Space Invaders – 2D Vector Movement Algorithm - c++

Question

I'm building Space Invaders in C++ (using the MBed platform) for a microcontroller. I've used a 2D Vector of object pointers to organise the invaders.
The movement algorithm is below, and runs in the main while loop for the game. Basically, I get the highest/lowest x and y values of invaders in the vector, and use those to set bounds based on screensize (the HEIGHT variable);
I also get the first invader's position, velocity, and width, which I apply changes to based on the bounds above.
Then I iterate through the whole vector again and apply all those changes. It sort of works – the invaders move – but the bounds don't seem to take effect, and so they fly off screen. I feel like I'm missing something really dumb, thanks in advance!
void Army::move_army() {
int maxy = HEIGHT - 20;
int Ymost = 0; // BOTTOM
int Yleast = 100; // TOP
int Xmost = 0; // LEFT
int Xleast = 100; // RIGHT
int first_row = _rows;
int first_column = _columns;
int firstWidth = 0;
Vector2D firstPos;
Vector2D firstVel;
for (int i = 0; i < _rows; i++) {
for (int n = 0; n < _columns; n++) {
bool state = invaders[i][n]->get_death();
if (!state) {
if (i < first_row && n < first_column) {
firstPos = invaders[i][n]->get_pos();
firstVel = invaders[i][n]->get_velocity();
firstWidth = invaders[i][n]->get_width();
}
Vector2D pos = invaders[i][n]->get_pos();
if (pos.y > Ymost) {Ymost = pos.y;} // BOTTOM
else if (pos.y < Yleast) {Yleast = pos.y;} // TOP
else if (pos.x > Xmost) {Xmost = pos.x;} // LEFT
else if (pos.x < Xleast) {Xleast = pos.x;} // RIGHT
}
}
}
firstVel.y = 0;
if (Xmost >= (WIDTH - 8) || Xleast <= 2) {
firstVel.x = -firstVel.x;
firstPos.y += _inc;
// reverse x velocity
// increment y position
}
else if (Ymost > maxy) {
_inc = -_inc;
// reverse increment
}
else if (Yleast < 2) {
_inc = -_inc;
// reverse increment
}
for (int i = 0; i < _rows; i++) {
int setx = firstPos.x;
if (i > 0) {firstPos.y += 9;}
for (int n = 0; n < _columns; n++) {
invaders[i][n]->set_velocity(firstVel);
invaders[i][n]->set_pos(setx,firstPos.y);
setx += firstWidth + 2;
}
}

Answer 1

It looks like you have your assignment cases reversed. Assignment always goes: right <- left, so in the first case you're changing the YMost value, not pos.y. It looks like if you swap those four assignments in your bounds checking it should work. Good luck!