
then it compile fine but this is what the program does

enter a number: 50
the number is now 50

do u think it might be my compiler? i'm using msvc++6

**DougDbug** · 07-31-2003

You might take a look as some other books to see if they explain things more clearly.

The only beginning book I have is "Teach Yourself C++ in 21 Days" by Jesse Liberty. I don't have it with me... and I don't remember the specific explainations of functions... but in general, the book is very clearly written.

Functions CAN be confusing at first. Here's something that had me confused: The variable names passed to the function can be different from the variable names inside the function.

For example, this is OK: (but incomplete)

Code:

{
     /...
     int X = 10; // Length
     int Y = 15; // Width 

      A = Area(X, Y);
      /...
}

int Area(int Length, int Width)
{
     return Length * Width;
}

This works because functions (normally) "pass by value". That means that the values10 and 15 are what gets passed-in.

**Elite** · 07-31-2003

Well its starting to make sense now but its just not compiling and when it does the program doesnt work

The book i'm using is called c++ interactive course which was recomended by lots of ppl at gamedev.net

**...** · 07-31-2003

yes, my apologies, i rushed through my reply.

do what salem said and your program will run fine...

scope is very important when youre thinking about functions... whenever you have a set of brackets (such as a loop, a function, or main) any variables you define inside of those brackets only has scope inside of those brackets, meaning no other part of the program can see that variable. when you pass a variable to a function, you arent passing that exact variable, but merely the value of that variable, and then the function makes a copy of it to modify.

when a function returns a value, you must have a variable to "catch" that value, because the copy that the function made dies when the function ends.

like so:

a = half(a);

**laasunde** · 07-31-2003

This should be correct.

Code:

#include <iostream>

int half(int letter);

int main()
{
	int a;
	std::cout<<"Enter a number: ";
	std::cin>>a;
	a = half(a); // one correction here
	std::cout<<"The number is now " << a << endl;
	system("PAUSE");
	return 0;
}

inline int half(int letter)
{
	letter /= 2; // another correction here
	return letter;
}

**Elite** · 07-31-2003

Ahhh, it all makes sense now, sorry i dont know how i missed salems post
Well thanks heaps for that

**DougDbug** · 07-31-2003

The /= stuff is shorthand...

This is the same thing to the compiler, and easier for humans to understand!

number = number / 2;

In C++ (and in most other programming languages) the following will add 10 to X:

X = X + 10;

Note that this is NOT an equation... it makes no sense in math. It is an assignment operation. It means "Assign a new value to X, which is 10 more than the current value."

The shorthand for this in C++ is:
X += 10;

In fact, you will see the += shorthand used in almost all "real" programs. But, to the compiler, they are EXACTLY the same.

**Elite** · 07-31-2003

Thanks for the tip DougDbug

you guys rule

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I dont get functions

Maybe your book sucks!

Notice what ... did with your return-calculation statement:

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