Why exactly does this code work???

BattleGuard · 2003-06-18T16:20:10

Ok, this code works: int _tmain(void) { const int max = 100; long primes[max] = {2, 3, 5}; int count = 3; long trial = 5; bool isprime = true; do { trial += 2; int i = 0; //try dividing the candidate by all the primes we have do { isprime = trial % *(primes + i) > 0; }while(++i < count && isprime); if (isprime) *(primes + count++) = trial; }while(count < max); for (int i = 0; i < max ; i++) { if (i % 5 == 0) cout << endl; cout << setw(10) << *(primes + i); } cout << endl; return 0; } I don''t get it, when they referenced this: *(primes + i) they hadn''t declared "primes" as a pointer, only as a long value. Oh.. And btw this code is from a book in the context of teaching pointer!! BattleGuard My hobbies: Martial Arts, Computer/laptop, Listening to Linkin Park, Computer/laptop, programming, game-dev (that''s why the heck I''m on this site in the first place!!! And did I mention my computer/laptop???

For Beginners

Started by BattleGuard June 18, 2003 03:48 PM

12 comments, last by BattleGuard 20 years, 10 months ago

noixtirdoe

157

June 18, 2003 04:07 PM

primes[0] = 2
primes[1] = 3
primes[2] = 5

if primes is the address of the first element in the array, &primes[0], then *primes is the value that is stored at primes[0], which is 2. the '*' operator dereferences the pointer so you can get the actual value, not the memory address.

the expression *(primes + i) means add i elements to the element primes is pointing to and dereference it, getting the value at the element pointed to by primes + i.

if primes is pointing to the first element, primes[0], then primes+1 = primes[1]. dereference primes[1], *primes, and you get the value 3.

-noix-

In this world gone mad, we won't spank the monkey; the monkey will spank us.

[edited by - noixtirdoe on June 18, 2003 5:09:38 PM]

In this world gone mad, we won't spank the monkey; the monkey will spank us.

Thunder_Hawk

314

June 18, 2003 04:09 PM

quote:Original post by Tiffany Smith
Maybe im missing something but that code doesnt look like it would work to me because "setw" is not defined and there''s no header file.

cout is in there as well...It can be assumed he''s including iostream and has the line "using namespace std;" (or some equivalent construct)

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