Using C preprocessor concatenation to get number in exponential notation

Question

Why does code:

#define EXPONENT(num, exp) num ## e ## exp
EXPONENT(1,1)
EXPONENT(1,-1)
EXPONENT(1,+1)

after preprocessing changes into:

1e1
1e- 1
1e+ 1

and not into

1e1
1e-1
1e+1

? I suspect it might be because -1,+1 are parsed as two tokens (?). However, how in that case obtain the latter result?

I think you need another level of indirection... see here http://stackoverflow.com/questions/1489932/c-preprocessor-and-concatenation — , Jul 12 '12 at 13:43
I think another level of indirection is necessary, when trying to concatenate with value of another #define, which is not the case here. — kwitek, Jul 12 '12 at 13:55
@OliCharlesworth: Sorry for the noise, forgot to check with negative value. — Jo So, Jul 12 '12 at 14:06
This is indeed strange. If the macro is just `e ## exp`, then no spaces appear. — Oliver Charlesworth, Jul 12 '12 at 14:10
@OliCharlesworth: see here: http://stackoverflow.com/a/216975/195488 — , Jul 12 '12 at 14:27

score 4 · Answer 1 · answered Jul 12 '12 at 14:37

4

You're right, -1 and +1 are two preprocessing tokens, hence only the first is pasted to the e.

For me,

#define EXPO(num, sign, ex) num ## e ## sign ## ex

EXPO(1,,1)
EXPO(1,-,1)
EXPO(1,+,1)

worked with gcc-4.5.1.

answered Jul 12 '12 at 14:37

Daniel Fischer

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This works. However, to solve my problem you have changed its description ;-) I need a macro that takes only two arguments, number and exponent... – kwitek Jul 12 '12 at 14:41
If that's an unchangeable requirement, I have no idea how to do it. It may be impossible. – Daniel Fischer Jul 12 '12 at 15:02

score 3 · Answer 2 · edited May 23 '17 at 12:07

3

I think the problem you are running into is considered undefined behavior. Per gcc 4.3.2 documentation on concatentation:

However, two tokens that don't together form a valid token cannot be pasted together. For example, you cannot concatenate x with + in either order. If you try, the preprocessor issues a warning and emits the two tokens. Whether it puts white space between the tokens is undefined. It is common to find unnecessary uses of ## in complex macros. If you get this warning, it is likely that you can simply remove the `##'.

See also this answer on SO which exhibits the same problem.

Edit:

I managed to get this to work, but you'll need two macros for + and -

#define E(X) 1e-##X
int main()
{
  double a = E(10); // expands to 1e-10
  printf("%e", a);
  return 0;
}

edited May 23 '17 at 12:07

Community

1
1

answered Jul 12 '12 at 14:35

Thanks. Does that mean that it is impossible to solve my problem with C preprocessor? – kwitek Jul 12 '12 at 14:43
@kwitek: Did you try a variant of the last code sample in that answer? – Jul 12 '12 at 14:46
Yes. #define EXPO(x, y) x e y leaves whitespaces, and #define EXPO(x, y) xeb results in string xeb ;( – kwitek Jul 12 '12 at 14:52

score -1 · Answer 3 · answered Jul 12 '12 at 15:18

-1

#include <stdio.h>
#include <stdlib.h>

#define EXPONENT(num, exp) atof(#num "e" #exp)

void main(){
    double x = EXPONENT(1,-1); 
    printf("%f", x);
}

answered Jul 12 '12 at 15:18

BLUEPIXY

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I think Daniel's suggestion is better. – BLUEPIXY Jul 12 '12 at 15:23

Using C preprocessor concatenation to get number in exponential notation

3 Answers3