Changing code inside an unexecuted if block alters my program - c++

Question

I am making a physics-based where balls (gdi+ ellipses) will collide. There are 4 conditions I need to program to make the collisions work correctly. To do this I'm using if statements.
I noticed when debugging that in some cases, the code inside the if block is NOT executed as if the condition was not met - however - changing the code inside that if block will still alter the way my program works.
I've extensively debugged to ensure this is what's happening. I've also switched up the way I'm writing my if statements to ensure the logic is correct, but the problem persists.
When the game is started, I add a couple of balls to a vector.
/**
* Start a new game
*/
void CGame::NewGame()
{
this->mBalls.clear();
shared_ptr<CBall> cueBall = make_shared<CBall>(this);
cueBall->SetX(410.0);
cueBall->SetY(379.0 - (25 / 2) - 110);
cueBall->SetVelocityX(0);
cueBall->SetVelocityY(160.0);
this->mGameItems.push_back(cueBall);
shared_ptr<CBall> ball1 = make_shared<CBall>(this);
ball1->SetX(420.0);
ball1->SetY(379.0 - (25 / 2));
this->mGameItems.push_back(ball1);
}
They are then drawn on the screen by iterating through the vector CGame.mGameItems and calling a Draw function on each one.
/**
* Draw the ball with as a circle
* \param graphics The GDI+ graphics context to draw on
*/
void CBall::Draw(Gdiplus::Graphics* graphics)
{
SolidBrush colorBrush(Color(255, 1000, 1000, 1000));
graphics->FillEllipse(&colorBrush, Rect(this->GetX(), this->GetY(), this->diameter, this->diameter));
}
The redrawing and updating of the window is done in CChildView::OnPaint.
/** This function is called to draw in the window.
*
* This function is called in response to a drawing message
* whenever we need to redraw the window on the screen.
* It is responsible for painting the window.
*/
void CChildView::OnPaint()
{
CPaintDC paintDC(this); // device context for painting
CDoubleBufferDC dc(&paintDC); // device context for painting
Graphics graphics(dc.m_hDC); // Create GDI+ graphics context
mGame.OnDraw(&graphics);
// TODO: Add your message handler code here
// Do not call CWnd::OnPaint() for painting messages
if (mFirstDraw)
{
mFirstDraw = false;
SetTimer(1, FrameDuration, nullptr);
/*
* Initialize the elapsed time system
*/
LARGE_INTEGER time, freq;
QueryPerformanceCounter(&time);
QueryPerformanceFrequency(&freq);
mLastTime = time.QuadPart;
mTimeFreq = double(freq.QuadPart);
}
/*
* Compute the elapsed time since the last draw
*/
LARGE_INTEGER time;
QueryPerformanceCounter(&time);
long long diff = time.QuadPart - mLastTime;
double elapsed = double(diff) / mTimeFreq;
mLastTime = time.QuadPart;
mGame.Update(elapsed);
}
To handle collisions of the objects, a function calls a function called Update() on a CGame object which uses the following code to check if a collision happened.
/** Handle updates for animation
* \param elapsed The time since the last update
*/
void CGame::Update(double elapsed)
{
// Collisions of balls
for (shared_ptr<CBall> ball1 : this->GetBalls())
{
for (shared_ptr<CBall> ball2 : this->GetBalls())
{
double centerXBall1 = ball1->GetX() + ball1->GetDiameter() / 2;
double centerYBall1 = ball1->GetY() + ball1->GetDiameter() / 2;
double centerXBall2 = ball2->GetX() + ball2->GetDiameter() / 2;
double centerYBall2 = ball2->GetY() + ball2->GetDiameter() / 2;
double distance = sqrt((pow((centerXBall2 - centerXBall1), 2) +
pow((centerYBall2 - centerYBall1), 2)));
if (ball1 != ball2 && (distance <= ball1->GetDiameter() || distance <= ball2->GetDiameter()))
{
// The slope of the line that passes through the center of both
// balls
double slopeThroughCenters =
(centerYBall2 - centerYBall1) / (centerXBall2 -
centerXBall1);
// The angle between a vertical and the deflection direction of
// ball 1
double angleOfVelocity1 = 0.0;
// The angle between a horizontal line and the deflection
// direction of ball 2
double angleOfVelocity2 = 0.0;
double complimentaryAngle = atan(slopeThroughCenters);
if (centerXBall1 > centerXBall2 && centerYBall1 > centerYBall2)
{
angleOfVelocity1 = PI / 2 - complimentaryAngle;
angleOfVelocity2 = (PI / 2 - complimentaryAngle) + PI / 2;
}
double initialVelocityBall1 = sqrt(pow(ball1->GetVelocityX(), 2) +
pow(ball1->GetVelocityY(), 2));
double initialVelocityBall2 = sqrt(pow(ball2->GetVelocityX(), 2) +
pow(ball2->GetVelocityY(), 2));
double finalVelocityBall1 = ((ball1->GetMass() - ball2->GetMass()) * initialVelocityBall1) /
(ball1->GetMass() + ball2->GetMass());
double finalVelocityBall2 = (2 * ball1->GetMass() * initialVelocityBall1) /
(ball1->GetMass() + ball2->GetMass());
double velocityXBall1 = finalVelocityBall2 * cos(angleOfVelocity1);
double velocityYBall1 = finalVelocityBall2 * sin(angleOfVelocity1);
double velocityXBall2 = finalVelocityBall2 * cos(angleOfVelocity2);
double velocityYBall2 = finalVelocityBall2 * sin(angleOfVelocity2);
ball1->SetVelocityX(velocityXBall1);
ball1->SetVelocityY(velocityYBall1);
ball2->SetVelocityX(velocityXBall2);
ball2->SetVelocityY(velocityYBall2);
}
}
}
for (auto ball : this->mBalls)
{
ball->Update(elapsed);
}
}
When I run the debugger, the if block:
if (centerXBall1 > centerXBall2 && centerYBall1 > centerYBall2)
{
angleOfVelocity1 = PI / 2 - complimentaryAngle;
angleOfVelocity2 = PI - complimentaryAngle;
}
Doesn't seem to be executed, however, if I change the variable's values, for example,
angleOfVelocity1 = PI;
angleOfVelocity2 = PI;
My program actually changes according to the values of those variables. How is this possible if the debugger shows that the if block isn't executed in the first place?
Sorry if my code isn't minimalistic and concise enough, but I did not want to leave out anything important for solving this problem.