of looking far too closely at the engine physics source, is this a mistake?

trace_t SV_ClipMoveToEntity (edict_t *ent, vec3_t start, vec3_t mins, vec3_t maxs, vec3_t end)
{
��trace_t���trace;
��vec3_t����offset;
��vec3_t����start_l, end_l;
��hull_t����*hull;

// fill in a default trace
��memset (&trace, 0, sizeof(trace_t));
��trace.fraction = 1;
��trace.allsolid = true;
��VectorCopy (end, trace.endpos);

// get the clipping hull
��hull = SV_HullForEntity (ent, mins, maxs, offset);

��VectorSubtract (start, offset, start_l);
��VectorSubtract (end, offset, end_l);

// trace a line through the apropriate clipping hull
��SV_RecursiveHullCheck (hull, hull->firstclipnode, 0, 1, start_l, end_l, &trace);

// fix trace up by the offset
��if (trace.fraction != 1)
����VectorAdd (trace.endpos, offset, trace.endpos);

// did we clip the move?
��if (trace.fraction < 1 || trace.startsolid )
����trace.ent = ent;

��return trace;
}

Quoting from fitzquake source but I think it's unaltered. I've bolded the references to offset, because as far as I can see it never gets initialised. It doesn't actually matter in the call to SV_HullForEntity because that function never reads the offset parameter, just uses it as a local variable for internal calculations!

Still, since it's not passed by reference there presumably the same junk sitting there from the uninitialised starting state. Is it just by fortunate placement on the stack that it always gets zero initialised or something? It looks like it should at least mess up trace_endpos if it really contained garbage, or worse the whole SV_RecursiveHullCheck call... 

It's only evident when you check the definition of vec3_t in q_stdinc.h:

typedef float vec_t;
typedef vec_t vec3_t[3];

vec3_t is an array type, so it is always passed by reference in a function call. In this case, it looks like SV_HullForEntity always writes to the offset variable, so there's no problem.

That is really confusing, though - at first I though vec3_t was a type that is passed by value, like struct { float x; float y; float z}. 

that's what i suspected too, looking at that code. 

Do you write about code elsewhere? 

inertia: No, this is basically my one coding outlet! I've got a little article coming up on names in qc which lead me to look up this function and get confused.

ercw: Thanks! Been away from c coding too long I missed that vector thing. And I really should have seen it given the way that the VectorAdd function works, clearly taking a pass by reference in order to mutate the third parameter. Ah well... 

how does a for loop initialise an integer automatically?

int a;
for (a=1; a<150; a++){cout << a;}


Works, buuuut:


int a; cout << a;


throws a compiler error with


"Run-Time Check Failure #3 - The variable 'a' is being used without being initialized."


Now obviously i could start with:


int a=0;


But what gives the for loop the right to initialise the variable before it begins testing it? 

for (a=1; ...

The a=1 is the part that initializes the variable 

Hmmmmm. I kinda suspected that. I guess I'm mis-understanding the for part. I can't help but think of it as an if.

The excercise which has gotten me confused is the following:

int i, j;
bool isprime;
for(i=1; i<100; i++){
isprime = true;
// see if the number is evenly divisible
for(j=2; j<= i/2; j++)
// if it is then it is not prime
if ((i%j) == 0) isprime = false;
if (isprime) cout << i << " is prime. \n";


It's the modulus test part, and my understanding of prime numbers which has gotten me confused.

I inserted this after the second last 'if':

cout << "i=" << i << ", j=" << j << " ";


This way it shows me what the two variables are. I don't understand why the test works. 

cout << "i=" << i << ", j=" << j << " ";

looks suspiciously like some of the stuff i've been learning about linux and input redirection... 

What exactly don't you understand?
The if (isprime)? If you test a variable without explictly writing eg isprime == 42 it will test if the variable is true (or not false or something like that). 

%

% is what i dont understand lol.

Why is 2%3 equal to 2 ?

Why is 45%89 equal to 45 ? 

http://en.wikipedia.org/wiki/Modulo_operation 

The most useful bit of information I could find on that page was:

^ ISO/IEC 14882:2003 : Programming languages -- C++. 5.6.4: ISO, IEC. 2003. "the binary % operator yields the remainder from the division of the first expression by the second. .... If both operands are nonnegative then the remainder is nonnegative; if not, the sign of the remainder is implementation-defined".

The word "nonnegative" scares me a bit TBH. 

modulus is basically the remainder. like you do long division and stop before going into decimals.

so 2%3 is 2 because 2 can't fit into 3 at all, so you have 2 as the remained.
same with 45%89.

otoh, if you had 3%2, it's 1, because 2 fits into 3 once, and you have 1 left over. 

I'm sure you meant to say: 3 can't fit into 2 at all, that's why 2 % 3 = 2. Look at this, Ricky:

0 % 3 = 0
1 % 3 = 1
2 % 3 = 2
3 % 3 = 0
4 % 3 = 1

etc.

Just try a few examples and you'll get a feel for what the modulo operator does. 

I also figured he meant a three, but I was pretty cross-eyed already ;)

I think I'm starting to understand it. C++ is weird though, because sometimes you get a negative short. Which is weird. BUT I'm beginning to get my head round it. Which means I'm learning :D 

haha, apparently math doesn't fit at all into my head. :P 

You're probably messing up the types (using signed short instead of unsigned or something). 

A real gem from the walkmonster_start_go code I've never noticed before today:

self.ideal_yaw = self.angles * '0 1 0';


Why not just use self.angles_y? Because that wouldn't be vectorised and involve lots of awesome multiplication! 

...because then self.ideal_yaw_x and self.ideal_yaw_z may not be set to 0. 

ideal_yaw is a float though 

It's the dot product, so it returns the sum of all three components once you do the componentwise multiplication. Very handy some of the time, but a bit of a waste here. 

What is it that gives rocket (and grenades) their flight trails? 

There are flags you can set on models which select which (if any) effect that applies. If you open one with QMe it gives you a nice tickbox interface if you bring up the model properties box. 

Thanks Preach