Undefined variables have values???

Ok..I'm really confused on this one....why is the output of my program with the source:

Code:

---

#include <iostream>
#include <conio.h>

int main()
{
    int i;
    do
    {
        std::cout << i;
    }while(getch());
return 0;
}

---

output:
4370436

????

I didn't define i with a value.......why is this happening and what does it mean?

Because i actually refers to some location in memory. If it is not zeroed out first or initialized, it will contain spurious data or the data that is currently at the address of 'i'.

The compiler will not initialize variables to zero or any value for that matter - failure to init variables in code can produce errors and in some cases crashes.

>The compiler will not initialize variables to zero or any value for that matter
With the exception of global variables, I believe.

Soooo...basically what your saying is..my compiler is bailing out by defining 'i' with a value of 'i''s memory address?

your compiler isn't bailing out on anything, nor is it giving you the value of the memory address.

When you declare a variable, the compiler goes out and sets some memory blocks to hold the data. When you set it equal to something, it will set those memory blocks equal to the value that you set it to. If you never set it to anything, it can only use whatever was already in those memory blocks.

illustration:

Code:

---

  int i;      // i's memory block might look like this -> [334567]   
  int j;      // j's memory block might look like this -> [552001]

  int i = 23; //i's memory block now looks like this -> [000023]

  cout << i;  //output:  23
  cout << j;  //output:  552001

---

hopefully this will clarify what the others and me are trying to say. Of course, those values are made up, but you should get the idea.

o.o

If the memory block has no value....why is the result that value?

Quote:

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Originally posted by Tronic
o.o

If the memory block has no value....why is the result that value?

---

It does have a value. Just some random one. Maybe the program you ran earlier in the day used that address and that value is left over in there. It's just some garbage value.

Oh, I see :) Thank you.

Just to reinforce this, and the difference between local and global variables, run the following:

Code:

---

#include <iostream>
using namespace std;

int globalInt;
double globalDouble;

int main()
{
    int localInt;
    double localDouble;

    cout << "Local int: " << localInt << endl;
    cout << "Global int: " << globalInt << endl;
    cout << "Local double: " << localDouble << endl;
    cout << "Global double: " << globalDouble << endl;
}

---

You might want to get into the practice of initializing all variables to some meaningful default value, like 0. And for reference, Java and probably C# automatically initialize all variables to some default value if you don't. Even when you're using those languages, you don't want to get into the habit of relying on the compiler to do your work for you.

Quote:

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Originally Posted by UnregdRegd

You might want to get into the practice of initializing all variables to some meaningful default value, like 0.

---

If you're using C++, a better practice than initialising to a default value is to defer declaring your variables until you need them, then initialise at the point of declaration.