declaring variables inside loops

This is a discussion on declaring variables inside loops within the C Programming forums, part of the General Programming Boards category; And the only problem I have is that if you declare an initialized variable inside the for loop, it will ...

  1. #16
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    And the only problem I have is that if you declare an initialized variable inside the for loop, it will be initialized in every single loop. So... its not good in some cases.
    The variable will be reinitialized for each loop if you indicated to do so (number=0 below), this is
    not the same problem than reallocating it and except if you turn on the optimizer, there is nothing
    the compiler can (should) do here.

  2. #17
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    how about declaring it
    Code:
    static int number = 0;
    I guess (not sure) it would not initialize it each time it enters the loop. Somebody please confirm :-)

    maverix

  3. #18
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    Quote Originally Posted by maverix View Post
    how about declaring it
    Code:
    static int number = 0;
    I guess (not sure) it would not initialize it each time it enters the loop. Somebody please confirm :-)

    maverix
    Correct, it will not initialize static variables for each loop iteration (or for every call to a function for example). The compiler MAY, however, generate less good code with static variables, since there is (theoretically) a possibility that they are modified outside of the function without the compiler knowing about it. I haven't studied that much - but essentially, a static variable is a global. However, since it's limited scope, I suppose the compiler could know if the address of the static is ever exported out of the function, and thus whether it is susceptible to modification outside of the function or not.

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  4. #19
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    Some compiler optimizers might not be as good as others, and in debug mode it won't optimize anything.
    I use MinGW, how do I know if it is in debug mode? I don't use (or know how to use) a debugger, so I want to make sure that they are out of debug mode for now.

    how about declaring it

    Code:
    static int number = 0;
    I guess (not sure) it would not initialize it each time it enters the loop. Somebody please confirm :-)

    maverix
    Correct, it will not initialize static variables for each loop iteration (or for every call to a function for example).
    I supposed that the same goes for an if block right? Every time it enters the if, it won't reinitialize if I declare it static?

  5. #20
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    gcc (which is the actual compiler part of MingW) needs the switch -O to optimize, and -O2, -O3 and -O4 will make the optimization increasingly more "aggressive" (and sometimes means that code either won't work, or actually run slower). Debug mode (-g in gcc) is actually not really affecting anything other than generation of information that the debugger (gdb) can use to figure out where in the code you are executing and where to find the variables and such.

    Static variables are initialized (to zero if nothing else is given as initialization) when the executable file is loaded, that's it - never ever again.

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  6. #21
    and the hat of sweating
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    gcc Optimization Options
    -O3 is the highest one I see.

  7. #22
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    Quote Originally Posted by cpjust View Post
    gcc Optimization Options
    -O3 is the highest one I see.
    Yes, I think -O4 is actually equal to -O3 on older compilers - but gcc has moved on from 3.2.3 you know. The latest version is 4.4 or so (sorry, too lazy to look up the current latest version).

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