C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Why isn't srand working?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message David Shaw Guest Posted: Wed Mar 03, 2004 7:33 pm    Post subject: Why isn't srand working? Hi, I'm writing a fun little program to play "Petals Around the Roses" with... I need to seed my random numbers so that they won't be the same every time I run the program, but my compiler won't let me. Here's the output my compiler generated...: -- Compiler: Default compiler Executing g++.exe... g++.exe "C:Documents and SettingsrmarkoffDesktoproses.cpp" -o "C:Documents and SettingsrmarkoffDesktoproses.exe" -I"C:Dev-Cppincludec++" -I"C:Dev-Cppincludec++mingw32" -I"C:Dev-Cppincludec++backward" -I"C:Dev-Cppinclude" -L"C:Dev-Cpplib" C:/Documents and Settings/rmarkoff/Desktop/roses.cpp:7: ISO C++ forbids declaration of `srand' with no type C:/Documents and Settings/rmarkoff/Desktop/roses.cpp:7: `int srand' redeclared as different kind of symbol C:/Dev-Cpp/include/stdlib.h:362: previous declaration of `void srand(unsigned int)' Execution terminated -- And here's my code: #include iostream> // I took out the < #include stdlib.h> // in these #includes #include time.h> // so that they will work on the web. using namespace std; srand((unsigned)time(NULL)); int matrix[4]; void display(void) { // Start Display() int x; for(x=0;x<4;x++) { // Randomize matrix matrix[x] = (rand() % 6) + 1; } // End for() std::cout << "n----------------------------------" << std::endl; std::cout << "|" << matrix[0] << "|" << "|" << matrix[1] << "|" << "|" << matrix[2] << "|" << matrix[3] << "|" << matrix[4] << std::endl; std::cout << "----------------------------------" << std::endl; } // end display() int main() { // Start main() display(); std::system("pause"); } // End main .... thanks for any help you can give me. :) - David Shaw Back to top Lew Pitcher Guest Posted: Wed Mar 03, 2004 7:43 pm    Post subject: Re: Why isn't srand working? David Shaw wrote: Quote: Hi, I'm writing a fun little program to play "Petals Around the Roses" Aha! I wrote a k&r C version of this in the mid 80s. Kewl puzzle, ain't it? Quote: with... I need to seed my random numbers so that they won't be the same every time I run the program, but my compiler won't let me. Here's the output my compiler generated...: -- Compiler: Default compiler Executing g++.exe... g++.exe "C:Documents and SettingsrmarkoffDesktoproses.cpp" -o "C:Documents and SettingsrmarkoffDesktoproses.exe" -I"C:Dev-Cppincludec++" -I"C:Dev-Cppincludec++mingw32" -I"C:Dev-Cppincludec++backward" -I"C:Dev-Cppinclude" -L"C:Dev-Cpplib" C:/Documents and Settings/rmarkoff/Desktop/roses.cpp:7: ISO C++ forbids OK, you're off topic for comp.lang.c You are still on topic for comp.lang.c++, though [snip] Followups set to comp.lang.c++ -- Lew Pitcher, IT Consultant, Enterprise Application Architecture Enterprise Technology Solutions, TD Bank Financial Group (Opinions expressed here are my own, not my employer's) Back to top Christopher Benson-Manica Guest Posted: Wed Mar 03, 2004 7:46 pm    Post subject: Re: Why isn't srand working? In comp.lang.c David Shaw <spamthisemail (AT) mailinator (DOT) com> wrote: ^^^^^^^^^^^ Please don't post C++ code to comp.lang.c in the future. Quote: C:/Documents and Settings/rmarkoff/Desktop/roses.cpp:7: ISO C++ forbids declaration of `srand' with no type C:/Documents and Settings/rmarkoff/Desktop/roses.cpp:7: `int srand' redeclared as different kind of symbol using namespace std; srand((unsigned)time(NULL)); Your compiler is trying to tell you something, namely that you can't call functions outside of functions, as you are trying to do. It thinks you're declaring a prototype, and is consequently becoming confused. Quote: #include iostream> // I took out the #include stdlib.h> // in these #includes #include time.h> // so that they will work on the web. Any newsreader that can't handle something simple like <iostream> deserves to be shot anyway - don't bother with it in the future. -- Christopher Benson-Manica | I *should* know what I'm