Brackets in the function parts...

[Startering]

So we are learning functions and strings. I have lots of questions, but one that is bugging me and I can't find really is what does the variables (not sure if variables is right word) in the bracket of a function do?

  // example:
  // taken from this site, function section      //http://www.cplusplus.com/doc/tutorial/functions/
#include <iostream>
using namespace std;

int subtraction (int a, int b) // this part I don't understand, and sorry if its // a stupid answer 
{
  int r;
  r=a-b;
  return r;
}

int main ()
{
  int x=5, y=3, z;
  z = subtraction (7,2);
  cout << "The first result is " << z << '\n';
  cout << "The second result is " << subtraction (7,2) << '\n';
  cout << "The third result is " << subtraction (x,y) << '\n';
  z= 4 + subtraction (x,y);
  cout << "The fourth result is " << z << '\n';
}

[48T7M4Gy]

a) take line 16. z is the result of subtraction(7,2)

b) 7 and 2 replace a and b in line 6

c) r in line 8 is a new variable (memory element to store an integer)

d) line 9, r becomes a-b which is 7 - 2 which is 5

e) line 10 returns to line 16 (where subtract was called from) with r = 5. z takes on this vale so z = 5

f) line 17 prints out the value of z, thus 5

[Startering]

Thanks made it more clear, but what does the return thing do? Thanks!

[48T7M4Gy]

If you imagine you are the computer reading your program line by line it starts at line 13 then gets to line 16 which passes to line 6 via the function call along with the variable values as I described.

So then the computer (you in fact this time) follow the recipe from line 6 then line 7 etc until line 10.

At line 10 the return statement tells you to go back to the line where the function was called from i.e. line 16 and proceed from there. The first job is assign the r result to z and then on to line 17 etc.

[Startering]

So basically, it always starts at int main, and as it goes though the lines (14-21) it looks at the rest of the code to see if the other functions have returned it? like the same value? Again sorry for these questions, I'm still learning :3

[TwilightSpectre]

Your program will always start in main. You can think of the functions as 'shorthand' for code. For example:

void myFunc() {
    int x = 5;
    x = x / 2;
    std::cout << x;
}

int main() {
    std::cout << "5 / 2 = ";
    myFunc();
    return 0;
}

// === IS EXACTLY THE SAME AS === //
int main() {
    std::cout << "5 / 2 = ";

    {
        int x = 5;
        x = x / 2;
        std::cout << x;
    }

    return 0;
}

The return statement in a function is used so that you can use the function like a variable; i.e. say a value. For example:

int square(int x) {
    return (x * x);
}

int main() {
    int val = square(7);
    return 0;
}

// === IS EXACTLY THE SAME AS === //

int main() {
    int val = (7 * 7);
    return 0;
}

Does that help explain it?

[Startering]

Kind of, I understand it a lot better now thank you. But "int square(int x)" taken from your 2nd paragraph of code, line 1, I don't understand int x. I'm pretty sure its just like what you would do at the beginning of the program. For example int square(int x); the int x part; would be the same as,

(is it / code? first time)

[/code]
#include <iostream>

using namespace std;
int x; // samething?
int main()
{
for (x=1;x<=10;x++)
{
cout << x << endl;
}
return 0;
}
[/code]

[TwilightSpectre]

No. Think of the parameters in functions as placeholders; i.e. you take a value and call it x. The int specifies that the placeholder value has to be an integer. As an example:

void func(int x) {
    int square = x*x;
    int cube = x*x*x;
    std::cout << x << "^2 = " << square << std::endl;
    std::cout << x << "^3 = " << cube   << std::endl;
}

int main() {
    myFunc(4);
    myFunc(5);
    return 0;
}

// === IS EXACTLY THE SAME AS === //

int main() {
    {
        int square = 4*4;
        int cube = 4*4*4;
        std::cout << 4 << "^2 = " << square << std::endl;
        std::cout << 4 << "^3 = " << cube   << std::endl;
    }
    {
        int square = 5*5;
        int cube = 5*5*5;
        std::cout << 5 << "^2 = " << square << std::endl;
        std::cout << 5 << "^3 = " << cube   << std::endl;
    }
    return 0;
}

The x is part of the function; what it's called makes no difference to your code.

[48T7M4Gy]

Hi Startering

This thread has been hijacked a bit so it might be a good idea to mention the tutorials here that are worth having a look at.

This is one specifically and comprehensively addressing functions http://www.cplusplus.com/doc/tutorial/functions/