Variables have values after declaring them

Question

Im just starting to learn c++ and I've run into a little problem. After declaring variables they have value assigned to them.

#include <iostream>
#include <fstream>
using namespace std;

ifstream d ("d.txt");
ofstream r ("r.txt");

int plotas (int a, int b);

int main()
{
    int p,
        a,
        n,
        x1,
        x2,
        y1,
        y2,
        s,
        s1;
    d >> p >> a;
    d >> n;
    for(int i =0; i < n; i++){
        d >> x1 >> y1 >> x2 >> y2;
        s+= plotas((x2-x1), (y2-y1));
    }
    s1= plotas(p, a)- s;
    cout << s1;
}
int plotas (int a, int b){
    return a*b;
}

For example variable s is 1967866170. Shouldn't they all be 0? What am I doing wrong?

local atomic (i.e. not classes) variables are not initialized, unless you do it. therefore they may contain a garbage value, and you usually get some warning. — SHR, Apr 27 '15 at 17:01

score 4 · Accepted Answer · edited May 23 '17 at 12:29

4

Local variables that are not assigned any values have what is called Indeterminate Value ( also known as Garbage Value, it is the value that was previously stored in that memory location ( in c and c++) ) and accessing uninitialized variables leads to Undefined Behavior.

If you do not assign them a value, they will be having the garbage value.

But static and global variables have default value as 0

edited May 23 '17 at 12:29

Community

1
1

answered Apr 27 '15 at 16:59

Arun A S

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See Kerrek's comment. – jxh Apr 27 '15 at 17:02
1

@jxh , it's undefined Behavior, I admit that. But this is the general answer. The OP asks why the variables have a value after declaring them, and this is the answer. – Arun A S Apr 27 '15 at 17:04
I don't see anything wrong with your answer, Arun, and I've voted it up accordingly – Breandán Dalton Apr 27 '15 at 17:04
Thanks. What would be the tidiest way to set them all to 0? – PoVa Apr 27 '15 at 17:04
Your not wrong but it might not be useful in this case – Irrational Person Apr 27 '15 at 17:05
@PoVa , `variable(0);` – Arun A S Apr 27 '15 at 17:06
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@PoVa it can be `int x=0;` and also `int x(0);` the 2nd must be in the declaration line. – SHR Apr 27 '15 at 17:07
1

To nitpick, the correct term for "garbage value" is indeterminate value. – chris Apr 27 '15 at 17:27
Yes, you are correct @chris , Indeterminate value is the correct term, but it's just that people are more familiar with the term garbage value. – Arun A S Apr 27 '15 at 17:40

score 3 · Answer 2 · answered Apr 27 '15 at 17:03

http://en.cppreference.com/w/cpp/language/default_initialization

Default initialization is performed in three situations:

1) when a variable with automatic, static, or thread-local storage duration is declared with no initializer.

2) when an object with dynamic storage duration is created by a new-expression with no initializer or when an object is created by a new-expression with the initializer consisting of an empty pair of parentheses (until C++03).

3) when a base class or a non-static data member is not mentioned in a constructor initializer list and that constructor is called.

The effects of default initialization are:

If T is a non-POD (until C++11) class type, the constructors are considered and subjected to overload resolution against the empty argument list. The constructor selected (which is one of the default constructors) is called to provide the initial value for the new object.

If T is an array type, every element of the array is default-initialized.

Otherwise, nothing is done: the objects with automatic storage duration (and their subobjects) are initialized to indeterminate values.

score 0 · Answer 3 · edited May 23 '17 at 12:14

0

Accessing uninitialized variables will lead to undefined behavior. Also, see this answer.

You need to initialize 's' before performing the addition assignment s+= plotas((x2-x1), (y2-y1)).

edited May 23 '17 at 12:14

Community

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answered Apr 27 '15 at 17:06

Robert Blatner

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6

score 0 · Answer 4 · answered Apr 27 '15 at 17:14

0

Simply initialise the variables to prevent them from assuming any garbage value:

int p=0, a=0,n=0,x1=0,x2=0,y1=0,y2=0,s=0,s1=0;

You can also initialise them to any int other than 0 as your code desires.

answered Apr 27 '15 at 17:14

Ayushi Jha

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Variables have values after declaring them

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