talking about - if I ataru(at)cyberspace.org | don't, I need to know. Flames welcome. Back to top Andrey Tarasevich Guest Posted: Wed Mar 03, 2004 7:48 pm    Post subject: Re: Why isn't srand working? David Shaw wrote: Quote: ... #include iostream> // I took out the #include stdlib.h> // in these #includes #include time.h> // so that they will work on the web. using namespace std; srand((unsigned)time(NULL)); ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ What is this doing here and what did you mean by this? -- Best regards, Andrey Tarasevich Back to top Greg Schmidt Guest Posted: Wed Mar 03, 2004 7:48 pm    Post subject: Re: Why isn't srand working? On 3 Mar 2004 11:33:22 -0800, David Shaw wrote: Quote: Hi, I'm writing a fun little program to play "Petals Around the Roses" with... I need to seed my random numbers so that they won't be the same every time I run the program, but my compiler won't let me. You are trying to call srand outside of any function, which gcc is taking to be an attempt to declare the function prototype. Move the call inside of your main function, and all should be well. -- Greg Schmidt [email]gregs (AT) trawna (DOT) com[/email] Trawna Publications http://www.trawna.com/ Back to top Leor Zolman Guest Posted: Wed Mar 03, 2004 7:51 pm    Post subject: Re: Why isn't srand working? On 3 Mar 2004 11:33:22 -0800, [email]spamthisemail (AT) mailinator (DOT) com[/email] (David Shaw) wrote: Quote: Hi, I'm writing a fun little program to play "Petals Around the Roses" with... I need to seed my random numbers so that they won't be the same every time I run the program, but my compiler won't let me. Here's the output my compiler generated...: -- [snip] Best thing is just to copy and paste the code as-is right into your messages. Don't worry about "<" characters, they won't bite ;-) If you simply move that "srand" call into your main() function from where it is (out at file scope), your program works fine. -leor Leor Zolman BD Software [email]leor (AT) bdsoft (DOT) com[/email] www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix C++ users: Download BD Software's free STL Error Message Decryptor at www.bdsoft.com/tools/stlfilt.html Back to top Mike Wahler Guest Posted: Wed Mar 03, 2004 7:52 pm    Post subject: Re: Why isn't srand working? "David Shaw" <spamthisemail (AT) mailinator (DOT) com> wrote Quote: Hi, I'm writing a fun little program to play "Petals Around the Roses" with... I need to seed my random numbers so that they won't be the same every time I run the program, but my compiler won't let me. Here's the output my compiler generated...: -- Compiler: Default compiler Executing g++.exe... g++.exe "C:Documents and SettingsrmarkoffDesktoproses.cpp" -o "C:Documents and SettingsrmarkoffDesktoproses.exe" -I"C:Dev-Cppincludec++" -I"C:Dev-Cppincludec++mingw32" -I"C:Dev-Cppincludec++backward" -I"C:Dev-Cppinclude" -L"C:Dev-Cpplib" C:/Documents and Settings/rmarkoff/Desktop/roses.cpp:7: ISO C++ forbids declaration of `srand' with no type C:/Documents and Settings/rmarkoff/Desktop/roses.cpp:7: `int srand' redeclared as different kind of symbol C:/Dev-Cpp/include/stdlib.h:362: previous declaration of `void srand(unsigned int)' Execution terminated -- And here's my code: #include iostream> // I took out the Don't. Quote: #include stdlib.h> // in these #includes #include time.h> // so that they will work on the web. It's not 'the web', but some news readers 'mangle' such characters. That's their problem, not yours. Post compilable code if at all possible. Quote: using namespace std; srand((unsigned)time(NULL)); This statement doesn't do what you probably think it does. It does *not* invoke the 'srand()' function but (incorrectly) declares it. (Executable statements are not allowed outside of a function). This declaration conflicts with the correct one provided by <stdlib.h>. Remove this line. Call 'srand()' at the beginning of 'main()'. Quote: int matrix[4]; void display(void) { // Start Display() int x; for(x=0;x<4;x++) { // Randomize matrix matrix[x] = (rand() % 6) + 1; See