Issue with array (if (zeds[i].x == zeds[i].x)) - c++

Question

I've created a simple game in C++ in which a player must avoid being bitten by zombies to survive.
The zombies are stored as Z characters in an array called zeds.
I am trying to write a function that can check if any zombies collide, and act accordingly.
I've written this if statement:
for (int i; i < MAXZEDS; ++i);
if (zeds[i].x == zeds[i].x && zeds[i].y == zeds[i].y)
{
--zlives;
updateLives();
}
Obviously it doesn't work, I'm just wondering if I can somehow rewrite this to only decrement zlives if a zombie collides with another zombie but NOT itself.
Apologies for the title I couldn't find a better way to summarize my issue. Thanks in advance for any help.

Answer 1

You need a double loop so you can compare two different zombies.
Something like this.
for (int i = 0; i < MAXZEDS - 1; ++i)
{
// Looping from i + 1 ensures that we only test each pair once.
for (int j = i + 1; j < MAXZEDS; ++j)
{
if (zeds[i].x == zeds[j].x && zeds[i].y == zeds[j].y)
{
--zlives;
updateLives();
}
}
}

Answer 2

The simplest way is 2 loops.
for (int i = 0; i < MAXZEDS; ++i) {
for (int j = i+1; j < MAXZEDS; ++j) {
if (zeds[i].x == zeds[j].x && zeds[i].y == zeds[j].y)
{
--zlives;
updateLives();
}
}
}
If the object order in zeds is not important, and operator<() is defined correctly for the element type of zeds, then you could sort zeds and check consecutive elements to get lower complexity.

Answer 3

Try using nested for loops:
for (int i = 0; i < MAXZEDS; i++)
{
for (int j = 0; j < MAXZEDS; j++)
{
if (i == j) continue; // Don't compare the current zed with itself
else if (zeds[i].x == zeds[j].x && zeds[i].y == zeds[j].y)
{
--zlives;
updateLives();
}
}
}
This iterates through zeds twice and compares each item in the array with each other item. I am sure there are more efficient ways, but this is the simplest.