Constants

i have defined constants in a header file

//Define constants

#define Grids 70
#define Hori_L 1.0
#define delta_x Hori_L*pow((Grids - 1), -1) //m

However when i call them in my .cc files I need to place a bracket around them (delta_x). Why is that?

Cheers

Why you shouldn't use #define here.

What happens when you use constants without brackets? What error messages do you get?

If only the third one is the problem:

Code:

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#define delta_x Hori_L*pow((Grids - 1), -1) //m

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Then you need to remove the comment from it because it may cause problems like this:

Code:

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#define Grids 70
#define Hori_L 1.0
#define delta_x Hori_L*pow((Grids - 1), -1) //m

int main(int argc, char **argv) {
    printf("%d\n", delta_x);
    return 0;
}

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will become this after preprocessing:

Code:

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#define Grids 70
#define Hori_L 1.0
#define delta_x Hori_L*pow((Grids - 1), -1) //m

int main(int argc, char **argv) {
    printf("%d\n", Hori_L*pow((Grids - 1), -1) //m);
    /*notice that the end of the statement gets cut off*/
    return 0;
}

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and you should have brackets around that anyway, so that the order of operations doesn't get messed up


it should look like this:

Code:

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#define delta_x (Hori_L*pow((Grids - 1), -1))

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you could add C-style comments and be ok.

Code:

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#define delta_x (Hori_L*pow((Grids - 1), -1)) /*m*/

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you should use const int or const double in C++

For one thing, I note that x to the power of -1 is actually 1/x. So the expression:

Code:

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Hori_L*pow((Grids - 1), -1)

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can be simplified to:

Code:

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Hori_L / (Grids - 1)

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Then as vart suggested, you should prefer const to #define here, so it would be:

Code:

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const int Grids = 70;
const double Hori_L = 1.0;
const double delta_x = Hori_L / (Grids - 1);

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> However when i call them in my .cc files I need to place a bracket around them (delta_x). Why is that?
Because #define is just a dumb search/replace operation.

Eg.
#define ADD(x,y) x + y

Then you do
x = ADD(1,2) * ADD(3,4);
What the compiler sees is
x = 1 + 2 * 3 + 4;
At which point, the precedence is all screwed and you end up with something completely different to what you intended.

Anything more complicated that a simple constant should have () around the whole thing to prevent any kind of precedence mishap later on.