Same 'thing' gives different outputs

[crowandrabbit]

Hi, I'm having trouble with the int lab() part, specifically at the end, when I put in my calculations. What I'm trying to find is the average points of whatever the user inputs in lab().

For example, when I run the program and input in 7 for labs, and 4 for points, I get back 40, however when I try to set that equation into an int variable, I get 10, or sometimes a really large number.

Can anyone explain to me why that is and what is wrong with my code?

  #include <iostream>

using namespace std;

int lab () {
	int labs, point_value, how_many, points, r;
	r = (points*labs)/labs*10;	

	cout << "How many labs?" << endl;
	cin >> labs;

	cout << "Do the point values vary? Enter 0 for no, 1 for yes" << endl;
	cin >> point_value;

	if (point_value == 0){
	   cout << "Enter lab points: " << endl;
	   cin >> points;
	   cout << (points*labs)/labs*10 << endl;
	   cout << r << endl;	   
	   return r;

     }
}


int main () {

	int calculate_what, l;
	cout << "What would you like to calculate an average for?" << endl;
	cin >> calculate_what;
	
	if (calculate_what == 1) {
	l = lab();
	cout << "Your average is: " << l << endl;
}
	 
	return 0;

}

[Mantorr22]

For example, when I run the program and input in 7 for labs, and 4 for points, I get back 40, however when I try to set that equation into an int variable, I get 10.

And your code is :
r = (points*labs)/labs*10;
My impression is that when you enter 7 labs and 4 points, then r = (4 * 7) / 7 * 10 = 28 / 70 = 0.4 not 40.

You should let r become a double so that it can store floating-point values.

Also, the code above should be changed to :
r = (points*labs) / static_cast<double>(labs * 10);

Or :
r = (points*labs) / double(labs * 10);

Integer divided by another integer will always result in an integer not a double.

[Mantorr22]

if (point_value == 0)
{
	   cout << "Enter lab points: " << endl;
	   cin >> points;
	   cout << (points*labs)/labs*10 << endl;
	   cout << r << endl;	   
	   return r;
}

What if point_value is not zero? What value do you want to return?

[TheIdeasMan]

There is a golden rule that one should always initialise all the variables to something.

On line 7 r is some random value, and isn't changed anywhere, so the entire program is reduced to printing 1 random value.

[crowandrabbit]

My impression is that when you enter 7 labs and 4 points, then r = (4 * 7) / 7 * 10 = 28 / 70 = 0.4 not 40.

I think it follows the PEMDAS order of operations, so I believe it would do (4*7) first, then 28 / 7, and then finally 4*10 which would all equal to 40.

What if point_value is not zero? What value do you want to return?

Probably something like:

else if (point_value == 1){
	   while (labs > 0) {
	   cout << "Enter amount of points" << endl;
	   cin >> points;
           p += points;
           cout << p << endl;
           labs--;

and another else statement that loops the code if the user enters something other than 1 or 2.

[Mantorr22]

In addition to TheIdeasMan's post, you input only labs but not points.

cout << "How many labs?" << endl;
cin >> labs;

cout << "How many points?" << endl;
cin >> points;

r = (points*labs)/labs*10; // This line should be here 

[crowandrabbit]

Thank you, I just got it by adding
r = (points*labs)/labs*10
inside the if statement.