Thread: rand() & srand() not working as i want them to...O-o

  1. 06-27-2012 #1
    Max Winzar
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    Question rand() & srand() not working as i want them to...O-o

    Hey everyone it's meh the noob again... (you "sigh" and roll your eyes)

    I have abit of a problem with the rand() and srand()
    functions

    i looked through some forum searches for a solution to my first problem..

    Problem (1):
    When i use something like
    Code:
    int number_of_noobs_in_this_thread = rand()%3;
    it keeps coming up with the same number
    now i found out why (partly) it's because of something about the seed and that each hardware randoms up the same number (or something like that)

    so i look to the forums and found out about srand()

    i was like "Finally something that actually works"
    until this : 107|error: 'time' was not declared in this scope|

    i had used the srand(time(NULL));
    to see if that works and without it it goes

    note: declared here

    but on line 353 ??? wtf???

    so now im stuck and no i have only used srand() once and my
    #include <iostream>
    #include <cstdlib>

    are all i have in it ???

    Please help ?
  2. 06-27-2012 #2
    Salem
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    You also need
    #include <ctime>
    to prototype the time function.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
  3. 06-27-2012 #3
    Max Winzar
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    Quote Originally Posted by Salem View Post
    You also need
    #include <ctime>
    to prototype the time function.
    ...

    ...

    ...

    oh ... well it didnt say that anywhere?

    but thanks anyway

    hopefully i start to get better at this
  4. 06-27-2012 #4
    grumpy
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    Quote Originally Posted by Max Winzar View Post
    ...
    oh ... well it didnt say that anywhere?
    Virtually every bit of documentation I have seen about the time() function says it is in <time.h> (in C) or <ctime> (in C++).

    Granted, it is sometimes left out of examples about the srand() and rand() functions because - unfortunately - some implementations incorrectly declare it in <stdlib.h> (C) or <cstdlib> (C++).

    Basic lesson though: all functions and types in the standard library are declared in a standard header. So, if you are using a standard function as intended (right number and type of arguments, etc) and the compiler chokes, then you forgotten to #include the relevant header. From there it is not hard to find what the right header is by looking at relevant documentation (even google helps if you give it "C++ time", with a link to the ctime header).
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy or unhelpful in reply to you, or tell you you need to demonstrate more effort before you can expect help, it is likely you deserve it. Suck it up, Buttercup, and read this, this, and this before posting again.
  5. 06-28-2012 #5
    Elysia
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    You will have to remember that everytime you use a standard library function, you must look up in what header it resides in and add it to your list of includes.
    Also, if your compiler supports it, prefer using nullptr instead of NULL (MSVC 10+, GCC 4.6+ supports it).
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.
  6. 06-28-2012 #6
    Rodaxoleaux
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    Quote Originally Posted by Elysia View Post
    You will have to remember that everytime you use a standard library function, you must look up in what header it resides in and add it to your list of includes.
    Also, if your compiler supports it, prefer using nullptr instead of NULL (MSVC 10+, GCC 4.6+ supports it).
    C++11 to the rescue. dan dandan daaaaaaan!
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