CODEWITHSUNDEEP
c++
Quantum Dot
Quantum Dot

Reputation: 395

C++ Defining Global Variable

Very simple question: I was fiddling with basic C++ (being very new to programming) and I got into trouble while declaring a global variable to do some addition

 #include <iostream>
int x,y;
int sum(int, int)
{
    return x + y;
}
int main()
{
    using namespace std;

    cout << "The sum of 10 and 4 is: " << sum(10,4) << endl;
    return 0;
}

Changing "int x,y;" to "int x,y = 0" has the same result: The sum equates to 0. Could someone explain this odd behavior? Thanks!

Upvotes: 2

Views: 3230

Answers (4)

Vishesh
Vishesh

Reputation: 306

when you are just globally declare the variables x,y ,they implicitly set to zero value.in your function definition,you are just giving the datantype of args, not the args names.so when you returning the sum of x,y ,it returns zero.and the value passed by the main function goes nowhere. your program must look like this

#include<iostream>
int x,y;
int sum(x,y)
{
return x+y;
}
int main()
{
int v,a,b;
cout<<"values of a and b";
cin>>a>>b;

v=sum(a,b)
cout<<"their sum is"<<v;
}
    when you explicitly define the value in second case

i.e int x,y=0; you are just explicitly giving the value of value y to 0 while the x implicitly remains 0 and since you are not giving the args name,the ultimately result return biy the function is zero,

Upvotes: 1

Alfred Huang
Alfred Huang

Reputation: 463

Seems that you only need x and y inside your add function, so make them local to the function. There is no reason to make them global. Follow the "least accessibility" idiom to prevent other parts of your program from mistakenedly modifying variables.

You might need a global variable supposed you want to define a well known parameter that every function needs to know and yet modifiable during run time. If you want it fixed, then a global constant would be more proper.

Hope that helps.

Upvotes: 0

user3360398
user3360398

Reputation:

In order to fix the issue, the thing requires changing is the sum function.

int sum(int a, int b){

    return a+b;  //a,b here are referring to the inputs, while what you did was referring to the global variable..
}

Besides, try not to use global variables, usually you would end up with lots of troubles.

Another thing, I don't think your way of defining a function is correct. The inputs have to look like this instead:

int sum(int a, int b)

Unless you wanna declare the function first and provide the actual implementation later, you are not suppose to miss the name of the inputs!

Upvotes: 1

AnT stands with Russia
AnT stands with Russia

Reputation: 320719

Your function always returns the sum of global variables x and y, which are always 0. x and y are implicitly set to zero at the program startup. You never change their values, so they remain zero forever. The sum of two zeros is zero, no surprise here.

You pass 10 and 4 to your function, but the function itself completely ignores what is passed to it, i.e. it ignores its parameters (they are not even named). It always sums global x and y, which are always 0.

If you want your function to sum its arguments, you have to name the function parameters and use them 

int sum(int a, int b)
{
    return a + b;
}

And now you don't need any global variables at all. (main remains as is.)

Alternatively, if you so desire, you can get rid of the parameters completely and sum the global variables instead

int x,y;
int sum()
{
    return x + y;
}

but in this case you will have to pass the values to sum through those global variables, not as function arguments

int main()
{
    using namespace std;

    x = 10;
    y = 4;
    cout << "The sum of 10 and 4 is: " << sum() << endl;
    return 0;
}

This latter approach is here just for illustrative purposes. It is definitely not a good programming practice.

What you have in your code is a weird disconnected hybrid of these two approaches, which can't possibly work.

Upvotes: 3

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