should i use #define or const in declaring constant variables?? which one is better? when should i use #define or const?
for example when i declare a hexadecimal (0x01), should i use #define or const?
A constant variable will take up space, and finding its value may take more time. A #define will, as per the example you have shown, simply be a text replacement of the value 1 in source code. That is, it is directly encoded into the executable.
But with a const, you get type checking. This may be an advantage to prefering it.
For array sizes, I prefer neither. Using sizeof array / sizeof *array will get you the number of elements when array is in scope, and where it isn't it IMO should be passed as a parameter.
For bit masks, I would use a #define, if not a macro like this.
#define BIT(x) (1 << (x))
For data types other than integers, it too depends -- but I generally try to minimize the number of #defines in my code.