Thread: Small question: Why do I get a Debug error (variable not initialized)?

Notice the following program:

Code:

#include <iostream>
using namespace std;

int main(){

    for (int i = 0; i < 5; i++) {
        bool variable;
        
        if (i == 0)
            variable = true;

        if (variable)
            cout << "variable is true\n";
        else
            cout << "variable is false\n";
    }

    cin.get();
    return 0;
}

(I'm using Microsoft Visual Studios to compile)

Here, when I try to compile the program, I get a Debug error (attached below).


Now if I click on the "ignore" (which I would have to do 5 times, one for every time the compiler reaches the if statement), I get the desired output which is "variable is true", 5 times.

So technically the program is working but why do I get this debug error? What's an alternative and or how do I stop this kind of response and does it have a purpose?

Thanks for reading, Cheers!

You need to initialize the variable "variable" before you try to use it, which you're not doing in most iterations of the loop. Realize that this variable is a different instance every time you go through that loop and you only initialize the first instance.

By the way if you hit "Retry" instead of "Ignore" you would go into the debugger and then you might be able to see the problem.

Originally Posted by jimblumberg

You need to initialize the variable "variable" before you try to use it, which you're not doing in most iterations of the loop. Realize that this variable is a different instance every time you go through that loop and you only initialize the first instance.

By the way if you hit "Retry" instead of "Ignore" you would go into the debugger and then you might be able to see the problem.

But variable is assigned true in the first iteration itself.. So it would be true for all iterations, which it was when I ignored the debug error. I even tried declaring variable as false outside the loop to test what you said.. but got the same output.

Hitting retry just led to the if statement

Originally Posted by john.c

But what about the later iterations? Since you've declared it inside the loop body, variable is not guaranteed to hold it's last assigned value between loop iterations. According to the standard it's basically a new variable every iteration, although most systems probably implement it in such a way that your program would "work". But it's still technically incorrect and it could possibly fail on some systems.


If you want variable to retain it's last set value between loop iterations then you need to declare it (and preferably initialize it) before the loop.

Code:

#include <iostream>
using namespace std;
  
int main(){
    bool variable = true;
    for (int i = 0; i < 5; i++) {
        if (variable)
            cout << "variable is true\n";
        else
            cout << "variable is false\n";
        if (i >= 2)
            variable = false;
    }
    return 0;
}

Originally Posted by laserlight

To illustrate what john.c is saying, it might be helpful to initialise to false, then compile and run the program:

Code:

#include <iostream>
using namespace std;
 
int main(){
 
    for (int i = 0; i < 5; i++) {
        bool variable = false;
         
        if (i == 0)
            variable = true;
 
        if (variable)
            cout << "variable is true\n";
        else
            cout << "variable is false\n";
    }
 
    cin.get();
    return 0;
}

OHHH so what is happening is that 'variable' is being re-declared on the second iteration because of the statement 'bool variable'? So that means it will delete the old variable and create a new variable whose value isn't initialized?

Thanks a lot laserlight your example really helped! Thanks jimblumberg and john.c I really appreciate that you took your time to help me !

Okay back to the program, I intended on declaring the variable inside the for-loop so it would be local to the loop. So the variable would be deleted after the for-loop.
An alternative is declaring the variable outside the loop and forming a block which holds the declaration and the for-loop so that the variable is local to the block and gets deleted after the block ends (if I understand this correctly).

But I was thinking about instead using 'delete variable;' so does this make sense?

Any tips guys? The main reason I'm doing this is to save memory.