Using Switch With MACROS - c

Question

I am trying to simulate Switch statement using macros in my Header file (.h) .
I have some predefined macros:
#define MULTIPLY_BY_1 1
#define MULTIPLY_BY_10 2
#define MULTIPLY_BY_100 3
#define MULTIPLY_BY_1000 4
#define CHOSEN_FACTOR MULTIPLY_BY_100
I have a const result that takes a value according to CHOSEN_FACTOR (The user will define this macro). I am in the header file and I want to "simulate" the switch statement like this:
switch(CHOSEN_VALUE)
{
case MULTIPLY_BY_1:
const uint16_t result = 5;
break;
case MULTIPLY_BY_10:
const uint16_t result = 50;
break;
case MULTIPLY_BY_100:
const uint16_t result = 500;
break;
case MULTIPLY_BY_1000:
const uint16_t result = 50000;
break;
default:
break;
}
EDIT:
In the source file (.c), I want to use result like this:
uint16_t foo(void)
{
uint16_t myFoo = getMyFooValue();
return result * myFoo;
}
Is there any macros-based-solution to so ? Is there a more optimised approach to get the same result ?

Answer 1

Like in this answer suggests, i would suggest you to use enum for getting the selected value and based on the selection with the enum type, return the macro that you have specified. To do that, you can create a function and get the correct macro as shown below.
typedef enum {
MULTIPLY_BY_1
MULTIPLY_BY_10
MULTIPLY_BY_100
MULTIPLY_BY_1000
}multiplier_t;
uint16_t foo(multiplier_t multiplier)
{
switch (multiplier) {
case MULTIPLY_BY_1:
return 1;
case MULTIPLY_BY_10:
return 2;
case MULTIPLY_BY_100:
return 3;
case MULTIPLY_BY_1000:
return 4;
}
return 0; // just in case no code matches
}
Hope this helps you.

Answer 2

Assuming I understand what you're asking for, I think you're looking for something like this:
#define INIT_FACTOR(var, value) \
#ifdef CHOSEN_FACTOR \
#if CHOSEN_FACTOR == MULTIPLY_BY_1 \
const uint16_t var = value \
#elif CHOSEN_FACTOR == MULTIPLY_BY_10 \
const uint16_t var = value * 10 \
#elif CHOSEN_FACTOR == MULTIPLY_BY_100 \
const uint16_t var = value * 100 \
#elif CHOSEN_FACTOR == MULTIPLY_BY_1000 \
const uint16_t var = value * 1000 \
#endif \
#else \
const uint16_t var = value \
#endif
This will define a macro named INIT_FACTOR that takes two arguments, the name of the variable to define and the starting value. You'd add it in the body of your code as
INIT_FACTOR(result, 5);
and then, if CHOSEN_FACTOR is MULTIPLY_BY_100, that line would expand to
const uint16_t result = 5 * 100;
If CHOSEN_FACTOR is not defined, that line expands to
const uint16_t result = 5;
Remember that macro substitution happens at compile time, not run time. If you want a run time solution, this isn't it.

Answer 3

Selection in the preprocessor can be done by using a helper macro to expand a parameter and then pasting it with a token to kludge a look-up table:
#include <stdio.h>
int main(void)
{
#define MULTIPLY_BY_1 1
#define MULTIPLY_BY_10 2
#define MULTIPLY_BY_100 3
#define MULTIPLY_BY_1000 4
#define CHOSEN_FACTOR MULTIPLY_BY_100
#define Foo1 1
#define Foo2 10
#define Foo3 100
#define Foo4 1000
#define FooHelper(x) Foo##x
#define Foo(x) FooHelper(x)
printf("%d\n", Foo(CHOSEN_FACTOR));
}
Preprocessor abuse of this sort should be generally avoided and likely is not needed for the actual problem that motivated this question.

Answer 4

Thank you all for your answers. I was looking for a specific solution and I guess I found it.
#define MULTIPLY_BY_1 0
#define MULTIPLY_BY_10 1
#define MULTIPLY_BY_100 2
#define MULTIPLY_BY_1000 3
const struct
{
uint8_t index;
uint16_t value;
}myArray[] = {
{MULTIPLY_BY_1, 1},
{MULTIPLY_BY_10, 10},
{MULTIPLY_BY_100, 100},
{MULTIPLY_BY_1000, 1000}
};
#define CHOSEN_VALUE MULTIPLY_BY_10
const uint16_t result = myArray[CHOSEN_VALUE].value;
void foo(void)
{
printf("%d", result); // 10
}