the C FAQ for a better way to generate random numbers. Quote: } // End for() std::cout << "n----------------------------------" << std::endl; std::cout << "|" << matrix[0] << "|" << "|" << matrix[1] << "|" "|" matrix[2] << "|" << matrix[3] << "|" matrix[4] << std::endl; std::cout << "----------------------------------" << std::endl; } // end display() int main() { // Start main() srand((unsigned int)time(0)); Quote: display(); std::system("pause"); Note that while 'system()' is standard, its argument is not. Quote: } // End main ... thanks for any help you can give me. :) - David Shaw Finally, note that your code is topical for comp.lang.c++, but not for comp.lang.c. I've removed the latter from the 'newsgroups' list. -Mike Back to top Ben Pfaff Guest Posted: Wed Mar 03, 2004 7:55 pm    Post subject: Re: Why isn't srand working? [email]spamthisemail (AT) mailinator (DOT) com[/email] (David Shaw) writes: Quote: #include iostream> // I took out the #include stdlib.h> // in these #includes #include time.h> // so that they will work on the web. Don't post C++ to comp.lang.c. -- Go not to Usenet for counsel, for they will say both no and yes. Back to top Malcolm Guest Posted: Wed Mar 03, 2004 8:00 pm    Post subject: Re: Why isn't srand working? "David Shaw" <spamthisemail (AT) mailinator (DOT) com> wrote in message Quote: using namespace std; srand((unsigned)time(NULL)); Either move this to main() or, if you want to play with C++, declare an object class Randomiser { public: Randomiser(void) { srand( (unsigned) time(NULL)); }; }; Randomiser myengine; This will cause the call to srand to be executed some time at program startup. It's useful if you want to provide a library that relies on seeding rand(), but doesn't have access to main(). Back to top Keith Thompson Guest Posted: Wed Mar 03, 2004 9:51 pm    Post subject: Re: Why isn't srand working? [email]spamthisemail (AT) mailinator (DOT) com[/email] (David Shaw) writes: [...] Quote: And here's my code: #include iostream> // I took out the #include stdlib.h> // in these #includes #include time.h> // so that they will work on the web. You're not on the web. This is Usenet, which has no problem with '<' and '>' characters. Usenet is a plain text medium; HTML is largely irrelevant. And, as others have mentioned, your code is C++, so it's off-topic in comp.lang.c. -- Keith Thompson (The_Other_Keith) [email]kst-u (AT) mib (DOT) org[/email] <http://www.ghoti.net/~kst> San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst> Schroedinger does Shakespeare: "To be *and* not to be" Back to top David Shaw Guest Posted: Thu Mar 04, 2004 6:43 am    Post subject: Re: Why isn't srand working? <snippy> Many apologies, I really haven't used usenet all that much... when I had, it was usually about functions common to C and C++... sorry to the comp.lang.c readers who must have gotten a little miffed at me : Okay, that makes sense... I'll stick that back into a function, rather than having it in the very beginning of the program. On another note, what exactly does srand() *do*? All I know is that using rand() after it produces seeded random numbers, which are different each time the program's run, and it has something to do with the system clock... can anyone help me with this one? It's quite frustrating to not be able to fix things since you've no idea what you're supposed to be doing. Oh, and about the previous < withdrawl... I was using GoogleGroups, and forgot that... meh, it doesn't matter. Thanks for the help everyone, and sorry *.c users... heh. - David http://david.artoo.net/ Back to top Mike Wahler Guest Posted: Thu Mar 04, 2004 9:19 am    Post subject: Re: Why isn't srand working? "David Shaw" <spamthisemail (AT) mailinator (DOT) com> wrote Quote: snippy Many apologies, I really haven't used usenet all that much... when I had, it was usually about functions common to C and C++... sorry to the comp.lang.c readers who must have gotten a little miffed at me : Okay, that makes sense... I'll stick that back into a function, rather than having it in the very