Why does a calculation not work when the calculation is placed in the code before the input of the parameters?

Question

The correct answer for the value of x1=7, x2=3, y1=12, y2=9 is supposed to be 5. This code is giving me 5.9... I can't figure out what the problem is.

#include <iostream>
#include <cstdlib>
#include <cmath>

using namespace std;

int main()
{

    int x1, x2, y1, y2;
    double distance;

    distance = sqrt(pow((x2-x1), 2) + pow((y2-y1), 2));

    cout << "Enter x1: ";
    cin >> x1;
    cout << "Enter x2: ";
    cin >> x2;
    cout << "Enter y1: ";
    cin >> y1;
    cout << "Enter y2: ";
    cin >> y2;

    cout << "The distance between two points is: " << distance << endl;

    return 0;
}

Your program exhibits undefined behavior, due to doing calculations on non-initialized variables (i.e. You do your calculation before reading the values). Consider learning from a [good C++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) instead of coding randomly. — Algirdas Preidžius, Feb 08 '18 at 21:50
@MussieTesfay yes. Otherwise the calculations are done before the input on uninitialized variables. Remember, evaluation is done from the top-down. — Arnav Borborah, Feb 08 '18 at 21:52
@MussieTesfay What do you think, given the explanation, that I had provided? — Algirdas Preidžius, Feb 08 '18 at 21:53
The title of this question needs to be rephrased. "Can't figure out the issue" says absolutely nothing — Morgan, Feb 08 '18 at 21:53
@Morgan Someone has already edited it, but is in the review queue — Dijkgraaf, Feb 08 '18 at 21:54
It just didn't feel right to do that. I am used to defining all variables before dealing with anything else. — Mussie Tesfay, Feb 08 '18 at 21:55
When writing a program you do not write mathematical equations, but statements that are executed one by one, so you cannot change order any way you want. — Slava, Feb 08 '18 at 21:57
_I am used to defining all variables before dealing with anything else_ What you did was you `declared` the variables. You `define` them when you assign values to them from console input. — Killzone Kid, Feb 08 '18 at 22:02
Consider using `x * x` rather than `pow(x, 2)`. Often it is more accurate and faster. — Thomas Matthews, Feb 08 '18 at 22:15

score 3 · Accepted Answer · answered Feb 08 '18 at 21:52

Your expectation that:

distance = sqrt(pow((x2-x1), 2) + pow((y2-y1), 2));

will use the values of the variables that the user input is ill-founded. When that line is executed, the variables x1, etc are not initialized. Hence, your program has undefined behavior.

Move that line after the line where you read y2.

// Not good to be here.
// distance = sqrt(pow((x2-x1), 2) + pow((y2-y1), 2));

cout << "Enter x1: ";
cin >> x1;
cout << "Enter x2: ";
cin >> x2;
cout << "Enter y1: ";
cin >> y1;
cout << "Enter y2: ";
cin >> y2;

distance = sqrt(pow((x2-x1), 2) + pow((y2-y1), 2));

Why does a calculation not work when the calculation is placed in the code before the input of the parameters?

1 Answers1