random number between negative and positive number

This is a discussion on random number between negative and positive number within the C++ Programming forums, part of the General Programming Boards category; Hi I've seen some other threads on this and the faq, but I can't seem to find the answer to ...

  1. #1
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    random number between negative and positive number

    Hi

    I've seen some other threads on this and the faq, but I can't seem to find the answer to this one.

    I want to generate a random number (non-integer) between a negative number (such as -1) and a positive number (such as 1). So I want a random number of type double between -1 and 1, how do I do this? Appreciate it if someone could guide me or point me to some useful info. Thank you.

  2. #2
    and the hat of wrongness Salem's Avatar
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    Like for example
    rand() % 3 - 1
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    Registered User glUser3f's Avatar
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    anomaly is asking for a non-integer, one way would be:

    2.0f * ((float)rand() / (float)RAND_MAX) - 1.0f

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    Hi

    Thanks for the example code. Could you also explain the syntax to me? I'm fairly new to c++, so I don't completely understand the syntax yet. Thanks

    More specifically, why do you place an 'f' after a number?
    Last edited by anomaly; 12-06-2003 at 07:02 AM.

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    Registered User glUser3f's Avatar
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    the function 'rand()' returns a random integer between 0 and RAND_MAX, so to get a random number between 0 and 1 we can use rand() / RAND_MAX since rand() is always less than RAND_MAX, and you know, when using / with integers we get an integer, so we need to convert our integers to double or float first:
    (double)rand() / (double)RAND_MAX
    Now, this will give us a random between 0 and 1, to get a random number between -1 and 1:

    2 * (double)rand() / (double)RAND_MAX - 1

    hope you got it now.

    f in 2.0f is to tell the compiler that 2.0f is a float and not a double (this would be 2.0)

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    Thanks I already worked out how the code worked before, but thanks for shedding light on the 'f' syntax.

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    Registered User glUser3f's Avatar
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    np there are also u for unsigned, L for long.

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