Unexpected results when assigning calculated value to variable

[SkaMan814]

Hi,

I've been following a C++ tutorial, and I always try to play around with each one to get a better understanding. This program just takes a set of numbers and calculates the average. When I perform the calculation in a cout statement, I get the results I expect, however if I assign the calculation to a variable I get some extremely strange results. I have no idea why.

Program:

    int age;
    int ageTotal = 0;
    int numberOfPeople;
    
    cout << "Enter an age, or enter -1 to quit: ";
    cin >> age;
    
    while (age != -1) {
        ageTotal = ageTotal + age;
        numberOfPeople++;
        
        cout << "Enter an age, or enter -1 to quit: ";
        cin >> age;
    }
    
    cout << "Number of people entered: " << numberOfPeople << endl;
    cout << "The average age is: " << ageTotal / numberOfPeople;
    
    // If I add this code, I get unexpected results
    //int average = 0;
    //average = ageTotal / numberOfPeople;
    //cout << "Variable average is: " << average; 

Results without assigning average to variable:

 Enter first persons age or -1 to quit: 10
 Enter next persons age, or -1 to quit: 10
 Enter next persons age, or -1 to quit: -1
 Number of people entered: 2
 The average age of people entered is: 10

Results when I do assign the average to variable:

50 Enter first persons age or -1 to quit: 10
 51 Enter next persons age, or -1 to quit: 10
 52 Enter next persons age, or -1 to quit: -1
 53 Number of people entered: 4197138
 54 The average age of people entered is: 0
 55 Variable average is: 0

Why is this???

[helios]

You never assign any particular value to numberOfPeople before line 10, so its value is undefined. Undefined incremented is still undefined.

[SkaMan814]

@helios, thank you so much. Defining it did fix the problem.

I still don't totally understand why though. If an uninitialized incremented variable is undefined, then why does it return the expected values on lines 16 and 17? Does C++ store variable values in a stack like Java? Is it that it's referencing the value in the stack, but hasn't been assigned a space in memory?

Thanks so much for any info.

[helios]

then why does it return the expected values on lines 16 and 17?

An undefined value is undefined. There's no point in trying to derive meaning from it.

Does C++ store variable values in a stack like Java?

C++ stores some things on the stack, but not quite the same way Java does it.

Is it that it's referencing the value in the stack, but hasn't been assigned a space in memory?

No. The variable is stored on the stack. The stack is the place where its memory was allocated. It's just that the value hasn't been initialized.

[SkaMan814]

@helios thanks again, I appreciate the help!