Reverse an array with preprocessor

Question

I want to use pre-processor to fill some arrays in some ways. I can use preprocessor only for newly declared arrays. However, I need to change the array p which I declared and used before. Time optimization is very important for my purpose.

 #define Reverse(x) {x[63], x[62], x[61], x[60], x[59], x[58], x[57], x[56], x[55], x[54], x[53], x[52], x[51], x[50], x[49], x[48], x[47], x[46], x[45], x[44], x[43], x[42], x[41], x[40], x[39], x[38], x[37], x[36], x[35], x[34], x[33], x[32], x[31], x[30], x[29], x[28], x[27], x[26], x[25], x[24], x[23], x[22], x[21], x[20], x[19], x[18], x[17], x[16], x[15], x[14], x[13], x[12], x[11], x[10], x[9], x[8], x[7], x[6], x[5], x[4], x[3], x[2], x[1], x[0] }


    int main()
    {
    int p[64] =  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };

    int q[64] =  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };

    //doThings

    p= Reverse(q); // line A - Gives error

    int s[64] = Reverse(q); // line B - Works properly 

    //doThings
}

I got this error:

Error 11 error C3079: an initializer-list cannot be used as the right operand of this assignment operator c:\users\ferda\documents\visual studio 2013\projects\consoleapplication3\consoleapplication3\consoleapplication3.cpp 39‌3 1 ConsoleApplication3

You can't assign to an array in C! you have to use loop or memcpy — Garf365
– Garf365, Commented Jun 1, 2016 at 8:35
Hi I already attempted to use memcpy but it didn't work with the preprocessor. — Ferda-Ozdemir-Sonmez
– Ferda-Ozdemir-Sonmez, Commented Jun 1, 2016 at 9:19

marcinj · Accepted Answer · 2016-06-01 08:55:29Z

2

Built in array type does not allow to assign to it another array or use aggregate initialization on already initialized array, you would have to use memcpy or for loop to update it with new values. Your code will compile if you use std::array instead. It provides operator=:

operator= (implicitly declared) overwrites every element of the array with the corresponding element of another array (public member function)

http://coliru.stacked-crooked.com/a/8e664210b7f7f73b

i am not sure if this will work as fast as you expect, gcc will generate lots of mov instructions : https://godbolt.org/g/tzUqC3. I suppose it might be faster to use a for loop which will require less cache memory. As always profile your code.

answered Jun 1, 2016 at 8:55

marcinj

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Ferda-Ozdemir-Sonmez Over a year ago

Hi, std::array solved my problem. Thank you very much

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