beginning of the program. On another note, what exactly does srand() *do*? It seeds the random number generator. Quote: All I know is that using rand() after it produces seeded random numbers, which are different each time the program's run, Only if the seed is different. Also note that 'rand()' necessarily generates only 'psudo-random' numbers, which are sufficient for many applications. Quote: and it has something to do with the system clock... No it does not. You're getting confused because often folks use the system time as the seed. This is simply because it's the most 'random' value which can be generated with standard code. Quote: can anyone help me with this one? This one what? Quote: It's quite frustrating to not be able to fix things since you've no idea what you're supposed to be doing. What's broken? -Mike Back to top LNK2005 Guest Posted: Thu Mar 04, 2004 9:48 am    Post subject: Re: Why isn't srand working? "David Shaw" <spamthisemail (AT) mailinator (DOT) com> skrev i meddelandet news:10cd2710.0403032243.20b4793b (AT) posting (DOT) google.com... Quote: On another note, what exactly does srand() *do*? All I know is that using rand() after it produces seeded random numbers, which are different each time the program's run, and it has something to do with the system clock... can anyone help me with this one? What it does *exactly* (what algorithm is used) is probably implementation specific (I think?). Numbers like this aren't really random; rand() will produce an identical sequence of "random" numbers the next time you call srand() with the same argument. The system clock has nothing to do with generating the numbers, it's just that people usually pass the current time as an argument to srand(), to make rand() more unpredictable. For true non-deterministic random numbers, you'll need some source of "natural" randomness, like static noise from a radio, radioactive decay, blob movements in a lava lamp, etc. See www.random.org for more info. Just to give a hint what pseudo-random generator algorithms do, here's an extremely simple (and extremely crappy) one: - To startup, let randno = seed, where seed is some float between 0 and 1 - To get the next "random" number, let randno = fractional part of (147 * randno) However, I'm pretty sure this is *not* how srand() and rand() are typically implemented;-) Back to top Karl Heinz Buchegger Guest Posted: Fri Mar 05, 2004 2:28 pm    Post subject: Re: Why isn't srand working? David Shaw wrote: Quote: And here's my code: #include iostream> // I took out the #include stdlib.h> // in these #includes #include time.h> // so that they will work on the web. It would have worked anyway "on the web". Most importantly because this is *not* the web. This is usenet and we use text only. More to the point of your post. Don't mix old style and new style headers. This is a source of confusion. #include <iostream> #include <cstdlib> #include <ctime> Quote: using namespace std; srand((unsigned)time(NULL)); The above is a function call. But function calls (unless they are used to initialize some global variables) need to be in some function!!! int main() { srand((unsigned)time( NULL )); display(); ... } -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top CBFalconer Guest Posted: Fri Mar 05, 2004 4:48 pm    Post subject: Re: Why isn't srand working? Karl Heinz Buchegger wrote: Quote: David Shaw wrote: And here's my code: #include iostream> // I took out the #include stdlib.h> // in these #includes #include time.h> // so that they will work on the web. It would have worked anyway "on the web". Most importantly because this is *not* the web. This is usenet and we use text only. More to the point of your post. Don't mix old style and new style headers. This is a source of confusion. #include <iostream #include #include Please do not post c++ code to c.l.c. FUPs set. -- Chuck F (cbfalconer (AT) yahoo (DOT) com) (cbfalconer (AT) worldnet (DOT) att.net) Available for consulting/temporary embedded and systems. 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